Zoological Journal of the Linnean Society Volume 45 Issue 304 1964 [Doi 10.1111%2Fj.1096-3642.1964.Tb00488.x] Robert L. Carroll -- The Earliest Reptiles

8/10/2019 Zoological Journal of the Linnean Society Volume 45 Issue 304 1964 [Doi 10.1111%2Fj.1096-3642.1964.Tb00488.x

http:///reader/full/zoological-journal-of-the-linnean-society-volume-45-issue-304-1964-doi-1011112fj1096-3642196 1/24

J . Linn. SOC. Z ool ) ,45, no. 304, p.

61

With

14

tezt-figures

Printed in Great Britain

The earliest reptiles

BY ROBERT L. CARROLL

Accepted

f or

publication December 1963)

Communicated b y Errol I . White, F.L.S.

INTRODUCTION

In 1863, Dawson discussed several species of the genus

Hylonomus

from the Middle

Pennsylvanian (Upper Carboniferous) of Joggins, Nova Scotia, which he used as the basis

for the order Microsauria. He considered these animals to be either reptiles, or their close

ancestors. However, in later publications (Dawson, 1882, 1896), he referred to them as

amphibians. The modern concept of this order is based almost entirely on specimens from

other localities, and the forms originally designated by Dawson have been largely ignored.

The group as a whole is now generally considered to be among the lepospondyl amphibians,

and quite unrelated t o reptiles.

In the latest revision of the Microsauria, however, Romer

(1950)

suggests that Dawson

may have been correct in his description of

Hylonom us lyell i

as a reptile. Reinvestigation of

the Joggins fauna confirms that

Hylonomus lyel l i

is indeed a reptile, and indicates that

there are additional reptilian genera present as well. The other forms included by Dawson

as species ofHylonomus are apparently no t reptiles.

The manner of preservation at Joggins makes a systematic description of these reptiles

difficult. The specimens are very badly disarticulated, even to complete separation of the

component bones of the skull. In no case is there a complete skeleton of any of the reptiles.

There are three distinct types of humeri and parasphenoids, indicating the minimum num-

ber of genera present. Unfortunately, i t is not always possible to associate the remaining

skeletal parts with these elements. For this reason the following descriptions may be some-

what of a composite.

All these specimens come from the erect tjrees exposed along the sea cliff a t Joggins,

Nova Scotia. They are from the Joggins formation, whose age, based on plant remains, is

equivalent to the Westphalian B of Europe or the Upper Pottsville of the United States

(Bell,1944).The sectionisgiven in detail in Dawson, 1878. The only reptile of comparable

age is Cephalerpeton (Gregory, 1948)from the Westphalian C of Mazon Creek, Illinois.

It is not surprising that earlier workers (Dawson, 1896; Steen, 1934)did not recognize

reptiles a t Joggins.

It

is only because

of

recent knowledge of well-articulated specimens

from other Pennsylvanian and early Permian localities (Price,

1937

;

Romer Price,

1940;

Gregory, 1948; Peabody, 1952; and Watson, 1954) tha t the disarticulated pieces can be

recognized as reptilian and differentiated from microsaurs.

I wish to thank Dr Romer for suggesting this study, and for discussions with him during

the course of the work. I am grateful to him, as well as to

Dr

Margaret (Steen) Brough of the

University of Wales, and Dr Charig of the British Museum (Natural History), for reading

the manuscript. I very much appreciate the help of Prof Watson of University College,

University of London, and Dr Russell and Dr Langston of the National Museum of

Canada in the loan of specimens. Dr White, Keeper of Palaeontology a t the British Museum

(Natural History) and Mrs Turnham and Mrs Stevenson at Redpath Museum, McGill

University, where this work was carried out, have been helpful in providing working space

and other facilities. The research was financed by the National Research Council of Canada

and the National Science Foundation of the United States.

5



62

RO ERT

. CARROLL

Joggins

The following abbreviations are used

for

the institutions which have material from

BM(NH) British Museum (Natural History)

NMC National Museum of Canada

RM Redpath Museum, McGill University

Class

R E P T I L I A

Subclass

AN AP SI D A

Order

COT YLO SA UR IA

Suborder

CAPTORHINOMORPHA

Family

R O B E R I I D A E

HYLONOMUSawson, 1860

Hylonomus lyelli Dawson, 1860

Figures 1-10

Hylonomus lyelli

Dawson, 1860, p. 274.

H yl e rp tm curtidentaturn

Dawson, 1876,

p.

444.

Frh c h ia

curtidentuta

(Dawson)Dawson, 1882, p. 641.

PM

M

I crn

Q

Fig.

2. Hylonomw,

lyelli. Restoration of skull, based primarily on RM 12016a. One-and-one-

half times natural size. F, frontal; J, jugal;

L,

lacrimal;

M,

maxilla;

N,

nasal;

PA,

parietal;

PF,postfrontal;PM, premaxilla; P0,postorbital; PP,postparietal;PRF,prefrontal; Q,quad-

rate; QJ, quadratojugal;SQ, squamosal;ST upratemporal;

T,

tabular.



The

earliest

reptiles

63

Hylorwmus lyelli is the best preserved of the reptiles. At least

8

specimens can be

assigned to thi s species. Sufficient material is present to indicate tha t it is a captorhino-

morph, close to the ancestry of the Permian romeriids. The description is based on the

following specimens

BM(NH) R.4168. Holotype. Almost complete skeleton, but badly disarticulated, with

much represented only by impressions. Collected by Dawson from division 4, section XV,

coal-group 15.

BM(NH) R.4167. Vertebrae, pelvis, femur, phalanges, ribs and scales. Collected by

Dawson from division 4, section

XV,

coal-group 15.

NMC 10048. Vertebral column, pelvic girdle, both hind limbs and feet. Collected by Bell

from division 4, section XII , coal-group 26.

NMC 10047. Pterygoid, stapes, vertebrae, ribs, pelvic girdle, femora and scales. Collected

by Bell from division 4, section XX I, coal-group 10.

NMC 10046. Vertebrae, pelvis, femora, fibulae, humerus, ribs, scales and phalanges.

