J. Linn. Soc. (Zool.), 47, 311, p p . 53-98
With 3 figures
Printed i n Great Britcciii
October, 1967
Radotina and other tesserate fishes
BY T. STANLEY WESTOLL, F.R.S.
Department of Geology,
l l ~ iversit!y
i
of iVetccastle upoz Tyne
It is shown t,liat, tlie stmtigrapliiral assigniticnl of llcrdotimc needs considerahle revision.
Ilrrrloti?rrr fubercickdrr G r c is
~ ronspecific \\ ith ( ' o c c ~ o s t i ~p~r ~i m
s u q Barrande and should be
linownasR.prima (Barrande);tliissperiesdiffersfrotiiitsronteniporary R. tessekrtrr in thesarne
as extreme forms of K. kosorerrsis but, t l i c question of further synonymy is unsettled. The
ial bones of Rrrtlotitw are considered t o represent, a,modified pattern closely relatcd to that
of 3Iacropetalichtli~ida;to this general grouping tiiiiy be added the Rhenanida and Stensiocllidir. I t is highly probable that the ltadot,inida and Acarithot,horaci are synonymous.
Considerat ion of the t,esserate condition in arthrodires suggests that this is a secondary cond i t i o n in the known members. The Psarm noski& ;dso show the development' of secondary
tesseration among the Heterostraci. A I I hylmthesis lia,srd on Holmgren's principle of delamination is apparently illustrated by t,hese fc~rins.In cc:rte.in Rhenanida this delamination goes
so far as to produce rart,ilage bones i n the stinic? position, but at a deeper level, as compared
with dermal bones inearlier forms.
INTII.ODIJCiTIOX
R a d o t i m kosorensis was first named by Gross (1950) on the basis of a partial skull from
t,he Radotin limestone of t,he Loclikov (er)of Koso? in the Radotin valley near Prague, and
furt-her described in later works (Gross, 1958, 1959).Other species were named in 1958 a,s
R . tesselata and R. tuberculata, both from the Konieprus Limestone (fp), and Radotina sp.
from the Taunus Quartzite (Siegenian, Lower Devonian) of the Rhineland . An apparently
relatcd form from Kosoi: was briefly described by Gross in 1958, and more fully under the
na,me li-osomspispeckai in 1959. The import,ance of the Kosoi: specimens is that they occur
in the graptolite-bearing strata (zone of Monograptus hercynicus) bhen referred to t'he
Upper Ludlow Silurian by the Czechoslovak geologist,s.They were described as the oldest
known Arthrodira. However, the specimens from Konieprus are clearly of Lower Devonian
age, as is the Rhineland specimen. Gross (1958) noted that Kolymaspis sibirica (Buistrov,
1955) from the Lower Devonian of Siberia, is probably a relat'ed form. It will be shown
below that the material from Podolia well described by Stensio (1944)as Palaeacantltaspis
riasta Brotzen is probably related t o the Ratlotinida; it is from the Czortkow Stage,
originally referred to the Upper Silurim ( Liitllow equivalent) by ISoslowski and was
placed by Stensio in the new order Acanthothoraci.
Considerable doubt has long been expresscd on the ascription to the Ludlovian of many
European formations, notably hy Shirley ( 1 938, 1039), who drew attention to difficulties
in interpreting brachiopod faunas (see also Shirley, 1962, and Boucot & Pankivskyj, 1962),
and by Westoll(l951) on the basis of the occurrence of vertebrate faunas indicating Upper
Downtonian and Dittonian equiva1ent)sin t,hc Czortliow Stage. Numerous papers in Erben
(1962) have amply confirmed these views, and very recently BouFek (1966),for long a firm
upholder of t,he view that graptolites died u7it.h t>hcSilurian, and thus that the Lochkov
Stage is uppermost Silurian, has described a good monograptid fauna from the highest
beds of Dvorce-Proliop lirnest,one (upper I'rager-Stufe), which is definitely well within the
Emsian equivalents. The probable correlation of t,he strata with which we are concerned is
shown in Fig. 1 ;important horizons with fossil fishes mentioned in this paper are indicated.
T. STANLEY
WESTOLL,F.R.S.
84
From this it seems clear, first that the Kosoi: fossils are not the earliest known arthrodires,
being in all probability of middle Siegenian age ;second, that they are not much older than
the Konieprus species, and roughly contemporaneous with, or possibly a little older than,
the Taunus Quartzite specimens ; third, that the Palaeacanthaspis-Dobrowlania fauna of
t,he Czortkow of Podolia is probably older, Upper Gedinnian ; fourth, that the arthrodires
described by White (1961)from just above the ‘Psammosteus Limestone’ horizon, therefore in the lowest Dittonian as now defined by White and others, are probably at present the
oldest known arthrodires ;and fifth, that the Petalichthyida, Rhenanida and Stensioellida
from the Hunsruckschiefer are only slightly younger than the Kosoi: forms.
BORDERS
WELSH
-
-
?
Series
BOHEMIA
ih::t:rt
Prog
--
--
Anor
SlEGENlAN
hercYnicus%
__
-Calc.deLievin
Fa<\Q- ---
Lochkov
prehercynicus
__
L
GEDlNNlAN
(type)
ex
(extended)
ultimus?
-
--
__
“Old Red“
--
Czortkow
3-
Borsrczow
7
4
F
uniformis
tronsgndiens
-
-5x
4*
2
St.Hubert
Oiqnies
Gdournont
Mondrepuits
Haybos
-
atopus
U
M
--
Zlichov
3-%-.
--
--
PODOLIA
EMSIAN
L
--
~i~~~~ --Motringhem
Vimy
Series I-n Pernes
Ps.de Liivin
ps’Ls’ Mericourt
Downton
STAGES: Selected graptolites
--
-
-Clee
Series
ARDENNES
U
--
-
ARTOlS
Pridoli
42
f;
-f; Skala
Kopanino
Ludlow
Series
LUDLOVIAN
leintwordinensis
epl
?T
Malinovetski
Fig. 1. Correlation of fish-bearing horizons discussed in text. I n Podolia two tenable correlations are shown. The lower boundaries indicated reflect the view that the Skala is, at least in
large part, of Ludlovian age. The higher boundaries suggested would imply that the Skala is
post-Ludlovian; the Czortkow is essentially fixed in relation to the Lower and Middle Ditton
Series. LBB, Ludlow Bone-Bed; Ps. Ls., ‘Psammosteus Limestone’. The numbers refer to
important horizons : 1, earliest arthrodires from just above Psammosteus Limestone ; 2,
Radotina sp. from Taunus Quartzite; 3, Hunsriickschiefer ; 4, Kosor (Radotin); 5, Upper
Konieprus Limestone 6, Palaeacanthaspis horizon.
