/. Paleont., 72(1), 1998, pp. 133-136
Copyright © 1998, The Paleontological Society
0022-3360/98/0072-0133$03.00
THE FOREFIN OF CHENSAURUS CHAOXIANENSIS (ICHTHYOSAURIA)
SHOWS DELAYED MESOPODIAL OSSIFICATION
RYOSUKE MOTANI AND HAILU YOU
Department of Zoology, University of Toronto, 25 Harbord Street, Toronto, Ontario, M5S 3G5 Canada and
Department of Geology, University of Pennsylvania, Hay den Building, Philadelphia 19104
ABSTRACT—The
hitherto poorly known forefin of Chensaurus chaoxianensis (Ichthyosauria) from the Lower Triassic (Spathian) is
redescribed on the basis of the holotype and two new specimens. The humerus resembles that of Utatsusaurus hataii but is distinctive
in having an emarginated anterior margin. The anteroproximal prominence of the radius is well developed, unlike that of other ichthyosaurs. All three specimens have five metacarpals and many phalanges, but only three carpals, which are identified as the ulnare,
intermedium, and fourth distal carpal. These specimens show that delayed mesopodial ossification occurred in ichthyosaurs, at least in
an early evolutionary stage. Because delayed mesopodial ossification is common among diapsids and is unknown in Jurassic ichthyosaurs, it was lost during the evolution of the Ichthyosauria. The osteogenic developmental axis appears to have continued into the
fourth digit, as in other amniotes. The ossification pattern provides conclusive evidence to support the suggestion that the basal element
of the fifth digit in Early Triassic ichthyosaurs is a metacarpal, rather than a carpal.
INTRODUCTION
LIMBS are usually referred to as fins because
ICHTHYOSAURIAN
of their appearance. Because this aquatic adaptation involved
an extensive modification of limb bones, it is difficult to compare ichthyosaurian limbs with those of terrestrial amniotes. In
advanced Jurassic forms, such as Stenopterygius and Ichthyosaurus, the epipodial, mesopodial, and metapodial elements are
all similarly shaped, a condition that obscures the distinction
among these elements (McGowan, 1972; Johnson, 1977; Caldwell, in press a). On the other hand, the earliest ichthyosaurs
from the Lower Triassic (Spathian), such as Utatsusaurus, have
distinctively shaped mesopodial elements that are arranged like
those of some terrestrial diapsids (Brinkman et al., 1992; Motani,
in press).
In diapsids, the mesopodial region usually ossifies later than
other areas of the limb (Rieppel, 1992a, 1992b, 1992c, 1993;
Caldwell, 1994), a condition that is referred to as delayed mesopodial ossification. Caldwell (in press a, in press b) recently
described mesopodial development in Stenopterygius and reported no delay in mesopodial ossification. Whether the lack of
delayed mesopodial ossification is unique to Stenopterygius or
whether it is common to all ichthyosaurs was not determined
because no juvenile specimens had been reported from the Lower Triassic.
Reexamination of the holotype of Chensaurus chaoxianensis
revealed that its incomplete forefins showed some juvenile features. Additional specimens have been collected from the type
locality of C. chaoxianensis and nearby quarries since the study
of Chen (1985), one of which was briefly reported by Motani et
al. (1996). Among yet undescribed specimens are two nearly
complete forefins. The purpose of the present paper is to 1)
redescribe the holotype forefin of C. chaoxianensis to clarify the
diagnostic features, 2) describe new and well-preserved forefins
that are identified as C. chaoxianensis, and 3) discuss the juvenile features in these forefins.
replacement for Anhuisaurus of Chen (1985), which was preoccupied. The new forefins (IVPP VI1361 and VI1362) were
collected by H. You and J. Lu in 1991. They are from the same
biostratigraphic zone of the type locality, which is located at
Majia-Shan, Chao County, Anhui Province, PR. China. More
detailed stratigraphic information can be found in Chen (1985).
Specimens used for comparisons are IGPS 95941 and 95942,
the holotype and one of the paratypes of Utatsusaurus hataii
Shikama, Kamei, and Murata 1978. The holotype of U. hataii
is not fully mature in skeletal ossification (Motani, in press) but
is more mature than IVPP VI1361 and VI1362. Measurements
were taken with digital calipers and recorded to the nearest 0.1
mm.