Collected by Bell from division 4, section XII , coal-group 26.

RM 2.1 126. Type of Pritschia curtidentata (Dawson). Maxilla, premaxillae, lower jaws,

humerus, femur, ribs, phalanges and scales. Collected by Damson from division 4, section

XV, coal-group 15.

RM 12207. Palate , squamosal, dentary, articular, angular, coronoid, vertebrae, jugal,

tibia and phalanges. Collected by Dawson from division 4, section XV, coal-group 15.

RM 12016a. Most of skull roof, pterygoids, dentary, vertebrae, ribs and clavicle.

Collected by McNaughton from division 4, section XIII , coal-group 20.

Several additional specimens are mentioned in the description, but they are completely

dissociated and somay belong to different species.

Skull

(restoration, Fig. 2) The best preserved cranial material is RM 12016a (Fig. 3 .

A41mostall of the skull roof is present, and some of the palate. Most of the bones have

become disarticulated, but each is individually so well preserved tha t i ts position in the

skull is easy to determine. The back of the skull roof is preserved a s a uni t, exposed ven-

trally. There are prominent postparietals, almost entirely limited to the skull roof. Their

posterior margins are smooth and curved only slightly over the occiput. There is no

evidence of their being suturally connected to any of the bones of the occiput. They are

two or three times as wide as they are long.A single bone, the tabular, is visible a t he left

posterior corner of the skull roof.

It

is broken posteriorly and ventrally, where it was

presumably in contact with the paroccipital process of the otic capsule. The tabular is

thickened on its lateral margin. It extends anteriorly, slightly beyond the postparietal. It is

difficult to compare the configuration of the tabular of this animal with that of other early

reptiles, since only in th is specimen is the bone exposed ventrally. I f, as in

Protorothyris,

pelycosaurs, and other primitive reptiles, the supratemporal occupied a superficial position,

it would not be visible in ventral view.

Only the posterior portion of the parietals is preserved. They extend to the margin of

the skull table, and evidently rested upon the cheek region, rather than being suturally

connected with the squamosal. The area of the pineal foramen is not preserved. A single

frontal bone is present, also in ventral view. It is about four times as long as it is broad and

tapers slightly, anteriorly. Where it bordered the orbit, the lateral margin is reinforced by a

slight ventral ridge. The nasal bone is present in RM 2.1 126 (Fig. 4

a .

It was evidently

quite

a

bit shorter than the frontal and was overlapped for a considerable distance by i t.

The anterior end of the bone

is

unfortunately not well preserved and suggests nothing

of

its mode of contact with the premaxilla, nor of the dimensions of the external naris.

Much of the cheek region is missing in RM 12016a, but the squamosal is present in RM

12207 (Fig. 5

b ) .

Most of the bone itself is missing, but a n excellent cast of the inside sur-

face remains.

As

in other captorhinomorphs, the posterior margin is smooth and almost

perpendicular to the plane of the skull with no indentation for an otic notch. The bone

extends onto the occipital surface, particularly dorsally. The posterior portion

of

the bone



64

ROBERT

L.

CARROLL

Parietal

Dentary

Prefrontal

Parasphenoid

icle

) 1 /Tabular

Frontal

r imal

P o t t o r b i d

g

Parasphenoid

Fig. 3.

Hylonomwr

lyel l i . RM 12016a. One-and-one-half imes natural size.




65

extends ventrally, probably

to

the margin of the skull, almost,

if

not completely, separat-

ing the quadratojugal from the margin of the occiput. The quadratojugal has not been

identified

in

any

of

the specimens, but

its

configurationcan easily be determined from that

ofthe surrounding bones. The postorbital is present in RM 12016a;

t

bears a narrow medial

extension on the orbital margin.

It

is large enough to have extended posteriorly and dor-

sally to the parietal. The postfrontal has not been identified. The jugal, present

in

RM

12016a, s exposed laterally and exhibits a faint pattern of radiating grooves. The portion

of

the bone extending beneath the orbit is narrow, in contrast to the condition in Lower

Permian captorhinomorphs, and it is not expanded in front of the orbit.

Prernwi i

A

Corono id

I

I

crn

t

2 mm

D -

C

2

mm

E

Fig. 4. Hylonomus lyell i . (a)

M

.1126. Upper and lower jaws. Twice natural size

;

6 )Diagram-

matic cross-section

of

upper and lower jaws; (c)

BM(NH)

R.4169. Maxilla, medial and lateral

views. Twice natural

size;

d ) BM(NH)

R.446. Maxilla, medial view. Four-and-one-half

times naturd

size; (e)R M

2.1132. Maxilla, lateral view. Three times natural size.

The medial surface of the lacrimal bone is visible, exposing the lacrimal duct. A medial

extension on the orbital margin of the bone is pierced by two openings for the duct; these

open into a depression, anterior to which is a single passage enclosed in bone which extends

to the anterior margin. The bone appears triangular in shape, tapering rapidly in width in

front of the

orbit.

As preserved, the bone does not reach the external naris. This suggests

that

it

is probably broken anteriorly and dorsally. I n the same blockis a smallcranial bone

bearing a medial ridge on one margin, which may be the prefrontal.

Both of the maxillae are present in this specimen; each has room for

36

teeth, most of



66 ROBERT . CARROLL

which are present. On the right side, counting from the front, teeth 1,14,19,32 and 34 are

missing. On the left, 2 ,2 6 and 28 are lacking.

All

of the teeth are simple pegs, expanded

medially a t thebase and pointed at the tip. None shows any sign of labyrinthine structure;

some, however, have faint groovesnear thetip . Twoteeth,

the6thand%hfromthefront,are

considerably larger than the remainder and extend beyond them exactlyasdo the canines

in

Protorothyris.

The tooth row does not stretch the entire length of the maxilla; there

would be room for

3

or

4 additional teeth posteriorly and 2 anteriorly where the maxilla

extends dorsal

to

the premaxilla. The maxilla

is

widest above the 1 th and 12th teeth and

tapers to a point anteriorly and posteriorly. The anterior extremity of the dorsal margin is

recessed where it extends beneath the external naris. The bone extends laterally around the

canines. The external surface is marked by a series of pits near the ventral margin and by a

few faint grooves running perpendicular to the dorsal margin. The medial surface is

visibleas an impression in the type, and in the isolated maxilla BM(NH) R.4169 (Fig.4 c).