SPECIES O F R A DOTIN.4
Gross noted that many of the specimens of Radotina in the Prague National Museum had
been referred by Perner to other genera of fossil fishes. He was thereby able greatly to
reduce a most misleading list of genera. It is interesting that he failed to notice that
Coccosteus primus Barrande (1872: 640, pl. 29, figs 1-2) is clearly conspecific with
R. tuberculata, the correct name for which must clearly be Radotinaprima (Barrande).I am
greatly indebted, for the opportunity of studying this and other specimens,to Dr Zazworka,
Director of the National Museum in Prague, and to Dr R. Horn?, and also to Dr Horn? and
Dr I. Chlupah for showing me the type localities in the field.
A very remarkable fact, not sufficiently stressed by Gross, is the variety of conditions
shown in the dermal skull-roof of these forms. Thus of the material of R. kosorensis the
holotype, and the specimen in the British Museum (Natural History), No. P. 12829, both
show more or less extensive areas of tesserae between the larger dermal bones (Gross, 1958),
Radotina ant? other tcsserate fishes
86
whilc the new material figured later (Gross, 1959) has much more extensive large bones and
many fewer tesserae P e t they are referrctl--i\ itli full justice-to the same species.
The material from the Upper Konieprus Limestone shows precisely the same extremes,
though referred by Gross to two species, IZ.tuOrrculata and R. tesselata; it seems likely that
the latter is also conspecific XT itli R. prima (Barrande),but this cannot be proved without
further material. Exactly the same variation is seen nithin the material of the other radotinid form Kosoraspispeckai; the holotypc (Gloss, 1959, text-fig. 6D, pl. 6, fig. 4) haslarge
areaq of tesserae between the main derrnal bones nhile other specimens appear t o have
fen er or none.
A third condition of the dermal skull roof is best shomn in the Radotiiaa sp. from the
Taunus Quartzite (Gross, 1958, text-fig. UB) Herr (though the preservation is as a somem hat poor impression) the dermal skull-roof shov b no obvious sutures. A rather similar
ppckai (Gross, 1959. test-fig. BE, pl. 6,
condition is shonn by one specimen of Elosotcr~pi.~
fig. 5)
$~iiongthe material displayed in the Nat ional Illuseum in Prague are several specimens
of Radotana from Konieprusy (CF 1-6 :N o s %4arc counterparts, relabelled P 7916 a and b).
CF 1-4 are niedian dorsals n it11 shoit stout spines. inviting comparison nith Palaeacanthaspio wastu from Podolia. el:5 shows a rnctlian do1 sal, a good deal of the shoulder-girdle,
and what seems to be a remarkable synatcual section of the vertebral column. CF 6 is the
type of Coccosteus primus Barrande (Vol 1 . Supplement, pl. 29, figs 1-2), and includes a
nearly complete posterior part of the skull-roof. a i d thus permits completion of a sketchrestoration with the material described as X tuberculata by Gross.
As reconstructed, R. kosorensis, R prznm and K.peckai shom considerablc resemblance
in outline to the poorly preserved skull-roof of Palaeacanthaspis vasta (Stensio, 1944, pl. 1 ) .
suggesting that the area labelled ‘Eth. sh.’ by Stensio may perhaps represent the conjoint
nasal openings of Radotim. This comparison is strengthened by re-interpretation of the
shoulder-girdle material of R. kosorensLs described by Gross (1959, text-fig. 4 ) . The associated ,4L and scapulocoracoid are very rmiiniscent of the corresponding elements
in Palaeacanthaspis described by Stensio (1W4); the area doubtfully regarded by Gross
as for articulation with the pectoral fin is better regarded as for the origin of dorsal
fin riiusculature; the pectoral fin n a s probably associated with the notch ‘eb’ of Gross,
which is overhung by a process of AL recalling the ‘suprapectoral corner’ of Stensio. It
would seem that in this specimen the spinale and interolateral are either misbing or hidden.
The resemblances betu een the shoulder-girdles of Palaeacanthaspis and Kosoraspis are
even more striking, and Gross has already pointctl out this relationship. Some taxononiic
coniequences 17 ill be discussed belov
INTERPRETATION OF THE 1)ERRlAL SKVLL-ROOF O F R A D O T I S A
It is clear from the material figured by Gross that the dermal bones in some individuals
are large and mainly in contact, while in others they have a much smaller surface exposure
and are, at least externally, separated (or covered 2 ) by fields of tesserae. The tesserae differ
from the larger elements, hereafter ternied bone-plates, in being much thinner, with illdeveloped spongiosa, v ithout radiating vascular canals (both well-developed in the larger
plates) and no basal layer (also well-developed in the bone-plates). Frequently each tessera
bears a large stellate tubercle, occasionally more than one; one tubercle may overlie
another (with resorption effects?). On the large bone-plates the tubercles are very similar,
but more thickly set. Gross points out that the tesserae very closely resemble in character
the body-scales.
I n the strongly tesserate individuals the main bone-plates persist externally to varying
degrees. These bone-plates in all specimens include large paired elements, the centralia (C)
of Gross. These are, a t least externally, greatly decreased in size in individuals with welldeveloped tesserae. However, in poorly tcsserated individuals of R. kosorensis (Gross, 1959,
86
T. STANLEY
WESTOLL,F.R.S.
text-fig. 1, B-E) a very similar region is clearly indicated on C, forming a raised area surrounded by a broader marginal region bearing tubercles which are often different in size,
and more abundant, compared with those on the central raised area.
R. kosorensis (Fig. 2A) often shows a well-developedmedian bone behind C-C, clearly a
nuchal element. In some specimens,however, it seems to be replaced or covered by tesserae.
Paired postero-lateral elements called paranuchals by Gross are preserved in some individuals, separated from the nuchal by the centralia. I n well-preserved individuals there are
‘ circumorbital’ bones, seemingly three in number. There may also be clear indications of a
pineal plate, and, in occasional specimens, of a median element behind the pineal region,
associated with the convergence of the supraorbital sensory grooves. Finally there may be
two paired ‘lateral centra.ls’,intervening between the ‘paranuchal ’ and the most posterior
circumorbital plates. The latero-sensory ‘canals ’ appear as deep grooves ; where tesserae
occur, the grooves run between rows of tesserae which show some tendency to be in facing
pairs across the groove. It is an interesting fact that the deep grooves do not in general
~~
Fig. 2, Interpretation of the radotinid skull-roof. A, Radotina kosorensis, fully developed bones
shown on left, maximum area covered by tesserae on right; B, R . prima ( = R. tuberculata)
from Gross’s figure completed posteriorly from holotype of R. prima ; C, Kosoruspis peckai,
suggested interpretation showing on left side t,hat in some individuals PPNu is known to meet
Nu. Explanation of lettering : ?ASG, possible fused anterior aupragnathals; R, rostral;
P, pineal; PP, post-pineal; Nu, nuchal; C , central (Cl, Cz ‘fragmented’ central); PNa, postnasal; Pro, preorbital; PtO, postorbit,al; M, marginal; APNu, anterior paranuchal; PPNu,
posterior paranuchal; na. 0,bony nasal openings; SOC, supraorbital canal; ifc. b., central
canal; mpl, middle pit-line; ppl, posterior pit-line; ed, endolymphatic duct and fossa. Areas of
tesserae stippled.