DIAGNOSTIC FEATURES OF THE HOLOTYPE
Because Chen (1985) did not describe the forefin of Chensaurus chaoxianensis in detail, a redescription of the forefins of
the holotype of C. chaoxianensis is given below. Although the
areas around the forefins show some chisel marks, no detailed
preparation seems to have been conducted on the holotype since
its discovery. Two incomplete forefins are preserved nearly parallel to each other. The right forefin overlies the left (Figure 1.1)
but elements of each fin can be readily distinguished. No more
than three carpals are present in each forefin, and their small
size suggests that not all carpals were ossified. However, the
possibility remains that the absence of some carpals is a preservational bias.
The leading and trailing edges of the forefins are poorly preserved. An extreme example of this damage is the right humerus,
which appears as a long and narrow structure because the anterior and posterior parts are missing (Figure 1.1). The left humerus is not damaged, but it is concealed by the overlying right
radius posteriorly (Figure 1.1). On the basis of what is exposed,
the humerus is of a common Early Triassic type that has an
anterior flange (Wiman, 1933; Brinkman et al., 1992; Motani, in
press). The anterior flange is distinctive, however, in the presMATERIALS AND METHODS
ence of an emargination in the middle of the anterior margin
The abbreviations used for the institutions are AGM, Anhui (Figure 1.1, arrow). Although Chen (1985) described the huGeological Museum, Hefei, China; IGPS, Institute of Geology merus as having two expanded ends, this appears to be based
and Paleontology, Tohoku University, Sendai, Japan; and IVPP, on the broken right humerus. The radius is also similar to those
Institute for Vertebrate Paleontology and Paleoanthropology, Ac- of other Early Triassic ichthyosaurs in that there is an anteroademia Sinica, Beijing, China.
proximal prominence (Motani, in press). However, this promiThe holotype of Chensaurus chaoxianensis is stored at AGM nence is very well developed in Chensaurus chaoxianensis, with
and retains the field number given by Chen (1985), P45-H85- a long and round anteroproximal margin, unlike that of Utat25. The generic name was given by Mazin et al. (1991) as a
susaurus hataii (Figure 1.4). Therefore, there are two diagnostic
133
134
JOURNAL OF PALEONTOLOGY, V. 72, NO. I 1998
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7—Forefins of Chensaurus chaoxianensis and Utatsusaurus hataii. 1, forefins of the holotype of C. chaoxianensis (AGM P45-H85-25); 2,
left forefin of IVPP VI1362 (C. chaoxianensis), in ventral view; 3, right forefin of IVPP VI1361 (C. chaoxianensis), in dorsal view; 4, forefin of
U. hataii (laterally inverted from Motani, in press). Abbreviations: H, humerus; R, radius; U, ulna; i, intermedium; u, ulnare; 4, fourth distal carpal;
i-v, metacarpals; il-v2, phalanges. Bar scale is two centimeters in total. In Figure 1.1, the right forefin is colored in light gray, the left in white.
The element in dark gray is unidentified. Arrow indicates the position of the notch in the anterior flange of the left humerus. Right bracket indicates
the well-developed anteroproximal prominence of the left radius.
FIGURE
features for the forefin of C. chaoxianensis: 1) a notch in the
middle of the anterior flange of the humerus, and 2) a welldeveloped anteroproximal prominence of the radius with a long
and curved anteroproximal margin.
These two features are also present in Chensaurus faciles,
whose forefin is distinguished from that of C. chaoxianensis
mainly by its smaller size. The erection of C. faciles was largely
based on size: Chen (1985) estimated that C. chaoxianensis was
approximately 60 percent larger than C. faciles in body length
and about twice as large in the forefin length. Size difference
alone, however, is not taxonomically significant in the more
thoroughly studied ichthyosaurs from the Jurassic (McGowan,
1974a, 1974b, 1979); therefore, the validity of the species C.
faciles is doubtful. Moreover, the discrepancy between the body
and forefin lengths of the two species indicates a growth series
in which the forefin shows a positive allometry. It is beyond the
scope of the present paper to evaluate fully the status of C.
faciles.