It

bears a stout ridge which supports the tooth row. The maxilla of RM 2.1126 (Fig.

4 a

is

larger and bears about ten more postcanine teeth than tha t of RM 12016a.

Both premaxillae are present in RM 2.1126; both a re crushed, and partially covered by

other cranial bones, preventing determination of the dimensions or configuration of the

external nares. Each has room for

5 or 6

teeth. Two teeth are present in the left premaxilla;

they are intermediate in length between the canine and non-canine teeth in the maxilla.

The nasal process compares in width with th at of

Captorhinw,

but ismuch wider than that

ofPetrolawsaurus (Peabody, 1952).

It is

not possible to determine whether the premaxilla

extended ventrally

in

this species as

it

does in the captorhinids. The external surface is

marked by prominent pits. The septomaxilla has not been recognized.

The pterygoid is the only palatal bone that can be described adequately. It resembles in

most respects tha t of

Captorhinus.

There is a prominent transverse flange, a feature which

definitely distinguishes the Joggins reptiles from contemporary microsaurs. The flange is

accentuated by a shallow cleft anteriorly, which separates it from the palatine ramus. The

medial margin of the palatine ramus is straight as far anteriorly as

it

is preserved. The

nature of the juncture of the palatine ramus with either the palatine

or

vomer isnot known.

Like the transverse flange, i t is covered by uniform small teeth. Without adequate

know-

ledge of the remainder of the palate, it is not possible

to

determine the width of the inter-

pterygoid vacuities. The configuration of the basipterygoid process of the pterygoid is

identical to tha t

of Captorhinus,

but it is situated anterior, rather than posterior,

to

he

transverse flange. It also has a n anterior position in Petrolacosaurus.A s in the lattergenus,

the quadrate ramus is composed of horizontal and vertical flanges. The horizontal portion

is

essentially a posterior continuation of the palatine ramus.

It

extends medially from the

vertical flange, supporting, as suggested by Romer and Price in the pelycosaurs, the eu-

stachian tube and the middle ear. The vertical plate extends toward the skull roof, sloping

medially. The posterior portion of the quadrate ramus is not preserved.

It is not possible to determine whether there was a suborbital fenestra or whether the

ectopterygoid was present.

The left palatine and vomer are present in RM 12207, bu t are disarticulated from the

remainder of the palate and broken on their margins. Like the pterygoid, they are covered

almost completely by denticles and resemble in general their counterparts in Cuptorhinw.

Two

parasphenoids are present in the block with RM 12016a, bu t neither is likelytobelong

with the remainder of the skull because of their small size. A completely isolated para-

sphenoid, BM(NH) R.877 (Fig. 5e),may belong to this species.

It

resembles

that

of

Cup-

torh inw

in general, but differs in several particulars. The posterior plate, except for the

presence of denticlea, isvery similar, althoughit sbroken posteriorly, giving the mpression

of being somewhat shorter. The central portion of the plate

is

recessed in both genera. The

cultriform process is broken anteriorly but was evidently both longer and wider than that

of

Captorhinus,

and bears teeth a t

ts

base. Jus t anterior to the basipterygoid processes, he

cultriform process extends dorsally around the base of the braincase.



The earliest reptiles 67

The basipterygoid processesof the basisphenoid extend laterally and anteriorly, rather

than directly anteriorly as in Captorhinus.Each

is

grooved a t its base for the passage ofthe

palatine branch of the VII th nerve. The anterior end of the groove, which is overlapped by

the parasphenoid, s pierced bya foramenfor the palatine artery. Posterior and dorsal to the

basipterygoid process

is

a further extension of the basisphenoid, probably forming the

base of the prootic pillar. The opening for the internal carotid is apparently behind this.

This is all of the braincase tha t

is

known.

The quadrate has not yet been found.

cm

D

E

Fig. 5. Hy lonomwly e l l i . (a) M 12207. Palateand bonesoflowerjaw;

( b )

RM 12207. Squamosal

in lateral, posterior anddorsal views; (c)RM 12207. Dentary

;

d )RM 12207. Jugal; ( e )BM(NH)

R.877. Parasphenoid. All

twice

natural size.

The stapes is present in NMC 10047 Fig.

9).It

closely resembles that of the captorhinids

in the configurationof the greatly expanded footplate and a smaller dorsal process. There

is a conspicuous stapedial foramen, but the presence

of

the nutrient foramen described by

Price (1935) cannot be determined owing to the difficulty of preparation. The stem is re-

cessed distally and obviously

was

continued in cartilage.

Lower aw:

Disarticulated dentaries are present in the type, in

RM

12016a,

RM

12207and



68 ROBERT

. CARROLL

RM

2.1126. The jaw associated with NMC 10046 (Fig. 10) apparently does not belong with

the remainder of the specimen becauseof its relatively great size compared with the jaws of

the other specimens. There is room or 40 1 teeth in RM 12016a, the same number as in

theupper jaw. The 4th and 12th from the front are somewhat longer than the remainder in

this jaw, and the 5th and 13th in RM 2.1126. The condition of preservation in the other

two jaws precludes a count. Judging from the configurationof the skull, it isprobable th at

the dentary occupied about the same position relative

to

the other jaw bones as it

does

in

Captorhinus.

A series of irregular depressions

runs

along the middle

of

the lateral surface.

The dorsal rim of the dentary overlaps the tooth row laterally. Supporting the tooth row

is

a stout medial ledge of the dentary, the anterior extremity of which forms most, if not

all,of the symphysis. The area beneath this ledge must have been covered by an extensive

splenial,as n Ophiacodon (Romer

t

rice, 1940).