0
Radotina anti other fpsseratefishes
87
traverse the radiation-centres of the bones. in fact the groove ‘ifc. b.’ seems in R. kosorensis
to lie neatly between the ‘circumorbitals ’ and the central.
Tlie bone called ‘rostral’ by Gross is ccrtalnly not the hornologue of the rostral of
euart hrodires, and is possibly t o be regarded as fused anterior supragnathals. The ‘septum
internasale’ of Gross separating the nasal openings, seems to be homologous with the
internasal septum and/or its dermal covering of euarthrodires ;between this and the pineal
plate should lie any true homologue of the euarthrodiran rostral, and in one specimen
(Gross, 1958,fig. 2F) there seems t o be a recognizable rostral plate.
Turning to R. prima ( = R. tubemdata of Gross),the holotype material can be combined
with the figure by Gross to produce a reconstruction very similar to the non-tesserate
R. kosorensis, with the important difference that C-C are separated by a median element,
wit11 its radiation-centre a t the junction of the supraorbital canals, and therefore a considerably larger bone than the correspondlng element shown in some R. kosorensis (Fig.2B).
The pineal plate is also larger; circumorbitals could well be restored as in other species,
since this region is very badly preserved. Thr main difference is that the canal ‘ifc. b.’ runs
directly forward to the radiation-centre of the middle circumorbital. (The same condition
must exist in R. sp. from the Taunus Quartzite ) O n the other hand, the only described
specimen of R. tesselata resembles, in its general features, the more strongly tesserate
specimens of R. kosorensu, except again for the more forward course of the medial extension of ‘ifc. b.’, w4iich, however, does not pass t o the radiation-centre of the second ‘circumorbital ’.
There are several possibilities that could account for these facts. Since there is no significant difference in size of specimens, crude growth-changes may be discounted.
(a)There may be two species a t each horizon, one tesserate and one not. This is thought
t o be extremely unlikely. as the material from any one horizon shows so many points in
common.
(b)The extreme forms may be dimorplis, possibly sexual dimorphs. This may be possible, but if so would involve t,henext interpretation.
(c) The differences are to be underst’ood in tjerms of the varying incidence of different
skelet,ogenic processes.
The last possibility is the most rewarding t o consider, particularly since the interrelations of tesserae and bone-plates is an important problem, for example in other arthrodires (Macropetalichthyida, Rhenanida, Stensioellida, etc.) and in the psamrnosteid
Heterostraci, and has given rise t o considerable differences of interpretation. At one
extreme i t has been held that the tesserae represent primitive conditions and that the
larger dermal bones have formed by accret,ion or fusion of groups of tesserae ; at the other
it has been held that tesserae result from phylogenetic fragmentation of large dermal bones.
I n a recent work Stensio (1963: 19 (footnote); 403) has outlined a combination of these
views applied to Radotina, which is discussed below.
Before turning to this problem and suggesting a new approach, it is necessary t o see
whet,her the pattern of bone plates is significantly similar to that of any other Arthrodira.
Here we meet a difficulty. I n many groups of artlirodires, as in many other groups of bony
fishes, the canals of the latero-sensory system tend to have a very closely established
relationship with the dermal bones SO that t,he major canals traverse the radiation-centres
ofthe bones, or at least, wherever they terminate, are directed towards such centres. The
more superficial elements of the latero-sensory system, usually termed pit-lines, may be
however, the very wide grooves only occasionmore variable in several groups. I n Radot im,
ally t,raverse, or are directed towards. the radiation centres of the bones, and in particular
the canal ‘ifc. b.’ shows a great variation in course. It seems therefore probable that the
appearance of the larger dermal bones is only slightly, if a t all, controlled by the laherosensory system, though it is possible that, an original morphogenetic linkage is obscured by
88
T. STANLEYWESTOLL,F.R.S.
later ontogenetic development. It may not be a coincidence that somewhat similar difficulties arise in the coelacanths, where again the latero-sensory canals are unusually large.
We may start with the paired element ‘ C ’. There is every reason to identify this with the
centrale of Euarthrodira ; its characteristic surface moulding clearly suggests that it lay
over the ear-capsule with the semicircular canals, and it has the correct relationship, allowing for ‘wandering’ of some parts of the latero-sensory system, to the canals ‘ifc. b.’ and the
posterior pit-line equivalent. The raised area in the middle of C may indicate the shape of
early ontogenetic stages of this bone; the posterior extension may thus be a ‘secondary’
effect. The element ‘Nu’ is clearly a niichal, and there is no difficulty in recognizing a pineal
plate. Behind this in some R. kosorensis is a median element wedged apparently between
the front of C-C and related to the convergence of the supraorbital canals; the corresponding element in R. tuberculatu is larger, reaching the nuchal. A median bone in a corresponding position occurs in several Euarthordira, but rarely attains the relative size shown
in R. tuberculata; it is usually termed post-pineal or medio-central. A very striking
point is that this element and the nuchal together greatly resemble the large ‘centronuchal’ element of macropetalichthyids. The nature of the ‘rostrale’ of Gross, and the
position of the true rostra1 if present behind the internasal septum, have been discussed
above.
The best material of R. kosorensis (cf. Gross, 1958, fig. 1 , or 1959, fig. 19) S ~ O M Sthree
bone-plates termed ‘ circumorbitals ’ by Gross. I n the B.M.X.H. specimen (Gross, 1958,
fig. 2F), the middle plate is clearly a dermal bone of the same character as the central,
and there is no reason to suppose that the other two were not present. The holotype of
R. tesselata clearly shows the same three elements ; the specimen of ‘R. tubercuhta ’ as
figured by Gross shows only one, but the posterior element would have lain in the lost
lateral region and the anterior element may well be present-details are poor in the broken
anterior region. These are clearly not sclerotic plates. The anterior, in its relations to the
nasal openings and the rostro-pineal region, is closely comparable with the postnasal bone
of Euarthrodira, and allowing for the rather anomalous courses of the latero-sensory elements, has appropriate relationships to the infraorbital and supraorbital canals. The middle
of the three is clearly homologous with the preorbital. The posterior element has all the
characters of a postorbital, except that the junction of the infraorbital and central (ifc. b.)
canals lies just posterior to the element; the postorbital process of the endocranium partly
underlies it. The postorbital bone, and the two ‘lateral centrals’ next to be discussed have
their territory occupied by, or covered by, tesserae in the specimens figured by Gross
(1958);but in 1959 (fig. 1F) he figured a specimen with dermal bones labelled ‘C11’and ‘Cla’
which are clearly homologous with those in ‘R. tuberculata’ (Gross, 1959, fig. 4A). Behind
these, and postero-lateral t o the central plate, is a strongly ossified element forming the
projecting corner of the skull-roof, which may in some cases be partly ‘replaced’ by
tesserae; it is shown both in R. kosorensis and in the holotype of R. prima, and is traversed longitudinally by the postotic section of the latero-sensory system. Between this
element and ‘(312’ a branch runs medially and then forwards towards the radiationcentre of &clearly the homologue of the posterior pit-line. The otic section of the infraorbital canal runs forward from this junction immediately lateral t o ‘Clz’ and ‘Cll’, or at
least lateral to their radiation-centres ; in strongly tesserate individuals it runs between
tesserae, the bone-plates are not seen.
