zoological Journal of the Linnean Sociep (1986), 86: 279-307. With 14 figures
New land tortoises (Testudines:
Testudinidae) from the Miocene of Africa
PETER MEYLAN* AND WALTER AUFFENBERG
Florida State Museum, Gainesville, Florida 32611, U S .A.
Received April 1984, revised manuscript acceptedf o r publication JanuaT 1985
The described fossil testudinids from the Miocene of Africa are reviewed. Ceochelone stvomeri
sp. nov. is described from Lower Pliocene (Langebaanweg) and Miocene (Namib) specimens.
Kinixy erosa, an extant species, is reported from Songhor Hill. Chersina sp, is reported from
Arrisdrift. Impregnochelys pachytectis gen. et sp. nov. is described from Rusinga Island,
Kenya, and is unique in having the anterior shell opening orientated ventrally and in having struts
on the internal surface of the neurals, but shares with Kinixys a unique epiplastron shape, a high
number of axillary scutes and unique orientation of the head of the femur.
KEY WORDS:-Geochelone
Hill - Arrisdrift - Miocene
-
Kinixys CherJina
new taxa.
-
-
Impregnochelys Testudines
-
--
Rusinga
Songhor
-
CONTENTS
,
. . , . , , , .
Introduction . .
Methods . . .
. . . . . . . . .
Systematic palaeontology . . . . . .
.
.
Geochelone Fitzinger 1835 . . . . .
.
.
Geochelone crassa Andrews 1914 . .
. . . .
Geochelone namaquensis Stromer 1926. . . . .
Geochelone stromeri sp. nov. . . . . .
Kinixys Bell 1827 . . . . .
.
. . .
Kinixys erosa Schweigger 1812 . .
. . . .
Chersina Gray 1831
. . . . . . . .
Chersina sp. . . .
. . . . . . .
Impregnochelys gen. nov. . . . . . .
Impregnochelys pachytectis sp. nov. . . .
Discussion . . . . , . .
. . . . .
The relationship of Impregnochelys to Kinixys: parallelism
The behaviour of Impregnochelys
. . . . .
Palaeoecology . ,
.
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. . .
Land tortoise diversity in Africa . . .
. .
Acknowledgements
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Referenres.
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INTRODUC'I'ION
Seven genera and at least 16 species of land tortoises now occur on the
African continent making its extant testudinid fauna the most diverse in the
world. Unfortunately, little is known regarding the evolution and fossil history
*Present address: Department of Vertebrate Paleontology, American Museum of Natural History, Central
Park West at 79th Street New York, NY 10024, U.S.A.
279
0 1986 'Ihe Linnean Society of London
0024-4082/86/030279 29 $03.00/0
+
280
P. MEYLAN AND W . AUFFENBERG
of this fauna. T o date only six fossil tortoises, all of the genus Geochelone (sensu
Auffenberg, 1974), have been described from Africa.
Of the fossil tortoises known from Africa, only two have previously been
recorded from the Miocene: Geochelone crassa (Andrews, 1914: Karungu Beds near
Lake Victoria, Kenya) and Geochelone namaquensis (Stromer, 1926: Namib Desert,
Namibia). This study of fossil land tortoises in major museum collections reveals
the existence of Miocene representatives of two additional genera, Kinivs and
Chersina, as well as a n underscribed genus which has features of Kinixys but
differs sufficiently to be recognized as distinct. Furthermore, additional material
of Geochelone available from Kenya and South Africa reveals the presence of a
third species in the late Tertiary of South Africa and provides further data
bearing on the diversity of the Testudinidae in the Miocene of Africa.
METHODS
All African Miocene land tortoise fossils in the British Museum (Natural
History), Kenya National Museum and South African Museum were examined.
Quantitative and qualitative data were taken following Auffenberg ( 1976).
These data were compared to those taken from a large assortment of fossil and
Recent testudinids and relationships were assessed using shared-derived
characters where possible.
Nomenclature for bones and scutes follows Loveridge & Williams (1957) with
one exception: the small scutes lying anterior to the axillaries on the ventral
surface of the anterior peripherals in some tortoises (i.e. Kinixys), called
submarginals by Loveridge & Williams, are considered to be supernumerary
axillary scutes.
Abbreviations preceding museum catalogue numbers are: BMNH, British
Museum (Natural History); KNM, Kenya National Museum; SAM, South
African Museum; TM, Transvaal Museum; UF, Florida State Museum.
Abbreviations used in the text are: PL, plastron length; SCL, carapace length.
SYS'TEMA'TIC PALAEONTOLOGY
Order Testudines Linnaeus 1758
Suborder Cryptodira Cope 1868
Family Testudinidae Gray 1825
Diagnosis: Following Auffenberg (1974) and Crumly (1984) the family can be
diagnosed as crytodires with no more than two phalanges in the digits of the
manus and pes; only four externally visible digits on the pes; greater and lesser
trochanter of the femur joined by an intertrochantral ridge; coracoid blade
expanded; suprapygals usually two with the upper enclosing the lower; ethmoid
fissure wide; quadrate enclosing stapes; skin of head and limbs scale-covered;
prefrontal scales paired; axillary and inguinal glands as well as cloaca1 bursae
absent.
Ceochelone Fitzinger 1835
Diagnosis: An almost cosmopolitan tortoise genus with triturating surface of
maxilla strongly ridged; median premaxillary ridge absent; maxillary not
TESTUDINIDS O F AFRICAN MIOCENE
28 1
entering roof of palate; anterior palatine foramina small, concealed in ventral
view; prootic typically well exposed dorsally and anteriorly; quadrate usually
enclosing stapes; surangular subequal in height to prearticular; neck with
second, third or fourth centra biconvex.
Carapace never hinged; typically the anterior neurals alternately octagonal
and quadrilateral; outer side of third costal scute about as long as, or longer
than that of the fourth; one axillary scute; two suprapygals, the anterior larger,
bifurcating posteriorly to embrace the smaller posterior element which, in postEocene forms, is crossed near its middle by the sulcus between the fifth vertebral
and the supracaudal.
Plastron not hinged; gular region more or less thickened and produced; gulars
single or paired, longer than broad.
Remarks: The genus Geochelone is not a monophyletic taxon and as such is not
easily diagnosed. This large genus is being split up (Bour, 1980; Crumly, 1982,
1984) but refined limits for the generic name Geochelone have yet to be
established, thus we continue to use this name in the broad sense of Auffenberg
(1974).
As we have pointed out in the introduction, the genus Geochelone is by far the
most abundant testudinid in the African fossil record. Despite this abundance,
we still have a poor understanding of the evolutionary history of this genus in
Africa. In this section we report new material from the late Pliocene and the
Miocene which sheds some new light on that history.
Geochelone crassa Andrews 1914
Diagnosis: A large ( >700 mm PL) African form with the pectorals widest at
the midline and narrowing considerably on either side of the midline; gulars and
pectorals fall short of the entoplastron. This species is based on a partial plastron
from the early Miocene Karungu locality.
Material examined: The nearly complete but crushed shell of a large land
tortoise (more than 1.0 m SCL) KNM-KP 10052 can be referred to this species.
The specimen was collected at Kanapoi and is thus of Pliocene age (Cooke,
1978).
Description: Most of the shell surface is badly broken but scute sulci and bone
sutures are visible on the ventral surface of the anterior lobe of the plastron. The
pectorals are widest at the midline ( 1 10 mm) and narrow laterally (to 30 mm).
The gular scutes appear to reach the entoplastron, the pectorals do not.
Geochelone namaquensis Stromer 1926
Diagnosis: Epiplastra thick with distinct excavation; outline of the epiplastra
‘straight to slightly concave in front with a blunt corner in the hind third of the
lateral edge’; inguinal and axillary scutes large (larger than in G . p u r d a h ) ;
nuchal shield small.
Material examined: Despite the incomplete diagnosis of G. namaquensis,
epiplastra of this species can be referred with some confidence. An isolated left
epiplastron from the Namib Desert (SAM-PQ-N-141)
(Fig. 1B) and an
isolated right epiplastron from the Arrisdrift fauna (SAM-PQ-AD-2789)
(Fig. lA, C) are assigned to this species.
282
P. MEYLAN ,4ND W. AUFFENBERG
Figure 1. Ceochelone namaquensis epiplastra. A, C, SAM-PQ-AD-2789, Arrisdrift, South Africa. B,
SAM-PQ-N-141.
Namib Desert, South Africa. A, B, Ventral views; C, Lateral view of midline
surface.