A

bone which may be the angular is present, disarticulated, in RM 12207.It is in the form

ofa shallow trough, with both sides about equal in height. It is deeper in the middle than

either posteriorly or anteriorly, but the posterior portionis almost certainly broken. It was

apparently almost as long as the dentary. The surangular has not been found. Isolated

anterior coronoids are present in RM 12207 and RM 2.1126. They are narrow and covered

with small teeth. Judging from the extent of a roughened area on t he medial side of the

dentary, the coronoid must have extended anteriorly

to

about the level of the 10th tooth

Fig.6.

Hylonomw

lyelli.

Restoration

of

skeleton,

behind the symphysis.

It

would have covered a distance of a t least 20 teeth. Neither the

posterior coronoid nor the prearticular has been found.

An isolated articular

is

present in

RM

12207

;

t is narrow, and roughly triangular in

shape as viewed laterally. The dorsal surface is indented posteriorly to form a single condyle,

which faces medially and dorsally. There is a marked lip on the medial margin of the

condyle, below which is a greatly thickened area on the posterior margin of the bone, which

may have extended posteriorly in the form of a blunt retroarticulax process (thesurface of

the bone is broken here). The bone tapers anteriorly and ventrally. Much of the medial

surface is marked by small, deep pits for attachment of ligaments. The lateral surface is

almost flat, and completely smooth. The posterior margin of the lateral surface

is

indented

lateral to the area of the retroarticular process.

Postcranial skeleton (restoration, Fig. 6 ) I n none of the specimens is there a complete,

articulatedvertebral column. There are 26 scattered vertebrae anterior to the pelvis n the

type (Fig. l , ut one

or

two more may have been present originally. In none of the speci-

mens can a sacral vertebra be identified.

It

is doubtful whether there was more than

it

single principal sacral, judging from the configuration of the ilium and the condition ob-

served in other primitive reptiles. The total length of the tail is unknown. Thirty caudals

are present in NMC 10048 (Fig. 8),all with neural arches, but there were certainly more,

giving a

total

ofat least

50.

The vertebrae are all large and clearly reptilian. The pleurocentrum and the neural arch

form a solid

unit

in he large specimens, but there is occasional separation in smaller animals,

or as a result of crushing. The centra are spool-shaped, resembling outwardly those of




contemporary microsaurs. They are, however, more solidly ossified and the recesses for the

notochord are not as extensive. The external surface is for the most part devoid of con-

spicuous grooves

or

ridges. The ventral portion of the articulating surface

is

bevelled for

reception of the intercentrum.

The neural arches are supported by stout pedicles situated slightly anterior to the middle

of the centrum and extending for more than half its length. The anterior ventral portion

of

the pedicle, in conjunction with the centrum, forms the transverse process. The arch is not

swollen and is only slightly wider than the diameter

of

the articulating surfaces of the

centrum. The zygapophyses are small and lightly constructed and the articulating surfaces

appear to tilt at about 15 from the horizontal, although this is difficult to determine due

to crushing. The neural canal is large, exceeding in width the central diameter of the cen-

trum.

The neural spines show considerable regional differentiation. They are apparently quite

short in the trunkregion, althoughfew are known from this portion of the column. They are

triangular in lateral view and are located well posteriorly. Ju st behind the sacral region,

bestseeninNMC

10048,

the spines become longer and rectangular in outline. By the 6th or

7th postsacral the spines narrow, and become situated further posteriorly. I n the remainder

basedprimarily

on

BM(NH)

R.4168.

Natural

size.

of the caudal series they shorten and finally disappear in the vicinity of the 20th caudal.

Two intercentra are visible in NMC 10046,but none in any of the other specimens, except

for the axial intercentrum in the type. I n he few places where the vertebrae are

in

articula-

tion, i t is evident that the intercentra must have been small, as are those of Cephalerpeton

andPetrolawsaurus.If there were any doubt of the original presence of intercentra in these

animals, it

is

dispelled by the presence of strong haemal arches in many

of

the specimens.

These are particularly well displayedin the type where they have become disarticulated and

lie on end. NMC 10048(Fig. 8) shows them in place.

The atlas-axis complex is preserved in the type.

It

resembles generally that described

for

the pelycosaurs (Romer Price, 1940).The proatlas, atlas arch, axis arch and centrum are

in place, while the atlas centrum and the axis intercentrum lie close by. The atlas inter-

centrum is not present. Each side of the proatlas resembles an abbreviated neural arch,

with the anterior articulating surface facing ventromedially. The neural arch of the at las is

short, but not as abbreviated as those of the pelycosaurs; presumably

it

ispaired, but since

the specimen

is

viewed only from the side, this cannot be ascertained. The neural arch of the

axis is more strongly built and bears a very large neural spine, which overhangs th at of the

atlas. Both neural arches bear prominent transverse processes. The at las and axis have

essentially normal anterior and posterior zygapophyses, although they are somewhat

larger than those

of

the succeeding vertebrae. The atlantal centrum, like tha t of Petrola-

wsauru'us

and ophiacodonts, is crescentic in shape, open ventrally. The lateral walls taper

ventrally. The axial centrum is similar t o that of the succeeding vertebrae. The axial inter-

centrum is 2.5 cm. removed from its natural position, but

its

large size certainly precludes



70

ROBERT. CARROLL

it

from belonging with any of the trunk vertebrae. It closely resembles that of Petrola-

comurus.

Numerous ribs are present in the type. Few are directly articulated with the vertebrae,

but there is little difficulty in determining the portion of the column towhich they belong.

Those in the cervical region are relatively short, reaching a length of about twice that of the

vertebrae. The one beat preserved appears to have but a single head, but it is divided into

two, not particularly distinct, articulating areas. This rib

is

expanded into a flat

plate

distally. Posteriorly the ribs increase in length to four or five times the length of the centra

and the heads become more distinct. The capitulum is essentiallya continuation of the

shaft of the rib. The tubercular surface

is

at the end of a triangular extension of the dorsal

Fig.

7.

H y b n o m w

ZyeUi.

RM

.1126.

(a)

umerus;

( b )

Femur.