The most postero-lateral element is clearly a paranuchal. The element ‘C11’ has all the
expected morphological relationships of the marginal. ‘Clz’, so far as can be ascertained,
must lie above the true supravagal process of the endocranium, which is shown too small in
thereconstruction of Gross (1959,fig. 2A, B) and is the unlabelled process overhanging the
vagus foramen ; the ‘processus supravagalis ’ of Gross reaches under the posterior paranuchal plate and is probably homologous with the processus cristispinalis of Macropetalichthys (Stensio, 1925). Thus ‘Clz’ has most of the expected relationships of the
anterior paranuchal of macropetalichthyids, except that it is only adjacent to, and not
Radotina and otktr trsseratejshes
89
centred upon, the triradiation of the latero-sensory canals. (It is additionally noteworthy
that in Kosoraspis the endolymphatic duct opms just behind the posterior pit-line.)
The most significant results of this analysis are the recognition of direct homologues of
bones well-knon n in other arthrodires, and in particular the identification of both anterior
and posterior paranuchals, which is in strong contrast with euarthrodires and a compelling
link M ith inacropetalichthyids.
Kosoraspis is far less completely k n m n, h i t Gross ( 1959) has figured the known material.
Briefly, it may be re-interpreted as follou s (Tc.xt-fig. 2C). X large nuchal clearly meets large
posterior paranuchals, in those mdividuals i t ithont M ell-developed tesserae. The area
normally occupied by the centrals in not \I cll shon n Iii a tesserate specimen (Gross, 1959,
fig GD) there are t u o apparently scparatc ccsntral plates on either side; those on the
left side are separated hy a narrou zone ot te.sscrac. The latero-sensory canals are much
narroiter than in Radotina. and seein to traverse major bones, almost certainly close to
their radiation centres. The posterior pit-lint. leaves the main canal on a plate lateral t o
tliest,centrals,andextenrlsontotliepo\tt~i~oi
ctwtral, Lz (Czoft1iispaper)inthisspecimen;
i n another (Gross, 1939,fig. 6E) i t runs to\\ RNIS thc. opening of the endolymphatic duct.
A little further forward the middle pit-linc cro s the other ‘central’ plate L1 (Cl of
f the posteiior paranuchal are other
this paper) Lateral to the centrals and in froi
‘lateral bone-plates’ \T liich i t is difficult t o Interprctt in detail on the known material, particularly since there are cracks affecting tho boncs It is here suggested that an anterior
paranuchal and a marginal niay be recogn~zecl; the anterior pit-line is directed ton ards the
suture betxi eeii them Still further forv arrl is an incomplete postorbital, and the transverse
structure running inedially across it and the fiic,ltl of tesserae is here interpreted as the
groove for the central pit-line (ifc. b.) 111 other figured specimens the fields of tesserae are
apparently far less extensive and the silt urec, may be ‘ closed ’.
These considerations strongly suggest that ftrrdotina and Kosoruspis can best be
compared with Macropetalichtliyida. It I \ interesting that Gross (1961) has recently
~ l i nothat
~ the rheeli-region of the oldeht \\ c.ll-knoun member of that group, Lunaspis, is
tesserate The central (C1 of Gross) in L/trici+pshas different relationships which may be
regardcd as due t o ‘plastic ’ deformatioii of pattclii, it enters the orbit, and is nellseparated
from the back end of the slinll-roof. The postrrior extension of C in Radotina may be
‘secondary ’, as noted above A single element covers the territory of the nuehal and ‘postpineal’ or medio-central of radotinids; its ciirioiiq shape has suggested t o many earlier
n orliers that it may be of compound origin [t may be added that the plate labelled ‘Pm’
(post-marginal) in the account of Lunaspi 5 (Gi‘osq. 1961, fig. 2 ) is apparently an opercular
element, in euarthrodires it is more 11kc4y to be represented by the usually smaller
submarginal, but the corresponding eleinciit in otlier plaeoderms may be quite large. A
large oval plate (Gross, 1959: 6, pl. 1, fig 1) in R. kosorerwk is probably the same eleinent ,and is called postsuborbital by Gro after the bone sonamed by Stensio in Jagorina
and Gemuendina.
It is only possible very briefly to indicatc mine other resemblances. The Rhenanida are
represented by the Lower Devonian (Hurisruckschiefer, see Fig. 1 ) Gemuendina, the
Middle Devonian Asterosfeus, and the V1)per TIcvonian Jugorina. All of these are strongly
tesserate. A re-interpretation of the very lieaut ifiil specimen of Gemue~dinafigured by
Broili (1933, pl. 1 , fig. 1 ) is possible bccaiise of the superb quality of the plate, and is
shown in Fig. 3. From this it is seen that a irietli~nnuchal and paired centrals, posterior
and anterior paranuchals can very reasoilably he identified ; the presence of both paranuchals is again strongly suggestive of relationship 11 ith inacropetalichthyids. It is also
very noticeable that the latero-sensory caiials havc not the exact relationship with major
dermal bones that is normal in many pltteodcrnis. The rlienanids have much in common
\\.it11 radotinids in the position and relationships of the nasal openings. The large cheekplate called postsuborbital by Stensio is clearly not lioniologons with the same bone in
euarthrodires, which is intimately connwtetl n it 11 the cluadrate; i t is probably an enlarged
90
T. STANLEY
WESTOLL,
F.R.S.
submarginal. The relationship between Radotinida and Rhenanida has already been
suggested by Gross.