ADDITIONAL SPECIMENS
The forefins of IVPP VI1361 and VI1362 possess the diagnostic features of Chensaurus chaoxianensis and are similar in
size to those of the holotype of this species. The bones of both
new specimens are compressed. IVPP VI1362 is a partial skel-
eton that extends from the posterior part of the skull to the anterior dorsal region. The left forefin is preserved in ventral aspect
(Figures 1.2, 2.1) and is articulated with the shoulder girdle.
IVPP VI1361 comprises the mid-dorsal region and a forefin,
which is probably the right fin in dorsal view (Figures 1.3, 2.2).
The humerus of IVPP VI1362 is approximately 30 percent longer than that of VI1361 (Table 1). Both specimens have incompletely ossified mesopodial regions, which suggest osteological
immaturity. Measurements are summarized in Table 1.
The humerus is completely exposed in both specimens (Figure
2) and resembles that of Utatsusaurus hataii in having an anterior flange, a concave posterior margin, and a tuberosity in the
middle part of the shaft (Motani, in press). However, the anterior
flange bears a notch that is absent in U. hataii. The notched area
does not appear to be composed of perichondral bone because
surface striations are not parallel to the shaft of the humerus
(Figure 2.1) and, therefore, may be lost with growth. Another
difference between the humeri of Chensaurus chaoxianensis and
of U. hataii is size: the humerus of IGPS 95941 is more than
2.5 times longer than that of IVPP VI1362 (Figure 1). Although
this may be explained by growth, further study is required. The
head is not well developed, and the shaft surface is rough (Figure 2), as in young Stenopterygius (Johnson, 1977), which suggests that the criteria proposed by Johnson (1977) for judging
MOTANI AND YOU—ICHTHYOSAUR FOREFIN
135
TABLE 1—Measurements of the propodial, epipodial, and mesopodial ele-
ments of IVPP VI1362 and IVPP VI1361. In millimeters.
IVPP VI1362
IVPP VI1361
Max. Diam. Min. Diam.
3.5
2.9
3.2
2.9
1.4
1.5
Max. Diam. Min. Diam.
1.2
1.0
1.9
1.8
1.3
1.0
Length
Intermedium
Dist. Carpal 4
Humerus
Ulna
Radius
Width
Width
Length
Ulnare
16.7
15.1
15.9
11.2
8.3
9.8
12.8
11.1
12.1
10.1
7.9
7.7
and Brinkman, 1995). However, this is due to the breakage of
bones when the slabs were separated. The fifth metacarpal of
IVPP VI1361 also appears constricted on one of the slabs, but
it is lunate on the counter slab. The second to fourth metacarpals
are short compared to those of Utatsusaurus hataii, and the extremities are not very much expanded, which indicates that the
diaphyses are incompletely ossified.
The phalanges resemble the metacarpals in that there are both
lunate and flattened-cyUnder types. The lunate type occurs in the
fifth digit, which has only one phalanx in each specimen. The
preserved phalangeal formula is 0-2-3-3-1 for IVPP VI1362 and
1-2-2-2-1 for VI1361. These low phalangeal counts probably
reflect osteological immaturity, but the lack of phalanges in the
first digit of IVPP VI1362 may represent preservational loss
because a phalanx exists in the more immature individual (IVPP
V11361).
DISCUSSION
2—Chensaurus chaoxianensis, photographs of the forefins of the
new specimens. 7, the left forefin of IVPP VI1362; 2, the right forefin
of IVPP VI1361. Bar scale is one centimeter.
FIGURE
the relative age of Stenopterygius are useful for Early Triassic
ichthyosaurs. The deltopectoral crest is present but not well developed.
The ulna has a wide, fan-shaped distal end (Figure 2), which
is typical of Early Triassic ichthyosaurs, such as Utatsusaurus
hataii (Motani, in press). The radius is similar to but more robust
than that of U. hataii. The anteroproximal prominence of the
radius is very well developed, with a long and round anteroproximal margin. This contrasts the poorly developed condition
in U. hataii, which has a straight margin anteroproximally (Motani, in press). Both radius and ulna are associated with weak
but clear ridges that may be related to ossification (Figures 1,
2).