Twice

natural size.

surface of the shaft. It is always more extensive then the capitulum. A shallow notch

separates the two heads. The shaft

is

sharply curved just distal

to

the head and continues

with little change in diameter to the end. Frequently the ends appear flattened, but

this

is

apparently only a result of crushing. One dorsal rib is thickened in the middle of the shaft,

but this probably marks the position of a mended fracture. The ribs become shorter

and

single-headed near the sacrum. They extend posteriorly, rather than ventrally, and

are

pointed, rather than ending bluntly as do those further forward.

No

sacral rib has been

identified. Ju st behind the sacrum the ribs become fused to the transverse processes. There

areat least4postsacral ribs.

The only evidence of the endochondralshoulder girdle

is

a small fragment of the scapula,




visible medially, in RM 12016a.Not enoughis preserved

for

description. The clavicle

is

also

preserved in RM 12016a. The stem is incomplete, but the expanded ventral portion is

well preserved, and unsculptured.

It

is set a t almost right angles

to

the shaft, and is two

or

three times as broad. The anterior margin of the shaft has a th in extension which is con-

tinuous with the ventral plate. This type of clavicle is essentially the same as th at in the

Lower Permian romeriids. Fragments of both clavicles are present

in

the type, together

with the interclavicle.

It

has

a

typically reptilian stem and a broad plate. The stem

is

forked

a t it s tip , as in some pelycosaurs. The plate is known primarily from an impression of i ts

dorsal surface. It is roughly oval and larger, relative to the stem, than in most other

primitive reptiles. The anterior margin is striated, as in some labyrinthodonts. The cleithra

have not been recognized, but they may well have been confused with rib fragments.

A fairly complete left humerusis present in RM 2.1126. I ngeneralit resembles he humeri

of other captorhinomorphs. The ends are expanded and set a t about a 90 angle. On the

posterior side of the proximal expansion are two deep pits for the at tachment of ligaments.

A

large, oblong entepicondylar foramen is situated in the proximal half

of

the area of

distal expansion. It extends directly dorsally, rather than posteriorly, as it does in

Cap-

torhinus

and the secondof the Joggins reptiles. It is not particularly near the margin of the

bone. The portion of the humerus carrying the articulation with the radius is crushed into

the matrix and cannot be prepared. The distal articulating surface is preserved in the type,

as well as the impression of the remainder of the bone. The articulating surface is well

ossified,with clearly defined areas for contact with the radius and ulna. There is no sign of

a supinator process, present in one of the

other

Joggins reptiles.It is primarily a comparison

of the humeri which leads to the synonymizingof

Fritschia

and Hylonornus. The lower fore-

limb is not known in any specimen of this species.A few phalanges and/or metapodials are

present in the type and in RM 2.1126, along with the ulnare in t he type, but they are com-

pletely disarticulated an d of lit tle help in interpreting the structure of the foot, except to

indicate that the toes were quite long.

Pelvicgirdles are preservedin the type, in BM(NH)R.4167,NMC 10047,and NMC 10048.

An isolated ilium is present in NMC 10046. Sutures are clearly visible on the medial surface

of the pelvis separating the ilium from the puboischiadic plate, but i t is difficult to see this

separation laterally. None of the specimens shows the suture a t the junction of the pubis

and ischium. The two halves of the pelvis, although not articulated in any of these speci-

mens, apparently met a t a distinctly acute angle.

The main shaft

of

the ilium extends dorsally and posteriorly in a broad curve above the

acetabulum. The anterior margin continues dorsally as a thin, triangular plate of bone.

This area is best seen in the type and in BM(NH) R.4167 (Steen, 1934, fig. 21 and plate IV

fig. 3); it is broken a t

its

base

in

NMC 10048 and appears to be totally missing in NMC

10047. This gives the pelvis of the lat ter specimen an appearance quite different from th at

of the others and suggests a t f i s t consideration that it may belong to a separate species.

The similarity of the remaining portions of the skeleton to tha t of the type of

Hylonomus

lyelli,

together with a similarity in size, suggest th at

it

is not distinct; but rather th at there

is considerable age and individual variation in the extent to which the anterior and dorsal

margins of the blade region are ossified, as was noted in the pelycosaurs by Romer and

Price (1940,p. 126).

The posteroventral margin of the blade is straight above the neck region, and solidly

built. The external surface of the blade is smooth, except for a series

of

grooves just above

the ventral margin. At t he base of the medial surface of the blade is a recessed area for the

attachment

of

a (single) sacral rib. Dorsally and posteriorly the blade is marked by rugose

ridges for the attachment of ligaments and perhaps axial musculature. There is no dorsal

groove for the attachment of axial musculature such as is present in primitive pelycosaurs.

The medial surface of the base of the ilium is pyramidal in shape, with amedianridgeextend-

ing onto the puboischiadic plate. The acetabular area, visible only in BM(NH) R.4167, has

been described by Steen.



72

ROBERT.CARROLL

The posterodorsal margin of the puboischiadic plate

is

raised where it comes in contact

with the ilium. The contact between the ilium and the pubis is less conspicuous, and not

always determinable. Posteriorly, the ischium extends beyond the end of the iliac blade.

The anterior end of the pubisisseparated from the remainder of the plate by a continuation

of the ridge on the medial surface of the ilium. This separation

is

accentuated by crushing,

particularly in the right half of the pelvic girdle in NMC 10048. In the type ut not in the

other specimens, this ridge appears

to

continue medially

to

strengthen the symphysis,

as

in Captorhinus. The obturator foramen pierces the pubis just anterior

to

this ridge. The

external surface of the puboischiadic plate is continuous and concave.

Fig. 8

Hy ~ onomu8yelli.

NMC 10048.

Twice natural size.

Both posterior limbs are articulated in

NMC

10048 (Fig. 8).The femora, whose descrip-

tion is augmented from other specimens, are large and well ossified. They are slightly

longer than the puboischiadic plate. Like the remainder of the skeleton, they generally

resemble the corresponding bones in Captorhinus, although they are somewhat more light-

ly built. Astrong internal trochanter continues distally as a low adductor ridge. There are

numerous grooves, particularly near the distal end, for the attachment of ligaments.