The Stensioellida are known a t present only from the Hunsriickschiefer, and have
recently been well monographed by Gross (1962).Two of the four known genera, Pseudopetalichthys Moy-Thomas and Paraplesiobatis, are obviously very closely related and may
be congeneric, in which case the latter has priority. They show some striking resemblances
Fig. 3. Interpretation of Clewmendinn based on Broili (1933, pl. 1). Lettering as in Fig. 2 with:
SO, suborbital; SM, submarginal; AL, anterior lateral; ADL, anterior dorso-lateral; MD,
median dorsal. Note that SM and PPNu have been pulled away from the skull-roof; in natural
position the lateral-line groove on PPNu would continue the line carried on tesserae a t the
lateral margin of APNu towards ADL.
to macropetalichthyids, and also (except for the nasal region) to radotinids. These resemblances include the general arrangement of the latero-sensory canals on the skull-roof and
the large submarginal ( ‘postsuborbital ’) plate, and the curiously-shaped median plate
suggested in Pseudopetalichthys. The two large tubercles ‘ tb1-z ’ of Gross’s descriptions
mark separate bones lying in front of a clearly-defined anterior paranuchal ; they may
represent a divided central or a central and a postorbital. The element labelled LNm’
(median nuchal) by Gross might better be regarded as a partly-exposed median dorsal.
The two genera taken together most strongly suggest close relationships with macropetalichthyids.
Radotina and ofher tesseratefishes
91
To sum up, it would appear that the Radotinida, Rhenanida and a t least PseudopetaZichthys and Paraplesiobatis of the Stensibellitla, show unmistakable signs of being related
to the Xacropetalichthyida. The reniainiiig Stensiocllida (StensioeZla and Nessariostoma)
are so poorly preserved in so far as their qkull-roofs are concerned that their precise affinities remain uncertain ; the ne\v specimen of Stensiodla described by Gross (1965) taken
with the earlier account suggests the presmce of paired centrals (‘0s.c.’ of Gross, 1962),a
median paranuchal (’tub ’ of Gross, 1965, figs 1-2). and possibly of a large postpineal; the
t~ccipital-synarcualjoint reseniblcs that in Itlienanitla, and the occipital region is long.
I t may thus be asserted 11 ith some confidence that the dermal bones in poorly-tesserate
ratlot iriids may be closely homologized with those in other placoderms, and that the
reseniblances are significantly closest to the macropetalichthyicl pattern. The varied
relationship of bones to latero-sensory canals, even within the genus Radotina, clearly
undermines the proposition that there is a constant morpliogenetic relationship between
tlirni, at least so far as thew arthrodires are concerned, and thereby throws light on some
otlierwise puzzling clifferc~ncesbetween, say, cuarthrodires, and macropetalichthyids and
antiarclis.
KATURE O F TESSERAE AN I) T H E I R INTERPRETATION
Gross (1959) has commented that tlie tesswae of Radntina are essentially scales, and
sriggcsts that they rcpresent a hold-over of a phylogenetically primitive condition, and
that in effect dermal bones have arisen through fusion of such scales. But in many teleosts
and in some living genera of lungfishes thrrc are scales spreading forward on the head, and
it i s qiiitc impossible that scales of these types are primitive. It is necessary briefly t o reconsitler this problem, especially since (as in Lepisosteus) somen hat similar mosaics are
sometimes regarded as clue to ‘fragmentation ’ of ancestrally large bones.
It is far from certain that the main dermal hones are not all present even in the most
tesserate specimens. The central, for example, has a raised centre, surrounded by a broad
marginal flange at a relatively lower level, in non-tesserate individuals; it is quite conceivable that thin scale-like superficial tesserae could exist independently, or only lightly
attached to the main bone, over these flaiqys. Onlp well-preserved material can settle such
a question. It is, however, noteworthy that the exposed flange usually bears large numbers
of stellate tubercles, presumably formed in the more superficial parts of the dermis immediately below the epidermis ; if such a tiibcrcnlate surface were to be covered by tesserae
this would presumably involve increase i n t Iiickiitw of tlie dermis and probably partial
resorption of the earlier formed tubercles. ‘l’licre is nothing a priori impossible in this; but
t o strengthen such a supposition would nrcd f a r better and more abundant material, while
t o prove it would seem impossible, since it \\ oiild involve continuous observation of the
animal during grou th.
There would seem to be several processes of tlcrmal skeletogenesis in these forms :
( 1 ) the development of the large dermal hones, on a pattern rather closcly similar to
that of macropetalichthyids ;
( 2 )the development of separate thin tcsscrae. u hich may apparently become attached
to underlying flanges of dermal bones ( e g. Kosoruspis,Gross, 1959, fig. 6D, E);
(3)development of new generations oft ulmcles ;
(4)probably a late development of thc superficial layer, tending to obliterate all
sutures, a t least in some individuals
There is nothing in tlie known material of ratlotinids to show directly whether a phylogenetically somewhat more primitive condition involved only thin tesserae over the skullroof, or large dermal bones, and none to s h o w whetlier, in the latter case, the large bones
originated in some wag by ‘fusion ’ of tesserae
Turning to the Rhenanida, the well-spaced stratigraphical succession noted above may
92
T. STANLEY
WESTOLL,
F.R.8.
help to illustrate trends; however it is necessary to be cautious in view of so small a
number of forms. Gemuendina (upper part of Lower Devonian) is re-interpreted in Fig. 3; in
the skull-roof there are dermal bones identified as nuchal, and paired centrals, anterior and
posterior paranuchals, with large opercular submarginal plates, and suborbital plates in
the cheek. There is a very large development of tesserae; and there are indications on the
cheek-plates in particular of tesserae presumably fixed to the surface of the bones, which
presumably send flanges below the tesserae. Latero-sensory canals ’ are mainly shown as
c
very superficial grooves, often between paired tubercles, on tesserae or bones. dsterostezcs
from the Middle Devonian (Stensio, 1948, text-figs 68,69)is far less completely known, but
shows considerable but variable reduction and ‘fragmentation’ of the large dermal bones
of the skull-roof.Jagorina, from the Upper Devonian, has been studied by Stensio in detail,
but has only been described piecemeal in various papers mainlydevoted to other subjects
(e.g. Stensio, 1948, 1963, etc.). Some years ago Professor Stensio, with characteristic
generosity, gave the author photo-copies of reconstructions, some still unpublished, and
permission to draw upon this information. This allows the necessary slight addition to
published knowledge, so that it can be stated that the dermal exoskeleton of the skull
shows still further ‘ fragmentation ’ and the further development of tesserae. The opercular
(‘Pop 3 ’ of Stensio, 1963, fig. 78D) is identified by Stensio as the bone usually termed
postsuborbital, but is, like that of Gemuendina,better regarded as a submarginal. Externally it is largely covered by tesserae, more or less attached to a deep lamina of dermal bone.
The inner face of this lamina is intimately attached to, or continuous with, an ossified
cartilage (the postspiracular cartilage of Stensio-to the author a misleading homology).
The suborbital bone is apparently completed ‘replaced ’ by large tesserae ; under these is
another cartilage bone, termed antorbitalcartilage ’ by Stensio. Further comment on these
is made below. If the three known genera indicate a secular trend it is towards the increased development of tesserae, the ‘fragmentation’ of dermal bones, and in the cheek
their in-sinking to levels well below the epidermis. Backward extrapolation would suggest
an immediate ancestor with in the adult larger dermal bones and fewer tesserae, and the
pattern would much more closely resemble that of macropetalichthyids.