There are only three carpals in IVPP VI1361 and VI1362,
which clearly indicates delayed mesopodial ossification. The elements are identified as the ulnare, intermedium, and the fourth
distal carpal, based on topology. Ossification is probably more
advanced in the latter specimen because the relative size of each
element in comparison to the epipodials is larger (Table 1). The
ulnare is the largest of the three in IVPP VI1362 but is slightly
smaller than the intermedium in VI1361.
Five metacarpals are present in both specimens. The first and
fifth are lunate, and the second to fourth are flattened cylinders.
The two lunate metacarpals are not well preserved in IVPP
VI1362, and they look as if they are constricted in the middle,
as in the first metacarpal of Parvinatator wapitiensis (Nicholls
The delay in mesopodial ossification seen in Chensaurus
chaoxianensis shows, for the first time, that early ichthyosaurs
retained this feature. Such delay has been observed for living
(Rieppel, 1992a, 1992b, 1992c, 1993) and Permian (Caldwell,
1994) diapsids. Because it is absent in Stenopterygius (Caldwell,
in press a), this limb-ossification pattern must have been lost
during the ichthyosaurian evolution. Caldwell (1994) suggested
that the delay in mesopodial ossification may be related to the
difference in the timing in perichondral and endochondral ossifications. He later mentioned that the lack of ossification delay
in the mesopodial of Jurassic ichthyosaurs was possibly related
to a lack of perichondral ossification in the more distal elements
(Caldwell, in press b). Whereas Late Triassic and later ichthyosaurs, including Stenopterygius, all have mesopodials that are
similar in shape to the metapodials, all Early Triassic ichthyosaurs, Mixosaurus, and possibly Cymbospondylus have distinctively shaped mesopodial elements. It seems likely that only
those ichthyosaurs belonging to the latter group retained delayed
mesopodial ossification.
The first three carpals to ossify in Chensaurus chaoxianensis,
namely the ulnare, intermedium, and the fourth distal carpal, are
the same as in many Permian diapsids (Caldwell, 1994). Therefore, it is likely that the primary axis (Shubin and Alberch, 1986)
continues distally into the fourth digit, as in other tetrapods.
There has been much confusion in the identification of carpals
and metacarpals of Early Triassic ichthyosaurs (Shikama et al.,
1978; Mazin, 1986), but a consensus has been reached, at least
for the carpals (Carroll, 1988; Brinkman et al., 1992; Nicholls
and Brinkman, 1995; Motani, in press). The most controversial
element is the one that forms the base of the fifth digit, which
lies between the distal carpal and metacarpal rows of the other
digits (Figure 1). The element is shifted toward the distal carpal
row in Grippia longirostris (Wiman, 1933), and toward the metacarpal row in Utatsusaurus hataii (Shikama et al., 1978; Mo-
JOURNAL OF PALEONTOLOGY, V. 72, NO. 1, 1998
136
tani, in press). It is lunate unlike typical metacarpals and is at
least twice as large as any distal carpal. Shikama et al. (1978)
and Mazin (1986) described this element as one of the distal
carpals, but Brinkman et al. (1992) identified it as the fifth metacarpal and asserted that the fifth distal carpal was absent. The
juvenile fins reported here supply conclusive evidence for the
identification of the element. Whereas the controversial element
shows no delay in ossification, the carpals do. Accordingly, the
element is identified as the fifth metacarpal, as first suggested
by Brinkman et al. (1992).
ACKNOWLEDGMENTS
We are grateful to Z.-M. Dong for assistance to this study. C.
McGowan provided useful suggestions and financial support. J.
Lu and A. He provided support in Chaohu. K. Mori made the
type specimens of Utatsusaurus available, and I. Hayami borrowed them on behalf of the senior author. Y. Tomida arranged
this collaborative study. M. Caldwell made available his in-press
manuscripts. This study was funded by a grant from the Fujiwara
Natural History Foundation, Tokyo, to the senior author, and
Natural Sciences and Engineering Research Council grant to C.
McGowan (A9550).
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ACCEPTED 11 JUNE 1997