The tibia and fibula are about two-thirds the length of the femur, with the fibula slightly

exceeding the tibia

in

length. Both bones are expanded a t each end. The lateral margin of

the fibula is almost straight, while the median edge is strongly concave, particularly as a

result ofthe very large distal expansion. The distal articulating surface with the calcaneum



Th e earliest reptiles

Fig. 9.

Hylonomw ZyeUi.

NMC 10047.

(a)

nd

( b )

Counterpartsof skeleton. Twicenatural size;

c ) Stapes. Three times natural size.

73



74

ROBERT.

CARROLL

Fig.

9. Hylonomua lye . NM C 10047. (a) nd ( b ) Counterparts

of

ekeleton.

Twice

netural

size;

c )

Stapes.Thme times natural size.



Tlie earliest

reptiles

75

and astragalus

is

at about a 45 angle to the shaft. The proximal end

is

twisted posteriorly,

to lie against the side of the femur. The proximal end of the tibia

is

divided into two dis-

tinct areas of articulation with the femur; in

NMC

10048, the cnemial crest stands out

strongly, while the remainder of the head is crushed posteriorly. The distal articulating

surface, like th at of the fibula,is set at about a 45 angle to the shaft; it is expanded to a

much greater degree than in

Captorhinus.

The most distal end of the bone is flat, and does

not

appear to articulate with the astragalus.

I

Fibula

7fl

lntercentrum

Fig.

10.

HyZonomus ZyeZZi NMC 10046. a)

nd

a) Counterparts of

skeleton.

Twice

natural

size.

The tarsus

is

perfectly preserved, and agrees essentially with that of

Captorhinus.

The

astragalus, as in that genus, still shows very slight lines of demarcation between the areas

originally occupied by the tibiale, intermedium and proximal centrale. The area proximal

to the surface of articulation with the tibia is very little developed but, in contrast to

Captorhinus,

a small portion of the lateral surface extends distal to the area of contact with

the tibia. As in Captorhinus,only a single, large distal centrale

is

visible, articulating with

4

distal tarsals. Primitive pelycosaurs retain 2, small distal centralia of approximately the

same size. The 4th distal tarsal, as in most early tetrapods,

is

considerably larger than the



76 ROBICRT. CARROLL

remainder. It is in contact with both the astragalus and calcaneum, as well B with the

centralia and the adjacent distal tarsals. The other distal b a l s are of about equal size.

There is a small element distal to the calcaneum, not present

in

other reptiles;

it

may not

belong with the specimen. All 5 metatarsals are present, although the 1st is turned side-

ways, and somewhat beneath the 2nd. The metatarsals are about two-thirds the length of

the tibia, the 4th the longest, and the 1st the shortest; the 2nd, 3rd and 5th are of about

equal size. The 5th metatarsal

is

not hooked. Each

is

expanded proximally

to

articulate

closely with the tarsals. Anumber of phalanges are present, but they are too disarticuhted

and mixed with those of the right foot

to

attempt a restoration of the toes. The proximal

phalanges are about the length of the 2nd, 3rd and 5th metatarsals, the succeeding

row is

somewhat shorter, and the terminal phalanges, present in NMC 10046, are much shorter

still, pointed and slightly hooked. In

Captorhinus

and other early reptiles, the terminal

phalanges are longer than the subterminal. The total length of the foot would be approxi-

mately equal to the combined lengthof the femur and tibia.

Numerous scales, or abdominal ribs, are present in most of the specimens. They are of

the typical wheat shape associated with other primitive reptiles. Where complete, they

are in the form of small rods, flattened and enlarged a t one end where they fit over the next

in the series. In NMC 10046 (Fig. lo), they are arranged in a pattern essentially similar to

that of

Cephlerpeton.

Although not as clearly defined as

Hylo nom w lyel li ,

there is some material from two

additional reptilian genera. The first is knownprimarily from postcranial material and can

be differentiatedfrom

Hylonomus

on the structure of the humerus.

ARCHERPETONen. n.

Type species, Archerpeton anthraeos

Diagnosis :Primitive captorhinomorph. Entepicondylar foramen piercing posteroven-

tral margin of humerus, rather than extending directly dorsoventrally. Parasphenoid

primitive with wide cultriform process covered with denticles. Basipterygoid processes

widely separated. The generic name is from the Greek, meaning chief reptile.

Archerpeton anthracos sp. n.

Figures 11and 12

Diagnosis: Same as for genus. The specific name is from the Greek, meaning coal, in

Holotype RM 12056 maxilla, parasphenoid, scapula, coracoid, clavicle, humerus;

Paratype RM 12206 humerus, ulna, radius and foot; collected by McNaughton.

Horizon

Cumberland group, Joggins formation, division4, sectionXII, coal-group 26.

Locality:

Joggins, Nova Scotia.

Description:

The only skull bone of this genus which is sufficiently well preserved

to

warrant description

is

the parasphenoid (Fig. 11c ) . It is present in the type, together with

the shoulder girdle and humerus. The latter bone is unquestionably reptilian, and there is

no reason to think that the parasphenoid is not associated.

It

is, however, much more

primitive than in any other reptile. The posterior plate

is

very broad and flat with little

tendency to curve upward around the base of the braincase. The parasphenoid diminishes

only gradually in width anterior to the basicranial articulation. The cultriform process

is

less differentiated from the plate than in any other Palaeozoic reptile. The central portion

of the plate and process is covered with denticles. The basipterygoid processes extend

ventrolaterally from the plate and only slightly anteriorly. They are small and covered a t

their bases by the parasphenoid. The configuration of the parasphenoid suggests that the

interpterygoid vacuities were larger than in any other early reptile. A portion of the

reference to the coal forest habitat.

collected by McNaughton.



78 ROBERT

. CARROLL

parasphenoid similar to that of RM 12056.A broken maxilla is associated with the ptery-

goid, but

no

more can be determined from this bone than from the maxilla

of

RM

12056.

Several vertebrae and ribs are present in the type, but are too poorly preserved for des-

cription.No intercentra are visible.

Much of the shoulder girdle is present in the type. The scapulocoracoid s badly crushed

and shows only its outline and the glenoid region.It is exceedinglybroad anteroposteriorly,

and the coracoid, although incomplete ventrally, is very extensive. Any foramina which

may have been present are obliterated by cracking. The girdle is well enough preserved to

show that the depression anterior to the glenoid, common to microsaurs, is not present.