Stensio (1963 : 403) comments as follows on these arthrodires. ‘As in the Ostracoderms
and Teleostomian fishes, the dermal bones of the head and shoulder girdle in the Arthrodires had naturally arisen by fusion of scales. It is clear, however, that these bones in the
Arthrodires also underwent a gradual phyletic reduction which took place either in a
superficio-basal direction or in a basalo-superficial direction.’ (The first is well seen in
Pachyosteomorphs : the second) ‘ultimately resulted in a disintegration of the persisting
superficial parts of the bones into minor, frequently scale-like plates and thus led to a reappearance of a mosaic . . . reflecting more or less clearly the primary scale mosaic. . . .
Areas of such a secondary mosaic are found above all in the Acanthothoraci, Radotinida
and Rhenanida, but also to some extent in the Stensioellida.’ He goes on to say of Gross’s
interpretation of Radotinida that i t is ‘also decidedly contradicted by the actual conditions in Jagorina . . . for in this form it is apparent (a) that the scale-like plates of the head
occur only in those places where the basal and middle bone-layers are under reduction or
have completely vanished and (b) that both these scale-like plates and the trunk scales
proper all are low, hollow cones or spines made up chiefly or entirely of the superficial layer
with the tubercular ornamentation. That being so, it may be assumed on good grounds that
the disintegration of the dermal bones still was a t an incipient stage in the Acanthothoraci
and Radotinida, but that it had proceeded much further in certain rhenanid forms, e.g.
Gemuendina and Jagorina ’.
The present writer, with due respect to Professor Stensio, finds this a highly formalized
descriptive statement; it is not morphogenetically sufficient. The large dermal bones of
Arthrodira which retain the superficial layer give little or no evidence that they were
ontogenetically produced by aggregation of tessera-like units ; the large bones are clearly
produced by a process of ossification under quite different skeletogenetic controls than are
Radotina and other twseratejtshes
93
areas of tesserae; and i t is exceedingly difficult t o see how processes of reduction of the
basal components and of the superficial components could result in anything but complete
suppression of the entire bone, whereas it appears to be the case that a ‘basal’ lamina and
more superficial tesserae can co-exist. Stcwiiii’sconclusion that the tesserate condition is
secondary is very probably true, the meclianism lie suggests is clearly inadequate.
The psammosteiform Heterostraci affortl very interesting comparisons ; they have
recently been monographed by Tarlo (1905) We shall first consider the implication of the
sequence Drepana,spis (Sieger~ian-Enisian)-~sarr~,,~o~~t~us
(Upper Devonian). Adult
Drepanuspis (Gross, 1963) has an anterior exoslwlcton including twelve large elements
uhicli can generally be identified very definitely as liomologues of the ten larger elements
inPtcraspis and its allies ; the exception is tht. occuripnce of two pairs of plates (orbital and
postorbital) occupying the place of the orbital plates of Reraspis. Between the dorsal disc
and the more lateral plates is a field of tcwci ae, M liicli extends anteriorly at, the side of the
rostral, and a narrow- zone separates the rostra1 plate froin the pineal and dorsal disc. The
ventral disc is separated from the branchial and oral postoral plates by fields of tesserae.
However in very small individuals the dorsal fields of tesserae are either absent or far less
developed according to Gross, who descri I ~ e the
s tlevelopment of new tesserae with growth.
There cannot be any reasonable doubt that the major plates, which grow in size by peripheral addition, are the result of a morpliogenetic process that was essentially complete
before the greater part of the tesserae were formed. As in Radotina kosorensis there is some
indication of a central thicker area in a t lwst some of the larger plates, that seems t o mark
an early-formed rudiment. The smallest known specimen is 9 to 10 cm long; Gross calculates that the first rudiments of the main bone-plates n-ere already present at a total length
of c. 2.5 cm, a t which size neither tesserae nor body-scales were present.
I n the later members of the group some remarkable changes are seen in the approximate
lineage Drepanaspis --f Guerichosteus - - f S r h i x s t e u s +Psammolepis -+ Psammosteus, spanning tlie time-interval Siegenian to Famennian. The large dermal bones persist, but from
Psammolepis onwards, as the dorsal or vcntral niedian plate grows from a central area it
does so at a deeper level, forming a flange. to t lie exterior surface of which tesserae become
more or less attached. The central area is retlucecl in later species until inPsanimosteus the
whole of the external face of the plate is t e
.ate ; a similar development is found in at least
most of the other plates.
The resemblances between the Drepaiiaspiclidae and their descendants on the one hand.
Stensioellida on the other, is thus considerable. If
and the Radotinida + Rlienanida
these forms alone are considered it is extremely difficult to resist the deduction that the
Drepanaspididae originated from a pteraspicl ancestor with a flattened body-form, reduced
cornual plate and no tesserae (such pteraspids as Europrotaspis are morphologically and
stratigraphically suitable) ; and that the tesserate arthrodires had ancestors with fewer or
no tesserac, which must have had a bonc-pattern similar in important respects to that of
Macropetalichthyida.
Traquair was the first t o suggest that tlie Heterostraci developed their carapace by
fusion of smaller elements. Stensiij (1927) claborsted an earlier view of Jaekel that extensive bone-sheets were primitive, and frequently became fragmented in later members of
many groups of fishes. Later, however, lie introduced his lepidomorial theory (1958, 1961,
etc.),propounding the view that large dermal bones in early fishes arose from concrescence
of small scale-like units of several kinds. Tarlo has completely revised the general interpretation of heterostracan evolution on tlie lines of Stensitj’s theory. However, he finds it
necessary to ‘reverse the trend ’ when discussing the Drepanaspis-Psammosteus lineage
(Tarlo, 1965, and references therein) ; this compares with Stensio’s (1963) remarks on
tesserate arthrodires noted above. The present writer shares the view of Obruchev (1945,
and in discussion of Tarlo, 1961) that the siniilarity amounting to practical identity in the
pattern of the dermal bones of pteraspids and drepanaspids is extremely difficult t o understand if these groups are not intimately related ; it is certainly difficult t o believe that these
+
+
94
T. STANLEYWESTOLL,
F.R.S.
patterns have originated completely independently from a wholly tesserate ancestor. I n
his review of the psammosteiform Heterostraci, Tarlo (1965) considers that Tesseraspis,
Kallostrakon and Weigeltaspis represent tesserate ancestors of the drepanaspids, in which
the large dermal plates are not present, but may be foreshadowed by groupings of thicker
tesserae or their closer association into ‘fused ’ units. He also points out that the Ordovician
Astraspis,Pycnaspis and Eriptychius are largely or entirely tesserate, and therefore assumes
that this is a truly primitive condition ; Eriptychius in particular shows close histological
similarities to the structure of the psammosteid tesserae, but Tarlo points out (1962a : 254)
that these features are common to many ‘primitive’ Heterostraci. It is an open question,
therefore, whether the Tesseraspidida are conservative ‘primitively ’ tesserate forms, or
whether they represent a rather advanced stage of reduction from better ossified ancestors.
