The earliest reptiles

Transcription

1 J. Linn. SOC. (Zool), 45, no. 304, p. 61 With 14 tezt-figures Printed in Great Britain The earliest reptiles BY ROBERT L. CARROLL (Accepted for publication December 1963) Communicated by 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 to reptiles. In the latest revision of the Microsauria, however, Romer (1950) suggests that Dawson may have been correct in his description of Hylonomus lyelli as a reptile. Reinvestigation of the Joggins fauna confirms that Hylonomus lyelli is indeed a reptile, and indicates that there are additional reptilian genera present as well. The other forms included by Dawson as species of Hylonomus are apparently not 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 number of genera present. Unfortunately, it is not always possible to associate the remaining skeletal parts with these elements. For this reason the following descriptions may be somewhat of a composite. All these specimens come from the erect tjrees exposed along the sea cliff at 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, 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 at 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) that 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 at 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

2 62 ROBERT L. CARROLL The following abbreviations are used for the institutions which have material from Joggins : BM(NH) British Museum (Natural History) NMC National Museum of Canada RM Redpath Museum, McGill University Class REPTILIA Subclass ANAPSIDA Order COT YLOSAURIA Suborder CAPTORHINOMORPHA Family ROBERIIDAE HYLONOMUS Dawson, 1860 Hylonomus lyelli Dawson, 1860 Figures 1-10 Hylonomus lyelli Dawson, 1860, p Hylerptm curtidentaturn Dawson, 1876, p Frhchia curtidentuta (Dawson) Dawson, 1882, p PM s M Q I crn Fig. 2. Hylonomw, lyelli. Restoration of skull, based primarily on RM 12016a. One-and-onehalf 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,quadrate; QJ, quadratojugal; SQ, squamosal; ST, supratemporal; T, tabular.

3

4 The earliest reptiles 63 Hylorwmus lyelli is the best preserved of the reptiles. At least 8 specimens can be assigned to this species. Sufficient material is present to indicate that it is a captorhinomorph, close to the ancestry of the Permian romeriids. The description is based on the following specimens : BM(NH) R 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 Vertebrae, pelvis, femur, phalanges, ribs and scales. Collected by Dawson from division 4, section XV, coal-group 15. NMC Vertebral column, pelvic girdle, both hind limbs and feet. Collected by Bell from division 4, section XII, coal-group 26. NMC Pterygoid, stapes, vertebrae, ribs, pelvic girdle, femora and scales. Collected by Bell from division 4, section XXI, coal-group 10. NMC Vertebrae, pelvis, femora, fibulae, humerus, ribs, scales and phalanges. Collected by Bell from division 4, section XII, coal-group 26. RM 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 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 so may belong to different species. Skull (restoration, Fig. 2) : The best preserved cranial material is RM 12016a (Fig. 3). A41most all 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 that its position in the skull is easy to determine. The back of the skull roof is preserved as a unit, exposed ventrally. 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 at the 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 this specimen is the bone exposed ventrally. If, 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 (Fig. 4 a). It was evidently quite a bit shorter than the frontal and was overlapped for a considerable distance by it. 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 (Fig. 5 b). Most of the bone itself is missing, but an excellent cast of the inside surface 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

5 ~ 64 ROBERT L. CARROLL Parietal p )1$/ /Tabular Dentary Prefrontal Frontal rimal Parasphenoid icle Pottorbid (g Parasphenoid Fig. 3. Hylonomwr lyelli. RM 12016a. One-and-one-half times natural size.