Descriptions: The Namib epiplastron is 37 mm thick at the dorsal edge of the
epiplastral excavation and 88.5 mm from that point to the anterior edge. ‘The
epiplastral excavation is very well developed. T h e bone has straight anterior
and lateral edges. There is no constriction at the gulohumeral sulcus. The dorsal
surface of the bone is flat. The gular scute probably reached the entoplastron in
this individual.
The Arrisdrift epiplastron is nearly a mirror image of the Namib specimen. I t
is 45.5 mm thick a t the posterior edge of the epiplastral excavation and 83 mm
from that point to its anterior edge, T h e excavation is very well developed. The
front edge of the bone is straight, the lateral edge is nearly straight. Like the
Namib specimen, the Arrisdrift epiplastron has a flat dorsal surface. I n this
specimen the gular scute does not reach the entoplastron.
Comparisons: With the minor exception of the short gular scute of the Arrisdrift
specimen, both of these epiplastra are as described and figured for G. namaquensis
by Stromer (1926). This element is not diagnostic in comparison with certain
other species in the genus (i.e. G. pardalis). However, given the geographic and
stratigraphic proximity of this new material to the type, we feel our assignment
is justified.
Geochelone stromeri sp. nov.
(Figs 2 & 3, Table 1)
Holotype: SAM-L-13721, a partial shell including a nuchal, right peripherals
two through eight, left peripheral eleven, a pygal, neurals three and four and
parts of adjacent pleurals, the left forelobe of the plastron, the right
xiphiplastron and associated fragments.
Paratypes: SAM-PQ-N-140 plus SAM-PQ-N-147 (two specimens believed
to represent the same individual) constitute about three-quarters of a plastron
from the Namib desert.
TESTUDINIDS OF AFRICAN MIOCENE
283
Figure 2. Geochelone stromeri sp. POV. SAM-L-13721 HOLOTYPE. A, Nuchal, dorsal view.
B, Nuchal, ventral view. C, Left epiplastron, lateral view. D, Pygal. E, Xiphiplastron. F, Left half of
forelobe of plastron. G, Shell, right lateral view partially restored.
Type locality: The holotype was collected from the Pellatal Phosphate Member
(PPM) of the Varswater Formation, E Quarry Langebaanweg, South Africa
(see Hendey, 1981).
Diagnosis: The only medium-sized (300-500 mm adult P L ) member of the
genus from mainland Africa with epiplastra which are convex on their
anteriodorsal surface and are not excavated posteriorly.
Etymology: Following his description of Geochelone namaquensis, Stromer ( 1926)
mentions the occurrence of material in the Elisabeth Gold Fields of South
Africa which he describes as having “a simple convex front edge”. This was very
probably material of the species described here. We have chosen to honour the
apparent discoverer of this new form, E. V. Stromer.
Description o f hololype: The nuchal is wider than long and has a very well
developed cervical scute which is nearly square on the dorsal surface but widens
considerably on the ventral surface (Fig. 2A, B; see Table 1 for measurements).
There is a distinct notch in the dorsoposterior edge of the cervical scute. T h e
P. MEYLAN AND W. AUFFENBERG
284
0 10 20 30 40 50mm
3. Geuchelone strorneri. PARATYPE. A, Plastron, ventral view, composite horn
SAM-PQ-N-140 and SAM-PQ-N-I 47. B, Lateral view of epiplastron at the midline.
Figure
nearly complete series of right peripherals are all quite tall especially in the
region of the bridge (Fig. 2G), suggesting that G. stromeri had a highly domed
shell, somewhat like G. radiata. There is no lateral ridge on the bridge
peripherals. The available neurals are four- and eight-sided and adjacent
pleural fragments appear to have formed long acute triangles with truncated
apices (Fig. 2G). The pygal is half as broad again as it is tall and is broadly
concave on the dorsal edge (Fig. 2D).
The available epiplastron is not posteriorly excavated. Its anterior surface is
broadly convex in lateral view (Fig. 2C) and it has a slightly protruding
epiplastral lip (Fig. 2F). The gular scutes are contained on the epiplastra, and
the pectoral just barely reaches anteriorly to contact the entoplastron. T h e
xiphiplastron is wider than long and indicates the occurrence of a broad shallow
anal notch in this species.
Description of paraQpe: The anterior lobe is worn around the edges. It includes
both epiplastra, the entoplastron and most of both hyoplastra of a tortoise of
about 350 mm PL (see Table 1 for measurements). The epiplastra are rounded
in outline and have a convex dorsal surface (Fig. 3B). There is no pronounced
epiplastral lip and no posterior excavation of these bones. The posterior plastral
TESTUDINIDS O F AFRICAN MIOCENE
285
Table 1. Measurements of type material of Geochelone
stromeri (in mm). Those measurements marked with
an asterisk are likely to have been affected by wear of
the specimen
Measurement
Nuchal bone length
Nuchal bone width
Anterior cervical scute width
Dorsal cervical scute length
I’osterioventral cervical scute width
Approximate plastron midline length
Maximum epiplastron thickness
Midline epiplastron length
Midline entoplastron length
Midline hyoplastron length
Midline hypoplastron length
Midline xiphiplastron length
Midline depth anal notch
Width anal notch
Midline gular scute length
Midline pectoral scute length
Midline femoral s a t e length
Midline anal scute length
Holotype
93.5
125
26.5
26
44
425
28.5
52.7
93
79
65
25.5
71
40.5
28
17.5
Paratype
350
29*
22.5*
58*
99
75
24
85
37
11
71.5
45
portion consists of a left hypoplastron and associated left xiphiplastron. The anal
notch is a deep wide triangle.
Remarks: T h e only Geochelone species from the African mainland lacking a
posterior epiplastral excavation is G. laetoliensis (Pliocene of Laetoli, Tanzania)
(Meylan & Auffenberg, 1986). This giant species, from the Pliocene of
Tanzania, probably grew to more than 1 m PL. The type material of G. stromeri
represents mature tortoises, about one-third the size of G. laetoliensis.
The diagnostic characters of the epiplastra referred to above deserve further
comment. Epiplastron shape is frequently sexually dimorphic. Anterior
extension of these bones in males of Geochelone atlas, G. sulcata, Gopherus berlandieri,
Chersina angulata and others is sometimes quite remarkable. But the condition of
the epiplastral excavation is apparently associated with the function of the
forelimbs and head retraction (Szalai, 1930, 1933) and is apparently not subject
to sexual selection (Auffenberg, 1964). For this reason, the possibility that
G. namaguensis and G. stromeri are actually males and females of a single species is
ruled out.
Posterior excavation of the epiplastra is widespread in the Testudinidae, and
relative to the ancestral condition found in the Emydidae can be considered a
derived feature shared by most members of the family. However, the absence of
this excavation is not uncommon among tortoises (Meylan & Auffenberg,
1986). I t is absent or weakly developed in very primitive tortoises (Stylemys,
Homopus etc.) and in highly derived island endemics of the genus Geochelone (due
to total reduction of plastron; C. Crumly, pers. comm.). We consider the
absence of posterior plastral excavation in Geochelone to be a loss of the feature
and therefore derived within the genus (Auffenberg, 1964; Meylan &
Auffenberg, 1986). The loss of excavation in the undescribed form suggests that
P. MEYLAN AND W. AUFFENBERG
286
it is not ancestral to the generalized African tortoises (subgenus Geochelone) which
survive to the present (G. brachygularis, G. pardalis, G. sulcata).
K i n i g s Bell 1827
Diagnosis: Carapace of adult hinged; supernumerary axillary scutes present;
gular region greatly thickened and projected; gulars divided; posterior border of
the xiphiplastra rounded to accept the ventral margin of the posterior lobe of
carapace during closure.
Kinixys erosa Schweigger 1812
Diagnosis: Cervical scute absent (present in other species); u p to three axillary
scutes present (two or fewer in other species).
Material examined: A total of 28 specimens from the lower Miocene, Songhor
Hill locality near Lake Victoria, Kenya, has been studied. A partial carapace
in the BMNH (KNM-SO-544) has three axillary scutes (Fig. 4D) and lacks a
cervical scute (Fig. 4C). Twenty-seven isolated shell elements in the K N M
include two left epiplastra (KNM-SO-555 1 and 5557)) an entoplastron
(KNM-SO-5553)) a partial hypoplastron (KNM-SO-3837)) a xiphiplastron
(KNM-SO-5550) and parts of five pleurals and three peripherals.