It is possible that the order, and the intensity of expression, of the various osteogenic processes were affected to some degree by environniental as well as by genetic factors. It could
equally be argued therefore that the Tesseraspidida are not specially closely related to the
Psammosteida. I n the same work Tarlo radically separates the three groups of cyathaspids, pteraspids and psammosteids. Denison (1964)gives very strong reasons for deriving
pteraspids from cyathaspids (aprocess involving ‘fragmentation ’ of major bone-elements),
and there is, to the present writer, equally strong evidence for the extremely close relationship of pteraspids and psammosteids.
It, would therefore seem clear that reliance on either of two extreme and simplified
interpretations of the heterostracan carapace leads to difficulties.
(a)The assumption that tesserae wherever seen represent a ‘primitive ’ condition and
that large dermal bones have arisen by fusion of tesserae makes the interpretation of the
later psammosteids anomalous ; suggests that fragmentation of dermal bones does not
occur ; and implies the parallel evolution of essentially identical bone-patterns in wellseparated lineages.
(b) The assumption that the most primitive condition is one of very few but large
dermal bones and no tesserae is not in accordance with the nature of the Ordovician
lieterostracans, nor with some of the interpretations of the epitega and scale-like superficial components of cyathaspids.
It is necessary to add to the mere comparison of adult conditions and of the histology of
mature bones and tesserae some understanding of the ontogenetic sequence of events. This
is reasonably possible within the Heterostraci ; when well-preserved very small radotinids
or rhenanids are available it may be possible to extend in detail this kind of study to arthrodires but i t already seems likely that some rather considerable similarities occur.
The first skeletogenic capacity to appear in the ancestral forms was very probably the
ability to form tubercles. It is also very likely that, a t an early stage in the evolution of the
Heterost,raci the dermal tubercles became continuous with the primitive bone-like but
acellular aspidin developed immediately below them. Scale-like structures termed cyclomoria by Stensio (1961,1962)were probably also phylogenetically early; each cyclomorial
unit consists of a primary tubercle with its aspidin base, which undergoes growth by the
development of secondary tubercles which become ranged around the primary one.
But there now seems good reason to suppose that the larger dermal bones, however they
originated phylogenetically, involve a different morphogenetic process. They are typically
much thicker. and flanges from them may pass under areas in whichseparate tesserae occur.
The difficultiesinvolved in Stensio’s theory of two-way reduction of dermal ossification
have been noted above. While in some forms the superficial layer may ‘grow’ by the
addition of new isolated elements (tubercles, or cyclomorial or synchronomorial tesseralike elements) there is not the slightest evidence that the deeper lamina increases in the
Samemanner. Indeed, it is fairly obvious that the superficial layer of a dermal bone, so long
as it consists of simple or compound dentine-like tubercles, must increase during peripheral
Radotina and ofher tesserate $shes
93
growth by the addition of discrete units. Such growth must therefore be ' quantized', and
is in itself no proof ofsorne of the aspects of'Steiisiii's lepidoniorial theory.
This strongly suggests therefore that there is considerable independence between the
superficial layer (including tesserae) and the dt.eper dermal structures. Such an independence is not confined to Heterostraci (ancl ptobably to artlirodires) ; it is clearly shown by
the resorption-reformation cycles affecting tlic superficial cosmine of early Dipnoi and
certain Crossopterygii, for example.
C J U R ~ I C that
) ~
skeletogenesis began in
Studies of juvenile cyatliaspids by D ~ I ~ I S(1964)
these forms as superficial dentine tubercles o r ritlgcls, v itli tlie beginning of aspidin-formation extending inwards betn een tlie tubercles or ridges. I n typical adult cyathaspids the
inner aspect of the dermal bones shows no cvitlence of a conipound nature, but thc superficial layer of the dorsal disc, for example, s h o w areas (termed epitega by Stensio, 1958) of
more or less sharply-differentiated patterns of tlentine-ornament. Steiisio regarded a t least
someepitega as having formed simultaneously (bynclironomoria).These epitega rather precisely compare in shape and arrangement itli the median rostra1 (median dorsal + pineal).
and t,he paired orbital plates of pteraspids. I n some of the earlier cyathaspids (e.g. Tolypelepis) the epitega themselves are divided into tessera-like areas, each of which is regarded
by Stensio as a cycloniorial unit. Denison ( l!Kil. 158)has shown that a t least four interpretations of the mode of origin of the To1ypelppi.s exoskeleton are possible; the only way t o
exclude any of these interpretations would be by histological study of growth-stages. But
whatever the interpretation that may surc ive it 111have t o take into account the order of
action of several morphogenetic processes-tlie nat tire of the ( I cyclornorial) tesserae, for
example, may indicate their early formation and grow th over a period; the organization
into epitega on the dorsal disc may well be later; and unless there can be proved t o be
considerable resorption and re-formation of the deeper * honeycomb ' and lamellar aspidin
it is most likely that this continuous deeper bone marked a terminal stage of ossification
and growth. It may be suggested that s u ~ lai sequence is typical of most cyathaspids; the
retention of the habit of terminal-grow th ossification could have led to the loss of presumed
more primitive conditions, leading to tlie condition of poraspids. However, it must be
stressed that apart from the apparently \\'eiilockian Tol?ypelepi,issp. mentioned by Denison
there is little stratigraphical evidence to indicate m hether any one of the tolypelepidine,
cyathaspidine or poraspidine cyathaspids is more 'primitive ' than either of the others;
much more remains to be discovered of pre-Ludlovian forms.
The pattern shown in pteraspids is very c>lcarlyone permitting further growth from a
juvenile condition with separate ossifications. Psanimosteids share the same property.
White (1958) shows clearly that within tlrr pteraspids there is strong indication (from the
arrangement of the latero-sensory canals \\ ithiri the dermal dorsal disc) that early pteraspids acquired their definitive dermal bones not long before full growth, while their descendants could grow progressively very much larger after tlie progressively earlier entrapping
of the canals. It is difficult to avoid the conclusion that still earlier members must have
differed from cyathaspids in only one respct-the appearance a t full grow th of several
plates replacing the continuous dorsal disc of tlie latter. From this it would appear natural
t o suppose that the epitegal fields had developed the morphogenetic capacity to induce
separate dermal bones in the deeper dermis, replacing the terminal-growth condition of
cyathaspids. Once established, this pattern allow ecl earlier and earlier appearance of the
dermal bones.