6 The earliest reptiles 65 extends ventrally, probably to the margin of the skull, almost, if not completely, separating the quadratojugal from the margin of the occiput. The quadratojugal has not been identified in any of the specimens, but its configuration can easily be determined from that ofthe surrounding bones. The postorbital is present in RM 12016a; it bears a narrow medial extension on the orbital margin. It is large enough to have extended posteriorly and dorsally to the parietal. The postfrontal has not been identified. The jugal, present in RM 12016a, is 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. Prernwii A 4 Coronoid B I I crn t C D - 2 mm 2 mm E - Fig. 4. Hylonomus lyelli. (a) RM Upper and lower jaws. Twice natural size ; (6) Diagrammatic cross-section of upper and lower jaws; (c) BM(NH) R Maxilla, medial and lateral views. Twice natural size; (d) BM(NH) R.446. Maxilla, medial view. Four-and-one-half times naturd size; (e) RM 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. In the same blockis a small cranial 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

7 66 ROBERT L. CARROLL which are present. On the right side, counting from the front, teeth 1,14,19,32 and 34 are missing. On the left, 2,26 and 28 are lacking. All of the teeth are simple pegs, expanded medially at the base 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 exactly as do 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 lth 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 visible as 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 (Fig. 4 a) is larger and bears about ten more postcanine teeth than that of RM 12016a. Both premaxillae are present in RM ; both are 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 that of Captorhinw, but is much wider than that of Petrolawsaurus (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 that 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 is not known. Like the transverse flange, it is covered by uniform small teeth. Without adequate knowledge of the remainder of the palate, it is not possible to determine the width of the interpterygoid vacuities. The configuration of the basipterygoid process of the pterygoid is identical to that of Captorhinus, but it is situated anterior, rather than posterior, to the transverse flange. It also has an anterior position in Petrolacosaurus. As in the latter genus, 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 eustachian 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, but 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, but neither is likely to belong with the remainder of the skull because of their small size. A completely isolated parasphenoid, BM(NH) R.877 (Fig. 5e), may belong to this species. It resembles that of Cuptorhinw in general, but differs in several particulars. The posterior plate, except for the presence of denticlea, is very similar, althoughit is broken posteriorly, giving the impression 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 at its base. Just anterior to the basipterygoid processes, the cultriform process extends dorsally around the base of the braincase.

8 The earliest reptiles 67 The basipterygoid processes of the basisphenoid extend laterally and anteriorly, rather than directly anteriorly as in Captorhinus. Each is grooved at its base for the passage ofthe palatine branch of the VIIth nerve. The anterior end of the groove, which is overlapped by the parasphenoid, is pierced by a foramen for 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 that is known. The quadrate has not yet been found. D cm Fig. 5. Hylonomwlyelli. (a) RM Palateand bonesoflower jaw; (b) RM Squamosal in lateral, posterior anddorsal views; (c) RM Dentary ;(d) RM Jugal; (e) BM(NH) R.877. Parasphenoid. All twice natural size. The stapes is present in NMC (Fig. 9). It closely resembles that of the captorhinids in the configuration of 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 recessed distally and obviously was continued in cartilage. Lower jaw: Disarticulated dentaries are present in the type, in RM 12016a, RM and E

9 68 ROBERT L. CARROLL RM The jaw associated with NMC (Fig. 10) apparently does not belong with the remainder of the specimen because of 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 The condition of preservation in the other two jaws precludes a count. Judging from the configuration of the skull, it is probable that 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 in Ophiacodon (Romer t Price, 1940). A bone which may be the angular is present, disarticulated, in RM It is in the form of a 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 and RM They are narrow and covered with small teeth. Judging from the extent of a roughened area on the 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 at least 20 teeth. Neither the posterior coronoid nor the prearticular has been found. An isolated articular is present in RM ; it 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 (the surface 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 ofthe retroarticular process. Postcranial skeleton (restoration, Fig. 6) : In none of the specimens is there a complete, articulatedvertebral column. There are 26 scattered vertebrae anterior to the pelvis in the type (Fig. l), but one or two more may have been present originally. In none of the specimens 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 observed in other primitive reptiles. The total length of the tail is unknown. Thirty caudals are present in NMC (Fig. 8), all with neural arches, but there were certainly more, giving a total of at least 50. The vertebrae are all large and clearly reptilian. The pleurocentrum and the neural arch form a solid unit in the 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