0
20 mm
20 rnm
0
20 m m
Figure 4. Xiniys erom Songhor Hill, Kenya (lower Miocene). A, B, Right xiphiplastron
(KNM-SO-5550): A, ventral view; B, dorsal view. C, D, Anterior half of carapace
(KNM-SO-544): C, dorsal view; D, ventral view of left side. Stippled area (D) filled with matrix.
TESTUDINIDS OF AFRICAN MIOCENE
287
Descriptions and comparisons: KNM-SO-544 is the anterior one-third of a
carapace stored at the British Museum (Fig. 4C, D). I t includes both fifth
peripherals, both third pleurals, third neural and all carapacial elements
anterior to them. The nuchal bone lacks a cervical scute. The first three neurals
are four-, eight- and six-sided; the third having posterior contacts to the fourth
pleurals. There are three axillary scutes on each side (Fig. 4D).
This specimen is most important in assigning the Songhor Kinixys material to
K. erosa. It is the only specimen which includes a nuchal bone and anterior
peripherals. T h e remaining material does not differ from K. erosa, but only
KNM-SO-544 exhibits the derived features diagnostic of this species.
The long anterior projection of the Songhor epiplastra is typical of Kinixys.
Both fossil epiplastra seem to represent young individuals. Allometric expansion
of the gular region during growth of the Songhor Kinixys would have resulted in
the condition found in Recent adults. T h e short length in the gular scutes of
these fossils is also typical of Xznixys, for in only one of 16 Recent specimens do
these scutes reach the entoplastron. They fail to reach the entoplastron in all
available K. erosa. The anterior epiplastral margin in Kinixys belliana and
K. homeana is usually convex; however, this region is concave in four Recent
K. erosa, as in 5551. The hypoplastron consists of the medial third, and has the
inter-hypoplastral suture complete. T h e femoral scute covers 19% of the length
of this suture. This scute covers 0-15.2% of this suture in Recent K. belliana,
8.7-13.8% in Recent K. erosa and 1.4-15.4y0 in Recent K. homeana.
The most diagnostic plastral element at the generic level is the single
available xiphiplastron (Fig. 4A, B). The posteriolateral edge is a single
continuous curve. There is no anal notch or constriction at the femoro-anal
sulcus. Both the shape of the posterior xiphiplastral margin and the lateral
support of the xiphiplastron by the hypoplastron suggested by this specimen are
found in all Kinixys. Though the anal scute in the fossil is slightly shorter (4.676
of midline length) than in Recent Kinixys (82.5-54.1 yo),the extreme variability
of this scute (Auffenberg, 1976) suggests that this condition does not strongly
contradict the assignment of this material to X. erosa.
The remaining isolated elements include parts of at least five pleurals and
three peripherals. I n all of these elements scute sulci are marked by raised keels
rather than depressions. T h e pleurals are not as triangular in shape as those of
. . neural configurations. They are more
advanced tortoises with 4-8-4-8-4.
parallel sided as one would expect with the 6 > 6 > 6 > 6 . . . or 4-8-6>6 . . .
neural formula found in Kinixys. The peripherals include a third left and seventh
left and right. The kinetic hinge in the carapace of Kinixys develops along the
posterior margin of the seventh peripheral and fourth pleural as the tortoise
approaches maturity. T h e development of the kinetic surface is visible in the
preserved seventh peripherals and the fifth pleural. However, some evidence of a
suture is retained, suggesting that the individuals preserved were not full-grown
adults, This correlates well with the subadult condition seen in the epiplastra. I t
seems likely that at least some of the elements described above represent the
same subadult individual or individuals.
Remarks: With few exceptions, the characters of the Songhor Hills Kinixys
agree with those of Kinixys erosa. One difference is the 4-8-6 . . . neural configuration of KNM-SO-544.
Nearly all Recent Kinixys examined have
6 > 6 > 6 > 6 . . . anterior neural series. Variability in the neural series of Kinixys
288
P. MEYLAN AND W. AUFFENBERC
is greater posterior to the hinge (neurals six to eight), but variation also occurs
anterior to this region. For example, two specimens of K. belliana, T M 34680
figured by Broadley (1981) and U F 55477, have neural formulas of 6 > 6 >
5-7 . . . and a specimen of K. erosa U F 57109 has a neural formula of
6 > 5 > 6 < 7 . . .. Variability in the neural series of land tortoises is such that it
would be unwise to base a new taxon on a unique neural configuration
(Lydekker, 1889; Hay 1908; Auffenberg, 1976).
Other deviation of the fossil material from K. erosa is evident in the plastral
scute arrangement. T h e available hypoplastron indicates that the femoral scute
covered slightly more of this bone than in Recent Kinixys erosa. This, too, may be
considered minor variation and does not constitute a significant difference
between the fossils and K. erosa.
Chersina Gray 1831
Diagnosis: Anterior neurals usually with 4-8-4-8 . . . neural pattern; one
suprapygal; no shell kinesis; single gular scute covering a pair of long thick
epiplastra; deeply notched nuchal bone with nuchal scute usually long and
narrow (sometimes small or absent); anterior peripherals not recurved and
somewhat sinuate; one or two axillaries; inguinal large; weak ridge, low on the
bridge peripherals; matures at a small size (175 mm SCL).
Remarks: Fossils of Chersina occur commonly in the Pliocene phosphate
deposits at Langebaanweg, South Africa (Hendey, 1973, 1981). Unreported
specimens are present among Pleistocene material from Hopefield (SAM 3964,
3967 and 9254). The early Miocene Chersina discussed below is from Arrisdrift
on the Orange River in SW Africa and is thought to be about 16 Myr old. The
Arrisdrift fauna is only slightly younger than those of Rusinga and Songhor
(Hendey, 1978). Among the tortoise material from this locality are 20 isolated
elements of a small but mature tortoise which we attribute to the genus Clzersina.
We have not assigned it a species name pending publication of a more detailed
study of the genus by Roger Wood.
Chersina sp.
Material examined: T h e Arrisdrift specimens representing Chersina include two
nuchals (SAM-PQ-AD-I294 and 1984), two neurals (SAM-PQ-AD-898 and
1436), two peripherals (SAM-PQ-AD-I 505 and 2274), two pygals
(SAM-PQ-AD-3084
and one unnumbered), an entoplastron (SAM-PQAD-250 I ) , three hyoplastra (SAM-PQ-AD-73, 5 12 and 832), three hypolastra
(SAM-PQ-AD-876, 1251 and 2262) and two xiphiplastra (SAM-PQ-AD-1141
and 1148).
Descriptions and comparisons: In this section we compare the small Arrisdrift
tortoise in detail only to Homopus, Psammobates and Chei-sina. We have also
considered and rejected the possibility that one of the other small African genera
is represented. The fully ossified shell with complete sutures between elements
eliminates the possibility that Malacochersus is represented. The xiphiplastra are
too deeply notched for the fossil to be Kinixys. In Pyxis (including Acinixys, Bour,
1981) a plastral hinge is often in evidence at the posterior border of the anterior
lobe. The anterior margin of the pectoral scute and the anterior margin of the
TESTUDINIDS O F AFRICAN MIOCENE
289
hyoplastra form right-angles to the midline of the plastron. T h e pectorals have a
longer midline contact than in Chersina and are nearly rectangular in outline.
Furthermore, the humeropectoral sulcus in Pyxis always reaches the
entoplastron (Bour, 1981).
Both fossil nuchal bones (Fig. 5G, H) have large cervical scutes, one-quarter
to one-third of the length of the nuchal bone itself. T h e cervical scutes are wider
posteriorly than anteriorly both above and below. T h e fossil nuchals differ
substantially from Psammobates in their cervical scute size. I n all six Recent
Psammobates tentorius examined, the cervical scute is less than one-fifth of the
nuchal bone length or anterior width. I n some specimens of Chersina and
Homopus the cervical scute length is one-quarter of the total length and one-third
of the anterior width of the nuchal bone as is the case in the fossils.
One of the two neurals (Fig. 5C, D) from Arrisdrift (SAM-PQ-AD-898) is
eight-sided and is most likely a fourth neural. Eight-sided neurals occur in none
of the four Homopus examined, and in only one of six Psammobates. Contrary to
statements in Loveridge & Williams (1957) a 4-8-4-8 . . . neural configuration
is normal for Chersina. All 24 of the Recent Chersina available in the U F
collection have at least one eight-sided neural; nearly all have at least two, and
some have three.