The coracoid region is not clearly demarcated from the scapula, nor is there any sign of a

division

into procoracoid and coracoid. This region was obviously curved strongly medially.

The portion of the scapula that is preservedisvery short. The dorsal margin, however, does

not appear to have been broken, but was undoubtedly finished in cartilage. The glenoid

retains

he screw shape of the amphibians. There is

a

stout supraglenoid buttress, below

which the glenoid faces almost directly posteriorly. The coracoid portion of the glenoid

Fig. 12. Archerpeton.

anthracoe

gen. at

sp.

n.

(a)

aratype,

RM

12206. Forelimb,

humerus

n

posteriorandventralviews.

Twicenaturalsize;

b )RM

12099. Premaxilla, nanteriorandlateral

views. Three times natural

size; (c) R M

12069. Pterygoid and

maxilla

Three times natural

size; ( d )NMC 1004lb. Femur, in dorsal and ventralviews. Twice natural she; e ) R M 12202.

Maxilla, medial surfme. Twice natural size.

faces directly dorsally. Fragments of the cleithrum and clavicle are present

on

the surface

of the scapulocoracoid. An almost complete right clavicle is also present in the block. It

resembles closely the clavicle ofH y l o m u s . The stem, which tapers

to

a point laterally, is

complete and about as long as the ventral portion.

No

interclaviclehas been associated

with this species.

A

complete and perfectly preserved humerus is present in the type, articulating with the

scapulocoracoid.

It

resembles in a general way the humeri of

Capbrhinus

but the distal

end

is

less expanded, and

is

set a t less of an angle to the proximal end (the atter may be due

todistortion). The entepicondylar foramen s small and set close to the margin of the bone,

as inCaptorhinus but in contrast to

Hylonomus,

and passes through the posterior margin

rather than through the dorsal surface. There are prominent areas for the articulation of

the radius and ulna. A low ridge runs from the anterior end of the proximal articulating

surface to the entepicondylar foramen. There

is a

deep depression

on

the ventral side of the

proximal end which may be

a

result

of

crushing.

An incomplete humerus, the ulna, radius and part of the hand are present in RM 12206

(Fig.

12a).

The position of the entepicondyle indicates that the humerus belongs

to

the

same species asRM 12056,and differentiates t from

Hylonomus.

The ulna and radius were

probably about two-thirda the length of the humerus. The ulna exceeds the length of the

radius by the height

of

he olecranon, which

is

well ossified and about one-fifth ofthe length




79

of the entire bone. It has a distinct sigmoid notch and resembles generally the ulna of

captorhinids and primitive pelycosaurs. The distal end

is

slightly expanded. The radius is

expanded at both ends, particularly distally. The ulnare, intermedium and pisiform are in

position and several additional carpals are scattered among the metacarpals. There is alsoa

distinctly double-headed rib among the foot bones. The ulnare is only slightly larger than

the intermedium, rather than considerably larger as in pelycosaurs and Petrolacosaurus.

The intermedium has apparently been rotated 90 from

its

normal position. The pisiform

is

almost as long as the ulnare, but considerably narrower. The metacarpals are about half

the length of the ulna. The bones are remarkably well ossified for an individual of such small

size.

An isolated femur,

NMC

10041b (Fig. 12

d ) ,

may belong to this species. It clearly differs

from those of the other reptiles in the fauna. It resembles in general those of Captorhinus

from Fort Sill. The internal trochanter is prominent, rising away from the remainder of the

bone as an isolated process.

It

is connected with a ridge which runs along the anterior

margin of the bone to about the middle of the shaft. The posterior distal condyle extends

considerably beyond the anterior. The remainder of the rear limb is unknown.

The most interesting of the reptilian remains is that of a pelycosaur. The skull is not

sufficiently complete to determine whether i t had yet developed a lateral temporal

opening, but the configuration of the humerus is essentially the same as in all early pely-

cosaurs, and significantly different from that of any other reptile group.

Subclass

S Y N A P S I D A

Order

P E L Y C O SA U R I A

Suborder uncertain

Family uncertain

PROTOCLEPSYDROPS

en. n.

Type species,Protoelepsydro ps haplous

Diagnosis Primitive pelycosaur. Ectepicondylar ridge oriented a t right angles to the

distal surface of the humerus rather than extending slightly anteriorly. Areas for the

articulation with the radius and ulna very large. Basipterygoid processes widely separated.

The generic name refers to the animals probable relationship t o

Clepsydrops,

heretofore

the earliest known pelycosaur.

Protoclepsydropshaplous

sp. n.

Figures 13and 14

Diagnosis:

Same

as

for genus. The specific name is from the Greek, meaning simple

or

undifferentiated, in reference to the possibility that this animal is close to the ancestry of all

later pelycosaurs.

Holotype: RM 3166; central portion of skull roof, lower jaw, parasphenoid, vertebral

column, ribs, humerus, pelvic girdle, femur and foot; figured by Steen (1934, fig. 22c,

p.

490)

as

Hylonomus latidens;

collected by Dawson from division 4, section

XV,

coal-

group 15.

Paratypes: RM 2.1191a; distal end of humerus and scales; in same block as type of

Leiocephalikon eutheton

Steen; collected by McNaughton from division 4, section XII,

coal-group 26. BM(NH)

R.5778;

distal end of humerus, no data on exact place of collec-

tion.

D. M.

S. Watson collection

B.

239; distal end of h-imerus, no data on exact place of

collection.

Horizon

Cumberland group, Joggins formation.

Locality

Joggins, Nova Scotia.

Description: All

that

is

known of the skull roof (Fig. 13a) is the central portion of the



80

ROBERT .

CARROLL

parietrtls and impressions of their margins, and an impression of the right postfrontal. The

parietals differ considerably from those of H y l o m u s in the presence of very large lateral

lappets. The ventral surface of the parietals is markedly concave. There

is

apparently

almost no sculpturing of the skull roof.