10 The earliest reptiles 69 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 conspicuous 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 centrum. 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. Just 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. In the remainder basedprimarily on BM(NH) R 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. In the few places where the vertebrae are in articulation, it is evident that the intercentra must have been small, as are those of Cephalerpeton and Petrolawsaurus. 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 (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 intercentrum 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 atlas is short, but not as abbreviated as those of the pelycosaurs; presumably it is paired, 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 that of the atlas. Both neural arches bear prominent transverse processes. The atlas and axis have essentially normal anterior and posterior zygapophyses, although they are somewhat larger than those of the succeeding vertebrae. The atlantal centrum, like that of Petrolawsauru'us and ophiacodonts, is crescentic in shape, open ventrally. The lateral walls taper ventrally. The axial centrum is similar to that of the succeeding vertebrae. The axial intercentrum is 2.5 cm. removed from its natural position, but its large size certainly precludes

11 70 ROBERT L. CARROLL it from belonging with any of the trunk vertebrae. It closely resembles that of Petrolacomurus. 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 to which 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 essentially a continuation of the shaft of the rib. The tubercular surface is at the end of a triangular extension of the dorsal Fig. 7. Hybnomw ZyeUi. RM (a) Humerus; (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. Just behind the sacrum the ribs become fused to the transverse processes. There are at least 4 postsacral ribs. The only evidence of the endochondral shoulder girdle is a small fragment of the scapula,

12 The earliest reptiles 71 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 at almost right angles to the shaft, and is two or three times as broad. The anterior margin of the shaft has a thin extension which is continuous with the ventral plate. This type of clavicle is essentially the same as that 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 at its tip, as in some pelycosaurs. The plate is known primarily from an impression of its 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 In generalit resembles the humeri of other captorhinomorphs. The ends are expanded and set at about a 90" angle. On the posterior side of the proximal expansion are two deep pits for the attachment 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 Captorhinus and the second of 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 synonymizing of Fritschia and Hylonornus. The lower forelimb is not known in any specimen of this species. A few phalanges and/or metapodials are present in the type and in RM , along with the ulnare in the type, but they are completely disarticulated and of little 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 An isolated ilium is present in NMC Sutures are clearly visible on the medial surface of the pelvis separating the ilium from the puboischiadic plate, but it is difficult to see this separation laterally. None of the specimens shows the suture at the junction of the pubis and ischium. The two halves of the pelvis, although not articulated in any of these specimens, apparently met at 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 at its base in NMC and appears to be totally missing in NMC This gives the pelvis of the latter specimen an appearance quite different from that of the others and suggests at fist consideration that it may belong to a separate species. The similarity of the remaining portions of the skeleton to that of the type of Hylonomus lyelli, together with a similarity in size, suggest that it is not distinct; but rather that 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 the 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 amedianridgeextending onto the puboischiadic plate. The acetabular area, visible only in BM(NH) R.4167, has been described by Steen.

13 72 ROBERT L. 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 pubis is separated 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 In the type, but 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~onomu8 lyelli. NMC Twice natural size. Both posterior limbs are articulated in NMC (Fig. 8). The femora, whose description 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 lightly 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 at 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

14 The earliest reptiles 73 Fig. 9. Hylonomw, ZyeUi. NMC (a) and (b) Counterparts of skeleton. Twice natural size; (c) Stapes. Three times natural size.

15 74 ROBERT L. CARROLL Fig. 9. Hylonomua lye&. NMC (a) and (b) Counterparts of ekeleton. Twice netural size; (c) Stapes. Thme times natural size.