T h e two peripherals are a ninth or tenth and a third left (Fig. 5F). The
posterior peripheral is thickest in cross-section at its midpoint and tapers dorsally
and ventrally. Its lateral outline is broadly convex and the ventral edge is
smooth. Thus, the posterior peripherals of this tortoise were neither serrated nor
recurved. The third left peripheral would have joined the left hyoplastron to
C
D
Ot-...Ld
20 rnrn
Figure 5. Chersina sp. Arrisdrift, South Africa (lower Miocene). Hyoplastra, ventral views: A,
B, SAM-PQ-AD-73.
Neurals, dorsal view: C, SAM-PQ-AD-898; D,
SAM-PQ-AD-832;
SAM-PQ-AD-I 436. Pygal, interior view: E, a composite of SAM-PQ-AD-3084
and SAM
unnumbered. Third left peripheral, lateral view: F, SAM-PQ-AD-I 505. Nuchals, dorsal view; G,
SAM-PQ-AD-1984; H , SAM-PQ-AD-1294.
290
P. MEYLAN A N D W. AUFFENBERG
the carapace in life. A moderately developed ridge, which in primitive tortoises
is continuous with the margin of the anterior arid posterior peripherals, crosses
the fossil third peripheral in its ventral quarter. The portion of the bone above
the ridge is five or six times that below the ridge. The costomarginal sulcus is
evident at the dorsal edge of this element. In both Psammobates and Homopus the
lateral ridge on the bridge peripherals runs through the middle of the
peripherals. The ridge is also far more developed in these genera. Chersina, like
the fossils, has dorsally extended bridge peripherals with a moderate to weak
lateral ridge.
Both pygals are badly broken, but together they show that this tortoise had a
keystone-shaped pygal (Fig. 5E is a composite). T h e dorsal margin of 3084 is
nearly straight, not indented to allow the enclosure of a second suprapygal
between the first suprapygal and pygal. Thus, this tortoise probably had a single
suprapygal. Both Psammobates and Chersina normally have a single suprapygal
and a keystone-shaped pygal. Homopus has two or more suprapygals and a very
small pygal which is usually nearly square.
Neither gular nor pectoral scutes cross the only known entoplastron. I n all of
the Recent Psammobates examined the gulars cross the entoplastron.
Two complete (PQ-AD-832
and 73) (Fig. 5A, B) and one partial
(PQ-AD-5 12) hyoplastra of this small tortoise are available from Arrisdrift. All
of the available hyoplastra have a narrow pectoral scute (10--210/, of
hyoplastron at midline) and a substantial abdominal scute (54-730/, of midline).
The anterior margin of the pectoral scute is concave and lies well posterior to
the anterior margin of the bone in all three specimens. The width of the pectoral
is comparable to that of Psammobates, Homopus or Chersina. But in Psammobates the
area covered by the abdominal scute is smaller than in Recent Homopus, Recent
Chersina or the fossils. Homopus has a narrower entoplastron, relative to
hyoplastron midline length, than that seen in these fossils.
Psammobates is most similar to the fossil hypoplastra in the degree of femoral
coverage ( 13-25y0). Both Chersina (4-18y0) and Homopus (0-100/) have a
smaller portion of this bone covered by the femoral scute.
The two xiphiplastra are much wider than long. An anal scute covers the
posterior one-half of each, and there is little constriction at the femoro-anal
sulcus. T h e anal notch is moderately deep, well over one-third of the
xiphiplastron length, and very wide. The fossil xiphiplastra are wider relative to
midline length than those of the Recent Chersina, Homopus and Psammobates
examined. The anal scutes of all three of the Recent species cover more of the
xiphiplastron. The size of the anal notch is smaller in all three but less so in
Homopus and Psammobates than in Chersina.
Remarks: Both a small and a large tortoise occur in the Arrisdrift fauna. The
larger is Geochelone (discussed above). The smaller one shows obvious evidence of
maturity and cannot be a juvenile Geochelone. This evidence is in the form of
well-developed costomarginal sulci on the available peripherals and complete
sutures at the midline of hyo- and hypoplastra. I n juvenile tortoises the
costomarginal sulci overlie fenestrae and would not be preserved as in
SAM-PQ-AD-1505 and 2274. There is also a fenestra between the hyo- and
hypoplastra in juvenile tortoises. Evidence of this would be present in the
available hyo- and hypoplastra in the form of incomplete sutures.
TESTUDINIDS O F AFRICAN MIOCENE
29 1
Comparison of the various proportions of the bones and scutes of the small
Arrisdrift tortoise to Recent, small, mainland African genera leaves a clouded
impression of its relationships. But examination of derived features of shell
morphology leaves little doubt that the small Arrisdrift tortoise is Chersina. We
find the most useful features to be the presence of the eight-sided neural
(Pa-AD-898) and the tall third left peripheral (PQ-AD-1505), with a weak
lateral ridge.
In their adaptation to terrestrial life tortoises have undergone major changes
in shell morphology. As has been discussed by Auffenberg (1974), the neural
series of advanced tortoises is made up of alternating quadralateral and
octagonal bones. These neurals occur with pleurals which are alternating
triangles with wide ends ventral at eight-sided neurals and dorsal at four-sided
neurals (see Auffenberg, 1974: fig. 3). Associated with this apparent shellstrengthening adaptation is a trend to increase the volume of the shell. This
includes dorsal expansion of the peripherals, especially those of the bridge. This
is most evident in highly domed tortoises like Geochelone radiata or G. pardalis.
Psammobates and Homopus do not exhibit the most advanced features associated
with maximizing shell strength and volume. Neither Psammobates or Homopus
normally have eight-sided elements in their neural series. There is a single eightsided neural in one of the six Psammobates available to us. Most neural series
include asymmetrical five-seven combinations. None of the Homopus has eightsided neurals. Most have a predominance of six-sided neurals with occasional
asymmetrical five-seven combinations present.
Neither Psammobates nor Homopus have dorsally expanded bridge peripherals.
The lateral ridge on the bridge of these two genera lies in the middle of the
peripherals. This ridge is very strong in primitive tortoises (Manouria emys and
M . impressa) and their closest relatives among the Batagurinae, sister group to
the Testudinidae (Hirayama, 1985). It is reduced and then lost as the bridge
peripherals become dorsally expanded. Both Psamrnobates and Homopus have very
strong lateral ridges on their bridge peripherals. The fossil Chersina from
Arrisdrift and Recent Chersina have weak ones.
There is additional evidence that the small Arrisdrift tortoise is not
Psammobates. It does not show the derived condition of the cervical scute found
in that genus. The occurrence of a large cervical scute is primitive in the
Testudinidae. Its absence is considered derived. Though all Psammobates possess
a cervical scute, i t is quite reduced.
The relatively large, keystone-shaped pygal is further evidence that Homopus is
not represented. The available pygals are too large to be from one of the known
members of this genus. Calculations based on the proportions of Recent Homopus
show that an individual with a pygal the size of SAM-PQ-AD-3084 would be
about 400 mm in SCL, about three times the known maximum length for this
diminutive genus.
It is possible that more than one species of tortoise is actually represented by
this material, but because the diagnostic elements are best assigned to Chersina, we
consider it most prudent at this point to refer all of the material discussed above
to Chersina. Specific assignment of the Arrisdrift Chersina must await completion
of studies of the extensive material representing this genus from the Pliocene
faunas of Langebaanweg.
292
P. MEYLAN AND W. AUFFENBERG
Impregnochelys gen. nov.
Diagnosis: As for the type and only known species.
Zmpregnochelys pachytectis sp. nov.
(Figs 6-12, Table 2)
lioloppe: BMNH R 5708; an adult male, collected by Louis Leakey in 1935,
consisting of a nearly complete plastron (Fig. 6A, B, C) with associated pygal
(Fig. 7A, B), second suprapygal, right peripherals six, seven, ten and eleven, left
peripherals five, six, ten and eleven and some unassigned fragments,
Figure 6. Zmpreg~ochelyspachytectisgen. & sp. nov. BMNH R 5708 HOLOTYPE. Plasiron:
A, ventral kiew; B, dorsal view; C , lateral view of anterior lobe. Stippled areas reconstructed.
Figure 7. Impregnochelys pachytectis gen. & sp. nov. BMNH R 5708 HOLOTYPE. Pygal
region: A, exterior view; B, interior view of pygal. Stippled areas reconstructed; shape of first
suprapygal and dorsal half of eleventh left peripheral (dashed) hypotheskd from condition of
remainder of specimen.