I n the Psammosteida as defined by Tarlo (1965) it is therefore thought reasonable to
suppose a derivation from pteraspids with flatter bodies and smaller cornual plates than
usual in the group. Here the morphogenetic processes have continued t o change their relative importance. The definitive dermal bones show further minor subdivision as compared
with pteraspids, and formed a t a very early stage ; they grew marginal flanges a t a deep
level within the dermis which could be quite separate from the more superficial tesserae, in
the latest survivors. Fields of tesserae between tlie major bones seem clearly t o have
T. STANLEY
WESTOLL,F.R.S.
96
formed later than the larger bones; they clearly behave in much the same way as the body
scales. If these suggestions are correct, the question as to whether the tesserae in psammosteids represents a truly primitive condition is perhaps rather meaningless. What is indicated is rather a continuous potential ofthe outer dermis and epidermis to provide calcified
skeletal tissue, while the morphogenesis of the deeper bony plates is under different control,
and tends ultimately to provide sheets of aspidin with only a secondary connection with
any more superficial calcified tissue. The continued skeletogenic capacity of the outer
dermis epidermis is manifested by the repair of injury, etc.
It was noted above that a quite different process may be discerned in some radotinidsnamely, the development (presumablya t a late stage ofgrowth) of a moreor less continuous
‘superficiallayer ’ which thus masks all sutures. (arather similar condition is shown by the
continuous cosmine which may mask sutures in some Crossopterygii and Dipnoi.) Some of
the bizarre heterostracan forms from Siberia discussed by Tarlo (1962 : 266-8), as the Order
Amphiaspidiformes, in which the whole carapace is an undivided dorso-ventral structure,
may perhaps result from similar developments of the superficial layer. On the other hand
they, and such cyathaspidiform genera as Cfenaspis and Allocryptaspis (in which the
branchial plates are said to be ‘fused’ with the dorsal disc), probably became ossified only
a t full growth, and perhaps the basal part of the bone is strictly continuous also across the
positions in which sutures may be expected.
The tesserate Arthrodira may easily be interpreted in a rather similar manner. Thus
Radotina kosorensis may show dermal boneswith broad flanges,or the thicker central areas
may be separated externally by tesserae. It is suggested that a somewhat earlier ancestral
form had better-developed bones, in which case these tesserae may be a late-formed feature
in ontogeny. It is quite possible that sometimes the bony plates of the tesserae did not
develop, so that tubercles became directly attached to the flanges of the larger bones; it
is also possible that resorption and reformation of calcified tissue occurred in the tesseral
fields. There is no evidence a t present to show unequivocally that arthrodires arose from
ancestors with an exoskeleton solely composed of scale-like tesserae, though thisis possible.
What evidence there is most strongly suggests that a well-defined pattern of large dermal
bones was characteristic of all known early arthrodires, or their immediate ancestors, that
these were under special morphogentic control, and that two groups can readily be distinguished, divergently specialized in one important matter :
+
(a) The group represented by Macropetalichthyida, Radotinida, Rhenanida and
Stensioellida (at least in part). I n this group the occipital region is long and there are
both anterior and posterior paranuchals in the skull-roof.
(b) The Euarthrodira (including Ptyctodontida), and their probable derivatives the
Antiarchi. I n these the occipital region is short and there is only one pair of paranuchal
plates; but the earliest members (e.g. Kujdanowiaspis) have much longer occipital
regions than the later forms.
To the author i t seems highly unlikely that the bone-pattern in the various members
originated quite independently by fusion of tesserae.
A further most important point emerges. What are confidently recognizedin Gemuendina
as homologues of the dermal suborbital and submarginal bones appear, in the lineage leading to Jagorina, t o be progressively replaced by fields of superficial tesserae externally,
while internally they seem to be associated with, perhaps replaced by, what is indistinguishable from cartilage-bone. This may have resulted from ‘induction’ of cartilage in mesodermal material, or may represent a gradual transformation. Such an origin for the antorbital cartilage in Elasmobranchii is clearly in accordance with Holmgren’s comparison of
early-formed anlagen with the dermal elements of Arthrodira; it is an example of the
application of his theory of delamination (Holmgren, 1940: 197-202; 250-1, see also
Jarvik, 1959).
This tendency for dermal bones to develop a t deeper levels in later forms, and even t o be
~
Radotina and other tesserate $.shes
97
replaced phylogenetically by cartilage-bones, is probably much more widespread than is
usually accepted. It is hoped very shortly to publish elsewhere evidence suggesting that the
scapular blade of elasmobranchs and of tetrapods may be a development from dermal
shoulder-girdle elements. It n.ill also be sliotr n that in Euarthrodira the dermal elements
surrounding the nasal capsule may readily be rccognized as the fore-runners of important
parts of the cartilaginous secondary nasal CR iile of elasmobranchs. Such a change must
necessarily have involved secular modific.it ion of the skclctogcnic processes, and is capable
of explanation b y means of Holmgreii's dchiiiiiation theory.
To sum up : the tesserate arthrodires at it1 1ic.terostracans appear to provide evidence for
mucli more complicated processes than ar(* snggt d either by the simple fusion of
ancestral tesscrae or scales (Traquair, Striisiii sitice 1948. and Tarlo), or b;v the 'fragmentation' of primitively large bones. The contl i t ions seen in any group require for their explanation the evidence of arell-preservetl ontogcnct ic stagcs as n ell as of well-established stratigraphically controlled sequences of evolutiotiai y significance. It is held tlmt our present
evidence suggests that the macropetaliclit hyitl and ciiarthrodiran groups of Xrthrodira are
divergently specialized ; to the former bcloiig t lit, tesserate groups of Radot inida, Rhenanitla and Stensioellida (at least in part), anti all thrcc inherit a basic dermal-bone pattern
suggesting a macropetalichthyid ancertot How tlic large dermal bones originated is not
yet known ; the more distant ancestors could ha\ e been tesserate, or merely tuberculated,
without bonc-plates. However, the dcrniis retained its ponrr to produce dentine-like and
bone-like superficial elemeutq. n liilc thc ninin h n i y plates could come to lie deeper, and
c w n to evolve (by a process of delaniinat ion) cartilagr, at a still deeper level.
Ite-interpretatioii of the available evlclciiw leads to a some\\ hat similar picture of the
evolution of certain Heterostraci ; here, I i o n c x cr, 11 t' have 110 sign of delamination going so
far ah t o produce cartilage, while there is iiicwiii1)lctcevidence of much earlier evolutionary
stagcs suggesting that trsserae dere1opc.d earlj in phylogeny. I t is held that a linon ledge of
ontogeny is required before the relati\ v importance and the sequence of ontogenetic
tle~--clopmentof several processes lcadiri~to I m i t ~ - l i k etissue in any group of Heterostraci
can be assessed.
REFEREKVES
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