16 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 distinct 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 that ofthe 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 (a) and (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 ofthe 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

17 76 ROBICRT L. 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 als 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 sideways, 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 approximately equal to the combined length of 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 at one end where they fit over the next in the series. In NMC (Fig. lo), they are arranged in a pattern essentially similar to that of Cephlerpeton. Although not as clearly defined as Hylonomw lyelli, there is some material from two additional reptilian genera. The first is known primarily from postcranial material and can be differentiated from Hylonomus on the structure of the humerus. ARCHERPETON gen. n. Type species, Archerpeton anthraeos Diagnosis : Primitive captorhinomorph. Entepicondylar foramen piercing posteroventral 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 11 and 12 Diagnosis: Same as for genus. The specific name is from the Greek, meaning coal, in reference to the coal forest habitat. Holotype : RM ; maxilla, parasphenoid, scapula, coracoid, clavicle, humerus ; collected by McNaughton. Paratype : RM ; humerus, ulna, radius and foot; collected by McNaughton. Horizon : Cumberland group, Joggins formation, division 4, section XII, 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. 11 c). 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 at 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

18 The earliest reptiles 77 basioccipital is visible behind the parasphenoid. Although incomplete, it resembles in a general way that of Captmhinus. A partial maxilla (Fig. 11 f is present in this same block, somewhat separate from the other bones. What is present resembles the maxilla of Hylonomus. It is broken anteriorly, precluding determination of the presence of Lcanines. There is room for about 23 teeth in the preserved portion, somewhat less than in the same region in Hylonomus. An isolated maxilla, RM (Fig. 12 e), may belong to this species. The medial surface is exposed. The ridge supporting the teeth is limited to the anterior end of the bone, in the region ofthe canines, and does not extend the full length of the tooth row, as is the case in Hylomus lyelli. RM (Fig. 12 b) is an isolated reptilian premaxilla. Although there is no reason to associate it with this particular species, it will be described here. Five teeth are present; if intact, all would be of about the same length, and are obviously shorter than those in the n I cm D V Fig. 11. Archerpeton anthracos gen. et sp. n. Holotype, RM (a) Sketch. Twice natural size; (b) Humerus; (c) Paraphenoid and basioccipital; (d) Scapulocoraooid; (e) Clavicle; (f) Maxilla. Three times natural size. premaxilla of Hylononzus. The nasal process is wide; its dorsal margin is very thin and partially broken. The opening or the external naris is quite large. The maxillary ramus was evidently overlapped by the maxilla. 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. Anisolatedpterygoid, RM (Fig. 12 c), may belong to this genus. It differs from the pterygoid of Hylonomus in lacking denticles on the quadrate ramus of the pterygoid ; in this specimen they stop abruptly at the base of the transverse flange. The flange itself is more clearly differentiated from the quadrate ramus than in Hylonmus, extending abruptly ventrally rather than continuing in the same plane. The palatine ramus bears a carpet of denticles, as in Hylomus. The quadrate ramus is crushed, but appears to resemble that of the other genus. Unfortunately, the area of articulation with the parasphenoid is also crushed, making it impossible to determine a-hether it could have accommodated a 6

19 78 ROBERT L. CARROLL parasphenoid similar to that of RM A broken maxilla is associated with the pterygoid, but no more can be determined from this bone than from the maxilla of RM Several vertebrae and ribs are present in the type, but are too poorly preserved for description. No intercentra are visible. Much of the shoulder girdle is present in the type. The scapulocoracoid is badly crushed and shows only its outline and the glenoid region. It is exceedingly broad 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 preserved is very short. The dorsal margin, however, does not appear to have been broken, but was undoubtedly finished in cartilage. The glenoid retains the 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) Paratype, RM Forelimb, humerus in posteriorandventralviews. Twicenaturalsize; (b) RM Premaxilla, inanteriorandlateral views. Three times natural size; (c) RM Pterygoid and maxilla Three times natural size; (d) NMC 1004lb. Femur, in dorsal and ventral views. Twice natural she; (e) RM 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 of Hylomus. The stem, which tapers to a point laterally, is complete and about as long as the ventral portion. No interclavicle has 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 at less of an angle to the proximal end (the latter may be due to distortion). The entepicondylar foramen is small and set close to the margin of the bone, as in Captorhinus, 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 (Fig. 12 a). The position of the entepicondyle indicates that the humerus belongs to the same species as RM 12056, and differentiates it 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 the olecranon, which is well ossified and about one-fifth ofthe length