TESTUDINIDS OF AFRICAN MIOCENE
293
Paratypes: BMNH 9462 and 9463 from the type locality.
Type locality and horizon: Rusinga Island, Lake Victoria, Kenya; assumed to be
from the uppermost Kathwanga series: lower Miocene.
Diagnosis: Impregnochelys pachytectis differs from all members of the family
Testudinidae, except one or all species of the genus Xinixys, in possessing the
following features: three axillary scutes, a long, narrow anterior plastral lobe,
long thick epiplastra with non-divergent paired gulars and anteriorly extended
peripherals. It differs from all Xinixys species in lacking a kinetic carapace, in
having anterior peripheral bones with extreme ventral as well as anterior
projection, and in having either the ribheads, or struts for their support, on all
neurals.
Etymology: The generic name, Impregnochelys, is derived from impregnable
( = able to resist attack) and chelone (Greek = turtle). The specific name,
pachytectis, is derived from pachy (Greek = thick) , and tectum (Latin = roof or
shell). The name reflects our belief that adults of this species were probably
highly resistant to predation because of their very thick shells.
Description of holotype: The plastron is nearly complete and has the ventral
portions of right peripherals six and seven attached (Fig. 6A, B; Table 2). The
Table 2. Measurements and character conditions for holotype and paratypes of
Impregnochelys pachytectis (all measurements in mm)
~
Specimens
Character
Plastron length
Thickness of epiplastral lip
Dorsal length of epiplastra
Epiplastral excavation
Accessory scutes on epiplastra
Anterior plastron lobe width
Anterior plastron lobe length
Entoplastron length
Entoplastron width
Gulars enter entoplastron
Pectorals enter entoplastron
Length of pectorals on midline
Pectoral expands laterally
Bridge length
Posterior plastral lobe length
Posterior plastral lobe width
Shape of lateral edge of posterior
plastron lobe
Anal notch
Anal scutes on dorsal surface
of xiphiplastra
Accessory scutes on dorsal surface
of xiphiplastra
Supracaudal
Growth rings
Axillary scutes
Suprapygals
First suprapygal encloses second
against pygal
R 5708
(holotype)
620
73-80
147
well developed
present
267
203
123
134
Yes
no
35
slightly
267
165
2 78
63
129
well developed
present
280
196
111
I36
Yes
no
54
95
crushed
present
250
143
107
I10
Yes
no
26
Yes
172
314
straight
wide, shallow
straight
wide, shallow
extensive
extensive
Yes
single
absent
Yes
single
absent
absent
3
2
2
2
(?I
no (?)
294
P. MEYLAN AND W. AUFFENBERG
plastral formula is: abdominals > gulars > humerals > femorals > anals > pectorals. T h e entoplastron has an extremely strong ridge on the dorsal surface which
bifurcates anteriorly. T h e posterior lobe of the plastron is slightly concave below
and very thick in the region of the anal scutes. Fusion occurs between the hypoand xiphiplastra.
Partial right peripherals numbers six and seven are represented by their
middle one-third only. The sulcus between marginals seven and eight is very
close to the suture of these two peripherals. There is no lateral ridge on these
bridge peripherals.
The pygal region elements include a wide trapezoidal pygal, a rounded
second suprapygal, most of both triangular eleventh peripherals, and part of
both tenth peripherals (Fig. 7A). T h e second suprapygal rests within a rounded
notch in the pygal, and has the posterior sulcus of the fifth vertebral scute at its
dorsal edge. The eleventh peripherals project ventrally to one-third of their total
height at their medial (pygal) edge. The supracaudal scute is single and
rectangular and extends more than three-quarters of the way up the internal
surface of the pygal (Fig. 7B).
Scute sulci on the type are variable. They consist of deep troughs with raised
margins on the peripherals, simple troughs on the plastron and single ridges
around the inguinal scutes.
Description of paratypes: B M N H R 9462 includes portions of the carapace,
plastron and limbs (Figs 8-10; Table 2) of a male tortoise about equal in size to
the holotype. Plastral elements include both epiplastra, most of the entoplastron,
most of the right and part of the left hyoplastra, portions of both hypoplastra at
the inguinal buttress, and most of both xiphiplastra. Carapacial material
includes parts of left peripherals one, two, three, four, seven and eight and right
peripherals two, three, four, seven, eight and eleven; part of the nuchal bone; a
complete pygal and second suprapygal and a n assortment of girdle and limb
elements.
Anal scutes cover much of the dorsal surface of the xiphiplastra stopping at a
distinct ridge against which the ischium of the pelvic girdle would rest. This
ridge is continuous anterolaterally and supports a very well developed inguinal
buttress. Unlike the holotype, this specimen lacks the single large pit in this
region. The xiphiplastra are pitted ventrally a t their posterior margin. The
Figure 8. Zmpregnochelys pachytectis gen. & sp. nov. BMNH R 9462 PARATYPE. Antcrior
right peripherals: A, interior view; B, exterior view.
TESTUDINIDS OF AFRICAN MIOCENE
0
295
50
Figure 9 Zmpregnochelys pachytectis gen. & sp. nov. BMNH R 9462 PARATYPE Pygal
region: A, exterior mew, B, iiiterioi view
E
0
C
10 20 30 40 50 mm
Figure 10. Zmpregnochelys pachytectis gen. & sp. nov. BMNH R 9462. PARATYPE. A, B,
Scapula: A, lateral view; B, medial view. C, D, Femur: C, dorsal view; D, ventral view.
inguinal scute is narrower than high, reaching dorsally to cover the expanded
inguinal buttress.
The portion of the carapace from the midline to the left axillary buttress
includes parts of the nuchal and left peripherals one, two, three and four. T h e
first peripheral is considerably wider than the second; the second and third are
subequal. Marginal sutures are in the middle of the first peripheral, on the
anterior third of the second peripheral, and anterior quarter of the third
peripheral, Parts of three axillary scutes are visible.
The right anterior peripherals are a mirror image of those of the left (Fig. 8A,
B). Both right and left sides have the axillary notch reaching posteriorly to the
fourth peripheral. The anterior carapacial material has costomarginal scute
sulci which fall well below pleuroperipheral sutures. There is no ridge on the
bridge peripherals.
296
P. MEYLAN AND W. AUFFENBERG
The pygal, second suprapygal, and partial right peripheral eleven are as
described for the type (Fig. 9). The pygal is trapezoidal. The second suprapygal
is rounded with the posterior sulcus of the fifth vertebral scute barely crossing its
dorsal edge (Fig. 9A). Scute sulci are as described for the holotype.
Numerous fragments of the pectoral and pelvic girdles, femora, humeri and
possibly lower limb elements are present (most of the material is fragmented
making element identification impossible). A partial left scapula (with glenoid
fossa) (Fig. 10A, B), partial right and left humeri, partial right ilium, complete
right femur (Fig. lOC, D) and the head of the left femur are recognizable.
The partial left scapula has the main axis of the glenoid fossa oriented
dorsoventrally. The glenoid fossa has a large dorsal rim but small anterior and
posterior rims. The suture for coracoid attachment to the scapula is a t 35" to the
main shaft. The acromial process is at 116" to the scapular blade (Fig. 1OA).
Both humeri are poorly preserved. The left one consists of the proximal
second quarter of the shaft. The right one consists of the middle one-third of the
shaft. I n both, the region of insertion of the latissumus dorsi and teres major
muscles is on the anterodorsal surface of the shaft just lateral to the humeral
head and is greatly enlarged. This results in a rough-bottomed depression
bordered by a thickened periphery which supports the lateral and medial
processes of the humeral head. A large intertubercular fossa is evident in both
specimens.
The left femur is the only complete limb element available (Fig. lOC, U). It is
131.5 mm long, 17.5 mm thick at its narrowest point and 56 mm wide across the
trochanters. T h e latter are joined by a strong continuous ridge equal in height
to either trochanter and the femoral head. The femoral head is 48 mm wide and
35 mm high, with the main axis perpendicular to the femoral shaft. The ventral
surface of the intertrochantral ridge is well developed as a site for extensive
attachment of the puboischiofemoralis and caudi-iliofemoralis muscles
(Fig. 10D). The tibia1 condyle is well developed; the fibular condyle is damaged.