The parasphenoid

is

well preserved. It

is

roughly intermediate in configuration between

that of Archerpeton and that attributed to H y l o m u s . The posterior plate is quite wide,

but shows signs of dorsal curvature around the lateral margins and the development

of

basisphenoidal tubera. The central portion

is

slightly depressed and carries no denticles.

0

A

I cm

C

Fig. 13. Protockpydrqs haplow gen. et sp.

n.

Holotype, RM 3166. (a) keleton; a) Pelvis;

c) Femur; d )Foot. All twice natural size.

The cultriform process is broad a t its base, but isdefinitely narrower, relative to the width

ofthe plate, than is that of

Archerpeton.

As a result, the basipterygoid processes are begin-

ning

to

swing nto a more anterior position; hey remain far more laterally placed than those

of H y l o m u s . They extend distinctly ventrally. The cultriform process narrow8 rapidly

and is broken a t the end. The pterygoid described with Archerpeton may pertain

to

this

form. Additional bonesof the skullroof and palate are preserved either badly broken or as

impressions. None can be identified, except a left jugal, just anterior

to

the parietrth. The

lower jaw is represented only by a broken angular and perhaps part of the splenial. Neither

is sufficientlypreserved for description.

A short stretch of vertebrae is present, presumably all from the anterior trunk region. A

distinct suture is present between the neural arch and the centrum. There is no sign of

intercentra. The pleurocentra are long and narrow, and considerably constricted medially.



The

earliest reptiles 81

The neural arch is not swollen;it bears a short, rounded neural spine and transverse proces-

ses similar to those in the trunk region of

Hylonomus.

The zygapophyses are apparently

nearly horizontal.

A

few broken ribs are scattered within the block.

None of the shoulder girdle is present.

The humerus (Figs.

13a

and

14

is the most distinctive bone of the skeleton.

It

resembles

those of Varawsaurus and Clepsydrops, except

for

it s smaller size. A prominent supinator

Fig. 14.

Protockpsydrops

huplowr

gen.

et

sp. n. Paratypes. a) . M.

S.

Watson private collec-

tion B.239. Humerus;

( 6 )

BM(NH) R.5778.Humerus; (c) R M 2.1191a. Humerus. All twice

natural

size.

process distinguishes it

at

once from any captorhinomorph. Most of the area of the prox-

imal expansion is missing, although enough is present in

R 3166

to indicate tha t it was

set at about an 80 ngle to the distal. The shaft is very thin, although short. Alarge ente-

picondylar foramen lies close to the proximal margin of the distal expansion.

A

shallow

groove

is

visible in the larger specimens running from the entepicondylar foramen toward

the distal end

of

the ectepicondyle. The ectepicondyle is very well developed, situated a t




animal. Other specimens of each species are much smaller, indicating animals only4 or5 in.

in length. There is also a great deal of size variation within each speciesof labyrinthodont

and microsaur in this locality. The bones of even the smallest reptiles are highly ossified,

suggesting an essentially adul t condition. The high degree

of

ossification and the lack of

any truly aquatic

forms

in the fauna suggest tha t these reptiles were primarily terrestrial

animals. This tends

to

refute Romers

1946)

suggestion th at early reptiles were aquatic,

and explains why direct ancestors have not been found in the predominantly aquatic

localities

of

the Lower Pennsylvanian and Mississippian.

REFERENCES

BELL,W. A,, 1944. Carboniferous rocks and fossil floras of northern Nova Scotia. Mem. geol. Sum.

DAWSON,.

W.,

1860. On a terrestrial mollusk, a. chilognathous myriapod, and some new species of

DAWSON,. W., 1863.

Air-heeathersof thecoalperiod.

Dawson Brothers, Montreal. 81pp.

DAWSON,.

W.,

1876. On a recent discovery of Carboniferous batrechians in Nova Scotia. Amer. J.

DAWSON,.

W.,

1878. Acadiangeology. 3rded.,Macmillan andCo., London.

694pp.

DAWSON,

.

W.,

1882.

On the results of recent explorations of erect trees containing animal remains in

DAWSON,.W., 1896. Additional reports on erect trees containing animal remains in the coal formation

GREGORY,. T., 1948. The structure of Cephakrpeton and aEmities of the Microsauria. Amer. J.Sci.,

PEABODY,

.

E., 1952. Petrolacoaaurw, kanaenab Lane, a Pennsylvanian reptile from Kansas. Pabont.

PRICE,. . 1935. Noteson the braincase of

Captorhinw. Proc. BOstOnsOC.nat.

H i s t . , 4 0 : 377-86.

PRICE,. . 1937. Two new cotylosaurs from the Permian of Texas. Proc. New Engl.

zool.

Cl.,

16:

97-

ROMER, . S., 1946. The primitive reptile Lirnnosceliarestudied.

Amer.

J .Sci.,244: 149-88.

ROMER,

. S.,

1950. The nature and relationships of the Paleozoic microsaurs. Amer. J. Sci., 248:

ROMER, . S. PRICE,L.

I.

1940.

Review of the Pelycosauria.

Spec.

Pap, geol.

SOC.

mer.,

no.

28:

STEEN,M.

C.,

1934. The amphibian fauna from the South Joggins, Nova Scotia.Proc.

2001.

SOC.

ond.,

WATSON,. M. S., 1964. On Bolosaurua and the origin and classificationof eptiles. Bull.

Mue.

comp.

Can.,no. 238: 1-276.

reptiles, from the coal-formationof Nova. Scotia.Quart.

J.

geol.SOC.ond., 16: 268-77.

Sci.,

3) 12: 440-7.

the coal-formationof Nova Scotia.Phil. Trans..

173

:621-59.

of Nova Scotia.Proc.

roy.

SOC.59 :362-6.

246 :550-68.

Contr. Univ. Kans.. Vertehata,Art. 1: -41.

102.

628-54.

1-538.

1934

: 465-504.

2002Haw., :299-449.

Zoological Journal of the Linnean Society Volume 45 Issue 304 1964 [Doi 10.1111%2Fj.1096-3642.1964.Tb00488.x] Robert L. Carroll -- The Earliest Reptiles

Documents