20 The earliest reptiles 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 also a 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 it had yet developed a lateral temporal opening, but the configuration of the humerus is essentially the same as in all early pelycosaurs, and significantly different from that of any other reptile group. Subclass SYNAPSIDA Order PEL YCOSA URIA Suborder uncertain Family uncertain PROTOCLEPSYDROPS gen. n. Type species, Protoelepsydrops haplous Diagnosis : Primitive pelycosaur. Ectepicondylar ridge oriented at 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 animal s probable relationship to Clepsydrops, heretofore the earliest known pelycosaur. Protoclepsydrops haplous sp. n. Figures 13 and 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, coalgroup 15. Paratypes: RM a; 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 collection. 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

21 80 ROBERT L. CARROLL parietrtls and impressions of their margins, and an impression of the right postfrontal. The parietals differ considerably from those of Hylomus 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 Hylomus. 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 (a) Skeleton; (a) Pelvis; (c) Femur; (d) Foot. All twice natural size. The cultriform process is broad at its base, but is definitely narrower, relative to the width ofthe plate, than is that of Archerpeton. As a result, the basipterygoid processes are beginning to swing into a more anterior position ; they remain far more laterally placed than those of Hylomus. They extend distinctly ventrally. The cultriform process narrow8 rapidly and is broken at the end. The pterygoid described with Archerpeton may pertain to this form. Additional bones of the skull roof 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 sufficiently preserved 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.

22 The earliest reptiles 81 The neural arch is not swollen; it bears a short, rounded neural spine and transverse processes 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 its smaller size. A prominent supinator Fig. 14. Protockpsydrops huplowr gen. et sp. n. Paratypes. (a) D. M. S. Watson private collection B.239. Humerus; (6) BM(NH) R Humerus; (c) RM a. Humerus. All twice natural size. process distinguishes it at once from any captorhinomorph. Most of the area of the proximal expansion is missing, although enough is present in RM 3166 to indicate that it was set at about an 80" angle to the distal. The shaft is very thin, although short. Alarge entepicondylar 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 at

23 82 ROBERT L. CARBOLL the end of a prominent ridge. The ridge is oriented at right angles to the distal surface, and marked on either side by areas of attachment for extensor musculature. The posterior margin of the entepicondyle is very extensive, but shows little rugosity for the attachment of the flexor muscles. The areas for articulation with the radius and ulna are well differentiated and very large. Both areas are visible dorsally, as well as ventrally. The humerus, although the best preserved bone known from this species, does not allow determination of the family to which Protoclepsydrops belongs. In fact, there are no features which would prevent this genus from being close to the ancestry of the entire order. An isolated bone in RM 3166, called humerus by Steen, might possibly be the radius of this animal, but it could as well be a phalange or metatarsal of a labyrinthodont, otherwise represented in the block by isolated scales. The pelvic girdle (Fig. 13b) is present in a second block associated with RM Both sides are present, their external surfaces facing each other. The ilium, the only well preserved element, resembles that of Hylomus. The external surface of the blade is smooth, with no dorsal trough for axial musculature, such as is present in other earlypelycosaurs. The area of the acetabulum is not well preserved, but it appears to be located quite far dorsally. The supraacetabular process is fairly prominent. A right femur (Fig. 13c) is present in a further block. It is stocky, but longer, relative to the length of the puboischiadic plate, than is the femur of Hykmomus (this, however, may be due to poor preservation of the pelvis). The head is quite wide and the distal condyles are widely separated. The ventral surface is not exposed. The tibia and fibula are not known. An isolated hind foot (Fig. 13d) is present in one of the blocks of RM It consists of a number of phalanges and perhaps one or more metatarsals, and the calcaneum. The foot was somewhat shorter than that of Hylonomus but, as in that genus, the terminal phalanges are very short. A few scales are present in RM a, similar to those of Hylonomus. DISCUSSION From their descriptions, there can be no doubt that these animals are reptiles, and that Hylomus lyelli is fairly closely related to the primitive captorhinomorphs from Texas described by Price and Watson. The remains of Archerpeton, althoughlimited, suggest that it too is a primitive captorhinomorph. A more specific classification of these genera awaits the completion of work on the morphology of the Lower Permian romeriids, being undertaken by Mr Clark. There are no other Palaeozoic reptiles to which these genera can be considered directly ancestral, with the possible exceptions of Arawsce&? and Petrohaurus. Limnoscelis, considered by Romer (1946) as representing an ancestral reptilian stock, is clearly not closely related to any of these reptiles. Although the evidence is limited to the configuration of the parietals and the structure of the humerus, it appears likely that Protoclepsydrops is either a forerunner of the pelycosaurs in general or a very primitive ophiacodont. The remains of this genus are too limited to add significant information as to the relationship of the pelycosaurs and captorhinomorphs. On the question of their own ancestry, the Joggins reptiles shed little light. They are certainly not related to any microsaurs that have been described, nor do they have any features whichindicate evolution from that group. There are several microsaur genera from this deposit, all of which are as distinct from reptiles as are the better known later Pennsylvanian and Permian forms. At the same time, the Joggins reptiles cannot be related to any other group of Palaeozoic amphibians, or even to the seymouriamorphs. On the basis of vertebral construction, their ancestry can presumably be sought somewhere within the anthracosaurs, but nothing more definite can be determined at present. All the reptiles are small. The largest specimens of Hylonomus may have reached a total length of about 16 in. The two larger pelycosaur humeri indicate a somewhat larger