A second paratype, BMNH 9463, is a female about 25% smaller than the two
males (Figs 11, 12; Table 2) represented by the anterior lobe of the plastron
from the pectoral scutes forward, with both axillary buttresses present. T h e
0
50
100mm
,
Figure 11. Impregnochelys pachytectis gen. & sp. nov. BMNH R 9463 PARATYPE. 4,
Anterior lobe and anterior peripherals in ventral view; B, left peripherals ten and eleven, extrrior
view.
TESTUDINIDS O F AFRICAN MIOCENE
297
anterior peripheral series (including the nuchal) is complete between the
buttresses (Fig. 1 IA). Portions of the plastron including parts of both hypo- and
both xiphiplastra are available. Five isolated neurals (Fig. 12), left peripherals
ten and eleven (Fig. 11B), a first suprapygal, and many unidentified fragments
are assigned to this specimen.
The anterior plastral lobe is crushed and broken in the region of the
epiplastra. T h e epiplastra are less massive than in either male and lack any
distinct gular projection.
The anterior lobe of the carapace extends beyond the epiplastra. It is broadly
rounded in outline but concave in the nuchal region (Fig. 11A). T h e third and
fourth peripherals are ventrally directed. The second is more anteriorly directed
but still has strong ventral extension. T h e distal edges of peripherals three and
four and part of peripheral two descend to the level of the plastron. T h e distal
edges of the anterior peripherals are smooth, not serrate or recurved. T h e
nuchal bone has a distinct cervical scute which is rectangular (15 mm wide by
51 mm long) on the dorsal surface but triangular on the ventral surface (52 mm
wide posteriorly).
Left peripherals ten and eleven are unserrated and unrecurved (Fig. 11B).
The eleventh has an angled suture for a broadly trapezoidal pygal, but lacks the
ventral projection seen in the males.
The neurals are eight-sided ( 1), six-sided ( 1 ) and four-sided (3) (Fig. 12). All
of the complete neurals have raised ridges perpendicular to the neural arch of
the vertebrae (Fig. 12A, E). They reach the neural arch in one case (Fig. 12E).
The eight-sided neural and adjacent four-sided neural form a 45" angle in the
neural series (Fig. 12B). T h e neural posterior to the available six-sided neural is
attached at a high angle (60-90") (Fig. 12C, D). T h e first suprapygal is wider
than tall. It is straight dorsally and laterally but slightly concave along its
ventral edge.
Rderred material: A single badly broken specimen in the Kenya National
Museum (KNM-RU-5933 A-EE) collected at the type locality is referred to
this species.
A
C
E
Figure 12. I m p r e ~ ~ o c h e l y s p a c h y t e c gen.
t i s & sp. nov. BMNH R 9463 PARATYPE. Neurals:
A, C, E, ventral views; B, D, lateral views. Dashed area hypothesized, based on remainder of
specimen.
P. MEYLAN AND W. AUFFENBERG
298
Remarks: The important characteristics of Impregnochelys pachytectis can be
divided into three categories: those it shares with other testudinids, those it
shares with only Kinixys and those which appear to be unique to the species.
Character states which are widespread among testudinids (Auffenberg, 1974)
and are found in Impregnochelys include: a long bridge, two suprapygals, large
abdominal scutes, union of the femoral trochanters, gular projection and
posterior epiplastral excavation. A single supracaudal scute occurs in the
majority of tortoise species. Eight-sided neurals are typical of advanced tortoises,
but they also occur in turtles of other families (Pritchard, 1979; Pritchard &
Trebbau, 1984; Meylan, 1985). As in all tortoises and also sea turtles (Walker,
1973), the acromial process of Impregnochelys is set at an angle of over 90" to the
scapula.
Impregnochelys shares six character states with Kinixys which are not among
those listed by Loveridge & Williams (1957: 213, 214) as being primitive for the
family. Their relevance in determining the relationship between these two
genera will be elaborated upon in the discussion section.
( 1 ) Most obvious among these characters is the enlargement of the epiplastra
noted in both male I,pachytectis. I n the type of I.pachytectis and in all adult
Ilinixys the epiplastra are about one-quarter of the total midline plastron length
(Table 3). These bones are thick and long but not divergent anteriorly. The
enlargement in both genera may be responsible for the tendency for the gulars
to form extra scales on the dorsal surface of the epiplastral lip.
(2) Extra axillary scutes occur in both Impregnochelys and Kinixys. There are
three in Impregnochelys and X . erosa, and two in K. betliana and K. homeana.
(3) Both genera have lengthened anterior peripherals. I n Kinixys the
extension is mostly anterior while in Impregnochelys it is anterior and ventral
(Fig. 8A, B) .
(4) These genera also share a common condition of the anterior plastral lobe.
It is longer and narrower than those of other testudinids (Table 4).
(5) The posterior part of the neural series of Impregnochelys includes a highangle bend (60-90") making the most posterior part of the shell vertical in
lateral view. The only Recent tortoise in which such an angle occurs is Kinixys
homeana.
(6) In both genera the main axis of the femoral head forms a high angle to
the long axis of the femur (Table 5).
Table 3. Dorsal epiplastron length (EPL) divided by
plastron midline length (PML) for selected Old World
testudinids. The difference between succeeding pairs
of taxa (except Impregnochelys and Kinixys) is significant
(P<O.OOl). Species with a single gular scute or
divergent gular scutes have not been included
Taxon
Impregnochebs
Sample size
Kinixys
1
25
Geochdone elegans
G. pardalis
Psammobales
28
6
29
EPL/PML
(meanf 1
0.240
0.220k0.032
0.185+0.020
0.147 _fO.O19
0.105f0.011
s.D.)
TESTUDINIDS OF AFRICAN MIOCENE
299
Table 4. Anterior plastron lobe length (ALL) relative to anterior lobe
width (ALW) in selected Old World testudinids. The difference in
ALL/ALW between succeeding pairs of genera is significant (P<0.05)
except in the case of Homopus and Psammobates. There is also a
significant difference between male and female Geochelone. Species
with a single gular scute or divergent gular scutes have not been
included
Taxon
Sample size
Impregnochelys pachytectis
3
12
48
17
27
Kinixys spp. (3 species)
Geochelone spp. (5 species, both sexes)
Geochelone spp. (5 species, males only)
Geochelone spp. (5 species, females only)
Homopus areolatus
Psammobates tentoria
4
5
ALL/ALW
(meanf 1 s.D.)
0.773 f0.040
0.639 & 0.05 1
0.520k0.047
0.559 0.038
0.500k0.034
0.471 k0.016
0.438 & 0.047
Table 5. Angle between the main axis of the
femoral head and shaft of the femur in selected
Old World testudinids. T h e difference between
the angles of femoral head orientation is
significant at the P<O.1 level for Ki'nixys and
Psammobates and at the P<0.01 level for
Psammobates and Geochelone, and Geochelone and
Chersina (this angle was measured with a
protractor from tracings of the femora made with
the dorsal surface facing up)
Taxon
Impregnochelys
Kinixys
Psammobates
Geochelone pardalis
G. elegans
Chersina
Sample size
Angle (deg)
(mean+ 1 s.D.)
1
36
5
18
13
5
75
59.6k6.89
53.6&4.88
48.2 k6.07
47.8k6.10
38.2k 10.1
Six character states of Impregnochelys are unique among land tortoises including
Kinixys. Three result in modification of the anterior shell opening: the suture
between peripheral bones three and four lies in front of the posterior edge of the
anterior shell opening; the anterior lobe of the plastron is significantly narrowed
(Table 4) and anterior peripherals two, three and four project ventrally to a
remarkable degree. The net effect of these features is an anterior shell opening
with a posterior part which opens ventrally. Thus the forelimbs of Impregnochelys
were extended ventrally out of the shell rather than laterally (Figs 8A, B, 11A).
The opening is enlarged by a posterior displacement of the axillary notch
concomitant with a narrowing of the anterior plastral lobe described above.
These modifications may have allowed the forelimbs to be placed more directly
below the glenoid fossa to reduce the stress required to support the massive shell
300
P. MEYLAN AND W. AUFFENBERG
and/or the ventrally projected peripherals may have increased the protection to
the forelimbs. By simply lowering its shell to the ground this heavy turtle could
have kept potential predators from its forelimbs.
The posterior shell opening is also modified, at least in males. The pygal is
more ventrally located in Impregnochelys than in other tortoise species. Using the
sulcus between the fifth vertebral and supracaudal as a reference, we find that
the pygal is lowered b y half the height of the second suprapygal. T h e sulcus
which normally runs through the middle of the second suprapygal barely crosses
the top of this bone in the two available examples (Figs 7A, 9A). Furthermore,
that part of both eleventh peripherals adjacent to the pygal on either side
extends ventrally as two narrow processes to support this long, heavy bone from
either side. In the single female the available eleventh peripheral does not
project ventrally at the pygal suture (Fig. 11B). There is neither pygal nor
second suprapygal available for this specimen. However, the available material
suggests that a large curved pygal may be restricted to males, as in certain other
tortoises (i.e. Gopherus berlandieri).
Xiphiplastra are available only for the two male tortoises. They are
remarkably thick, especially at their posterior edges and are covered by large
anal scutes on their dorsal, posterior and ventral surfaces. The area covered by
this scale on the dorsal and posterior surfaces of this bone is far greater than that
covered on the ventral surface. (Fig. 6A, B). There is a tendency for these large
scales to break up on the dorsal surface forming extra scales in the region of thc
anal notch. The great thickening of this anal region suggests the possibility that
xiphiplastral ramming may have been a part of the behavioural repertoire of
this tortoise (see Discussion).
All complete Impregnochelys neurals have large struts extending laterally from
the neural arch of the vertebrae to the edges of the neural bones (Fig. 12A, E).
In some cases they are in contact with the centrum, in others they are not.
These are either supports for the rib heads or the rib heads themselves. Supports
for the rib heads which rise from the neural bone have been observed in one of
several specimens of Testudo marginata (UF 25791 ). In Psammobates, the rib heads
arise from the pleurals but are supported by small ventral tubercles on the
neurals. I n Gopherus the rib heads can rise from either the neurals or the pleurals
but in either case they are always thin slivers of bone quite unlike those of
Impregnochelys, There is a general trend toward reduction of the rib heads among
tortoises (Auffenberg, 1974). The condition of the neurals in Impregnochelys is an
apparent reversal of this trend.
The xiphiplastra and hypoplastra of the type are fused with no evidence of a
suture between them. It is not possible at present to determine if this condition is
peculiar to the type, characteristic of the species or simply an artifact of
preservation.
DISCUSSION
The relationship of Impregnochelys to Kinixys: paralleEism or common ancestry?
In recent phylogenetic studies, the identification of the polarity of character
transformation series is usually the major concern. Criteria for identification of
character polarity are typically carefully listed. For tortoises, recognition of
TESTUDINIDS O F AFRICAN MIOCENE
30 1
character polarity is not a major problem, as a n out-group is well established
and there is some agreement as to which features are primitive for the family
(Hay, 1908; Loveridge & Williams, 1957; Auffenberg, 1974). T h e critical
problem here is the identification of parallel evolution in homologous structures.
This is accomplished through complete character analysis and the use of a
cladistic hypothesis of relationship.
Impregnochelys and Kinixys share common conditions for six of the characters
examined. All of these are derived conditions for the family. But the possibility
remains that these two genera exhibit these character states because they have
responded to evolutionary pressures in parallel, not because they share a
common ancestor. We have found it useful to examine the functional and
adaptive value of the advanced state of each of these characters to determine
which, if any, may be likely to evolve in parallel. Hecht (1976) has offered a
rationale for such procedures. However, Cracraft (1981) has argued that there is
no concrete methodology for such analyses and that results are subjective in that
they depend on the experience of the investigator. In the case of the testudinid
shell the adaptive value of certain morphologies is quite clear and such analyses
have allowed a more accurate appraisal of the characters.
Close examination of one of the character states shared by Kinixys and
Impregnochelys reveals that it has evolved via different mechanisms and is thus not
homologous in a cladistic sense. The long, narrow anterior plastral lobe
(Table 4) occurs through different changes in shell morphology. T h e anterior
lobe of the plastron is measured from the axillary notches, which in Kinixys, as in
most tortoises, lie below the third peripherals. I n lmpregnochelys the axillary
notches lie below the fourth peripherals. Compared to the condition in most
tortoises, the morphology of Kinixys results from actual lengthening of the
anterior lobe. T h e elongation in Impregnochelys results, at least in part, from
reduction of the bridge. Thus, the long anterior plastral lobe evolved twice and
was not inherited from a common ancestor.
Two of the derived character states shared by Kinixys and Impregnochelys are of
such extreme adaptive value that they might easily occur in parallel. A clear
case can be made for the elongation and thickening of the epiplastra (Table 3).
I n any tortoise species which uses epiplastral ramming in its behavioural
repertoire, hypertrophy of these bones is likely to occur in males. There is a clear
advantage to large size and mass of this battering ram in such species. Because
epiplastral ramming is widespread among tortoises (Auffenberg, 1977), highly
developed epiplastra occur in unrelated forms. But the identical shape and
relative size of the epiplastra in these two genera suggest that in this case
common ancestry is indicated. The epiplastra of Impregnochelys may be
accurately predicted by the allometric growth trajectory of Kinixys (Fig. 13).
The single data point for Imfregnochebs does not deviate from a regression based
on 25 Recent Kinixys. Other species which have greatly elongate epiplastra have
either rounded edges on the lip and a single gular scute (Chersina angulata and
Geocheloneyniphora) or divergent epiplastra (Geochelone atlas, G . sulcata and Gopherus
berlandieri) .
Elongation of the anterior peripherals might also occur in parallel in these
two genera. Extension of this region of the carapace affords added protection for
the forelimbs and head. In any case where a predator learned to gain access to
the body of a tortoise via the anterior shell opening, there would be an
302
P. MEYLAN AND W. AUFFENBERG
Plastron length ( m m )
*,
Figure 13. Relationship of dorsal epiplastron length to midline plastron length in Kinixys and
Impregnochelys. A,Kinixys erosa; e, K. belliana; H, X. hotneana;
Impregnochelys pachytectis. The single
point for I , pachytectis does not deviate from a regression based on the 25 Kznzxys. The regression line
Cy=0.278x-8.64) and 95% confidence limits are shown.
immediate selective advantage for those individuals with longer peripherals.
Because this could occur in any tortoise lineage, its shared appearance in Kinixys
and Impregnochelys is not considered strong evidence that the two are related.
Both genera have as many as three axillary scutes under the anterior
peripherals on each side of the carapace. This derived character state suggests
that they do indeed share a common ancestor. There is no clear functional
correlate for this feature, and it is hard to dismiss as occurring in parallel.
Hutchinson & Bramble (1981) have provided nine postulates about the
evolution of the plastron of turtles. One of these (number 8) states that “new
scales are most likely to be added during increase in plastral size.” One might be
tempted to extend this postulate to the carapace and look for an association
between extra axillary scutes and elongation of the anterior peripherals on
which they occur. Considering Impregnochelys and Kinixys only, this extension of
Hutchinson & Bramble’s postulate seems to provide a sound explanation for the
occurrence of supernumerary axillaries. However, another tortoise with elongate
anterior peripherals, Chersina, usually has a single axillary. Psammobates, which
has extremely narrow anterior peripherals, has two axillaries. Elongation of
anterior peripherals alone does not explain the occurrence of three axillary
scutes in both Impregnochelys and Kinixys. Therefore, we view this occurrence as
strong evidence of common ancestry.
The high-angle bend in the neural series of Kinixys homeana and Impregnochelys
is clearly a derived condition for tortoises. But the most parsimonious cladogram
of relationships of Impregnochelys and the three species of Kinixys (Fig. 14) suggests
that the right-angle bend in the neurals of I. pachytectis and X.homeana occurs in
parallel.
TESTUDINIDS O F AFRICAN MIOCENE
lmpregnochelys
pachyteciis
Kinixys
belliona
Kinixys
homeano
303
Kinixys
erosa
Nuchol scute
R i b heads on neurols
Ventrally oriented
a n t e r i o r s h e l l opening
R i g h t - a n g l e bend i n neurals
Maximum o f three a x i l l a r y s c u t e s
Robust,squared o f f epiplastra
Femoral head a t high angle to s h a f t
Figure 14. A summary of proposed relationships of Impregnochelys and Kzniwys.