24 The earliest reptiles 83 animal. Other specimens of each species are much smaller, indicating animals only4 or 5 in. in length. There is also a great deal of size variation within each species of labyrinthodont and microsaur in this locality. The bones of even the smallest reptiles are highly ossified, suggesting an essentially adult condition. The high degree of ossification and the lack of any truly aquatic forms in the fauna suggest that these reptiles were primarily terrestrial animals. This tends to refute Romer s (1946) suggestion that 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,, Carboniferous rocks and fossil floras of northern Nova Scotia. Mem. geol. Sum. Can., no. 238: DAWSON, J. W., On a terrestrial mollusk, a. chilognathous myriapod, and some new species of reptiles, from the coal-formation of Nova. Scotia. Quart. J. geol. SOC. Lond., 16: DAWSON, J. W., Air-heeathersof thecoalperiod. Dawson Brothers, Montreal. 81 pp. DAWSON, J. W., On a recent discovery of Carboniferous batrechians in Nova Scotia. Amer. J. Sci., (3) 12: DAWSON, J. W., Acadiangeology. 3rded.,Macmillan andco., London. 694pp. DAWSON, J. W., On the results of recent explorations of erect trees containing animal remains in the coal-formation of Nova Scotia. Phil. Trans : DAWSON, J. W., Additional reports on erect trees containing animal remains in the coal formation of Nova Scotia. Proc. roy. SOC.,59: GREGORY, J. T., The structure of Cephakrpeton and aemities of the Microsauria. Amer. J. Sci., 246 : PEABODY, F. E., Petrolacoaaurw, kanaenab Lane, a Pennsylvanian reptile from Kansas. Pabont. Contr. Univ. Kans.. Vertehata, Art. 1 : PRICE, L. I., Noteson the braincase of Captorhinw. Proc. BOstOnsOC. nat. Hist.,40: PRICE, L. I., Two new cotylosaurs from the Permian of Texas. Proc. New Engl. zool. Cl., 16: ROMER, A. S., The primitive reptile Lirnnoscelia restudied. Amer. J. Sci.,244: ROMER, A. S., The nature and relationships of the Paleozoic microsaurs. Amer. J. Sci., 248: ROMER, A. S. & PRICE, L. I., Review of the Pelycosauria. Spec. Pap, geol. SOC. Amer., no. 28: STEEN, M. C., The amphibian fauna from the South Joggins, Nova Scotia. Proc SOC. Lond., 1934 : WATSON, D. M. S., On Bolosaurua and the origin and classification of reptiles. Bull. Mue. comp Haw., 111 :