The features of the pectoral and pelvic region of Impregnochelys and Kinixys are
difficult to assess without a more thorough investigation of these functional
complexes. Such an undertaking is not within the scope of this study and would
be difficult given the extremely poor representation of these regions in the
known material of Impregnochelys. The only complete limb element, a femur, can
be discussed in terms of what has been reported for testudinids. Zug (1971) has
pointed out that the femoral head is elongate in testudinids. With the femur
held perpendicular to the central axis of the tortoise, the main axis of the head
occurs along a diagonal axis from proximoposterior to anteriolateral. There is
actually some variation in the orientation of the main axis of the femoral head
among testudinids (Table 5). I n some forms it lies nearly parallel to the shaft
(Geochelone gigantea) allowing a maximum amount of movement during
protraction and retraction in a horizontal plane. Extreme protraction of the
femur occurs when the tortoise assumes a defensive posture. I n this posture the
femur is brought into a position medial to the inguinal buttress. I n Kinixys this
movement requires extensive protraction plus some levation to bring the long
femur up over the thickened rim of the hypoplastron. A thick rim also occurs on
the posterior edge of the hypoplastron in Impregnochelys. As the angle of the head
of the femur deviates from parallel there mav be greater possibility for levation.
Kinixys and Impregnochelys have femoral heads which lie at a high angle to the
shaft of the femur, 60.1" in Kinixys (X=35) and higher in the single known
Impregnochelys. Such high angles have not been found in other tortoises (Table 5),
and their occurrence in these two genera might support a hypothesis of common
ancestry.
In summary, the best evidence for the common ancestry of Kinixys and
Impregnochelys is the multiple axillary scutes and the unique orientation of the
head of the femur. However, the condition of the epiplastral lip is also
remarkably similar. The epiplastra of Impregnochelys are exactly what one would
predict for a Kinixys of 620 mm P L . Though there is strong evidence that tortoise
gular regions sometimes evolve in parallel, they may not have in the case under
consideration. Our current hypothesis of the relationships of Kinixys and
Impregnochelys is given in Fig. 14.
304
P. MEYLAN AND W. AUFFENBERG
Further circumstantial evidence for the common ancestry of Impregnochelys and
Kini.vs can be taken from the geographic and stratigraphic proximity of their
first known occurrence: two localities separated by less than 100 km and perhaps
1 Myr (Cooke, 1978). These data provide a biologically believable background
for the suggested divergence of Kinixys and Impregnochelys a minimum of 20 Myr
ago.
The behaviour of Impregnochelys
Auffenberg (1977) concludes that visual and auditory signals are of little
importance in the behaviour of land tortoises. They rely instead on tactile and
chemical signals. Important tactile cues are various forms of shell ramming,
especially by the males. Ramming may be of two types: epiplastral, which
grades into hooking and pushing (Auffenberg, 1978), and xiphiplastral, which
occurs only after mounting. Certain modifications of the shell, namely
enlargement and thickening of the epiplastra and thickening of the xiphiplastra
in the region of the anal scutes, are associated with these forms of behaviour.
The massive size of the epiplastra of male Impregnochelys certainly suggests that
epiplastral ramming was used in male-male and male-female interactions in
this species, as it is in all Recent tortoises whose courtship has been studied
(Weaver, 1970; Auffenberg, 1977). Xiphiplastral ramming behaviour is used by
mounted male tortoises apparently to immobilize their mates before copulation
occurs (Auffenberg, 1978). The fact that xiphiplastra of Impregnochelys are
thickened indicates that they were used in ramming. Also, there is deep pitting
in this region, suggesting that it was frequently traumatized and probably
repeatedly infected, a condition common in large adult male tortoises in which
this behaviour is typical (Auffenberg, pers. obs.), Vocalization and tail probing
occur in conjunction with xiphiplastral ramming in some species and may also
have been a part of the behavioural reportoire of Impregnochelys.
With the observation that these uniquely testudinid behaviour patterns
probably occurred in Impregnochelys, it is possible that they may be a t least
20 Myr old. However, similar scarring and pitting of the xiphiplastra also occur
in a large male Hadrianus of middle Eocene age (D. Bramble, pers. comm.),
suggesting that ramming behaviour may be nearly as old as the family
Testudinidae.
Palaeoecology
Though tortoises in general are adapted for life in savanna and other open
habitats, there are species which occur in forests. Kinixys erosa is one such species.
Loveridge & Williams (1957) consider it a rainforest form and cite several
references to its semiaquatic habits. Collecting localities for this species nearly all
lie within the broadleaf tropical rainforest as figured by Cooke (1978: fig. 2.2).
There are populations in the Mabiru Forest, just north of Lake Victoria
(Loveridge & Williams, 1957). T h e Songhor specimens provide sound evidence
that the region that now comprises the northeast shore of Lake Victoria was well
forested during the earliest Miocene. The Miocene and Recent records of this
species between the Eastern and Western Rift Valleys is best explained by the
hypothesis of Andrews & Van Couvering (1975) who suggest that the western
TESTUDINIDS OF AFRICAN MIOCENE
305
equatorial rain forest may have extended east nearly to the present Eastern Rift
during the Miocene.
As in other ectothermous vertebrates, the presence of growth rings in tortoises
results from life in seasonal environments (Gibbons, 1976; Meylan &
Auffenberg, 1986). Growth rings are absent from all shell material known for
Impregnochelys, which suggests that it lived in a relatively constant climate. T h e
broadleaf equatorial rainforest, hypothesized as the home of the Songhar
K i n i v s , could have provided a relatively aseasonal environment. Other
palaeontological evidence from Rusinga suggests large tracts of unbroken,
evergreen forest (Andrews & Van Couvering, 1975; Butler, 1978).
Land tortoise diuersity in Africa
Though we have been able to add considerably to the known diversity of land
tortoises in the early Miocene of Africa, this diversity does not yet approach that
of the extant fauna. However, our evidence does indicate that large land
tortoises ( > 500 mm SCL) were more speciose. The extant mainland African land
tortoise fauna includes only two that approach 500 mm (Geochelone pardalis and
G . sulcata). T h e remainder includes two genera under 400 mm (Kinixys and
Testudo) and four others 300 mm or less (Chersina, Homopus, Malacochersus and
Psammobates).
The early Miocene fauna, as presently understood, included three large
Geochelone (crassa, namaquensis, stromeri) and the equally large Impregnochels
pachytectis. Only one intermediate size tortoise (Kinixys) and one small tortoise
(Chersina) have been discovered so far. The absence of small taxa probably
reflects collecting and/or preservation bias.
There has been a complete loss of the largest mainland African land tortoises
since the Pleistocene. Geochelone crassa and Geochelone laetoliensis are Late Tertiary
species as large or larger than the extant Aldabran tortoise, Geochelone giganlea.
O n the mainland, land tortoises of this very large size survive until at least the
middle Pleistocene, as indicated by Geochelone species B from Olduvai Gorge,
Tanzania (Auffenberg, 1981) and by undescribed specimens from Koobi Fora,
Kenya (Meylan, pers. obs). However, they do not survive until the Recent. T h e
genus Geochelone in central and southern Africa is reduced to a single
representative between early Miocene and the Recent.
In spite of these reductions in the diversity of larger species, mainland Africa
has retained a striking overall land tortoise diversity. The Recent fauna of
Africa includes 16 species in seven genera, a diversity as great as that known
from any single continent during any single epoch (with the Miocene divided
into early and late portions). It is difficult to identify the reason for this
diversity. We can only point out that the evolution of land tortoises is a
sequence of adaptation of generally aquatic ancestral emydid stock to increasing
terrestriality. Most of the fossil and extant species of the family are found in
savanna or at least open environments. Africa is the only continent on which a
diverse Pleistocene savanna fauna (mostly mammalian) has survived,
apparently due to the continuing equability of the climate (Axelrod, 1967). I t
seems that African testudinids have survived as a part of this fauna, and that
additional living genera will probably be found as late Tertiary and Pleistocene
fossils.
306
P. MEYLAN AND W. AUFFENBERG
ACKNOWLEDGEMENTS
This research was funded by the National Science Foundation (Grant EAR
7926330 to W.A.), the Leakey Foundation (grants to W.A. and P.M.) and the
Florida State Museum. Alan Charig and Cyril Walker, British Museum
(Natural History) loaned us the Imfiregnochelys material for a n extended period.
Those two gentlemen, Richard Leakey, Louis Jacobs (Kenya National
Museum) and Q. Bret Hendey (South African Museum) were very helpful
during visits to their respective collections. At the Florida State Museum,
Rhoda Bryant assisted in preparation of the manuscript, Wendy B. Zomlefer
and Amy L. Petty prepared most of the figures, Mike Heinrichs translated some
difficult material for us. We thank Dennis Bramble, Chuck Crumly, John
Iverson, David Webb and two anonymous reviewers for reading and
commenting on our work. Portions of the manuscript were written by the senior
author during his tenure as Visiting Herpetologist at the Kenya National
Museum.
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