notes on the crossopterygian hyomandibular and braincase

NOTES ON THE CROSSOPTERYGIAN HYOMANDIBULAR AXD BRAINCASE

ALFRED SHERWOOD ROMER Depccrtwent of Biology and Museum of Compamttae Zooloqy, Haroard University,

Cambridge, Massachusetts

FOUR FIGlTRES

The cranial anatomy of the extinct rhipidistian Crossopte- rygii is of considerable morphological interest because of‘ the close relationship of that group to the ancestral tetrapods. Our knowledge of these forms was until recently mainly con- fined to superficial features; the braincase was poorly known arid almost no data existed on the visceral arches. I n the last few years, however, the situation has changed greatly. In 1936, Save-Soderbergh correctly identified most of the external apertures of the braincase. In 1937 Jarvik gave an excellent description of many of the features of the nasal region. And in the latter year the studp of an excellent suite of material of the Permo-Carboniferous form “Megalicbthys ”

nitidus enabled the writer to yrescrit a reasonably complete account of both internal and external features of the hrain- case. At that time it was hoped that a study of the remaining cranial structures of “Megalichthys ”-upper and lomcr jaws, visceral arches and dermal bones-could be completed and published fairly shortly. This has not proved possible. At this time, however, the writer wishes to describe the hyomandibular because it fills an important gap in the history of that element and its stapedial homologue, and to discuss certain points regarding the braincase which have been brought out by subsequent work on fishes and amphibians.

The material upon which the present work is based is the fossil form from the Permo-Carboniferous redbeds of Texas

141

142 ALFRED SHEHWOOL) ROMER

originally described by Cope as Ectosteorhacliis nitidus. This lias usually been assigned to the European genus Megalichthys (Parahatrachus). Dr. T. S. Westoll, howerer, has pointed out to me that there are a number of features of the skull roof pattern which suggest that Ectosteorhachis should be retained as a generic name. This is further advisable because of the confusion that exists as to the proper name for the related European type, some writers considering that l\legalichtliys should be used fo r the larger Carboniferous fish usually called Rhizodus. Since our present interests are, however, morpho- logic rather than taxonomic, the Texas form will be referred to below as “illegalichthys” nitidus.

DORSAL HEAD

Fig. 1 The hyomandil)ular of ‘ Megaliehthys ’’ nitidus. 1,eft s ide ; A, lateral view; B, medial view.

DESCRIPTION O F TIIE IIYOMANDIRULAR

Little had been kiiowri of the hyomandibular in crossopterygians before the present studies were undertaken. Traquair (1881, p. 171), Bryant ( ’19, pp. 11-12), VC7atsor1 and Day ( ’16, p. 16)’ and Watson ( ’25, p. 249) had noted that the element was present in Rhizodopsis and Eusthenopteron, but had been able to determine little of its structure. In my earlier paper on the braincase the presence of the complete hyomandibular was noted arid a few salicnt features mentioned. In a description of the external features of a braincase of Rhizodopsis published while that paper was in press, Save- Soderberg (’36) correctly identified the facets f o r the hyomandibular and predicted that this element would be found to be double-headed.

1 l Y OMAN DIB ULAK U P CI1OSSOPTET;YOlI 143

The account below (fig. 1) is based entirely on the sectioned specimen (MCZ 6494) previously used for the description of the internal anatomy of the braincase. This was ground at f mm. intervals, the material preserved by an adaptation of the palcobotanical peel method, and wax plate reconstruc- tions made at an enlargement of three times natural size. Both byomandibulars were preserved in a position close to that in which they were articulated in life. Due t o the fortunate fact that the skull was sectioned vertically at an angle nearly 45 degrees from the longitudinal axis of the head, the left hyomandibular was sectioned at nearly right angles to its long axis, the right element essentially longitudinally ; the two series check each other in a satisfactory manner.

The grinding of the sections used in this study and in that of the braincase was performed on a machine furiiislied by the Elizabeth Thompson Science Fund. I wish to express my thanks for this assistance and my regrct that similar aclmowl- edgment was inadvertently omitted from my earlier paper.

The hyomandibular is essentially a rod-like structure, ex- pancled proximally, tapering distally, with a pronounced postero-dorsal proeess, flattened medio-laterallj- in its distal portion; the hone is traversed for much of its length h~ a longitudinal canal and bears a prominent ridge along the proximal portion of its medial surface.

The proximal expansion includes thickened dorsal arid ventral portiom, the dorsal much the larger, which bear the articulations with the otic region of the skull ; between them is a thinner connecting ridge. The actual articular surfaces lack a perichondral bone layer and were presumably tipped hy cartilage. The ventral articular surface appears to have faced nearly directly anteriorly in life ; the much larger dorsal area tilted rather more medially. Both are somewhat convex. Al- though the ridge connecting the two heads appears not to have heen in actual articulation with the edge of the braiii- case, it seems to have been so nearly in contact with it that no nerves or blood vessels could pass fore or aft between

144 ALFRED SHRRWOOD ROMER

the two heads except by utilizing the “jugular canal” pene- trating the braincase beneath this area.

The proximal portion of the shaft, wliich in life appears to have extended backward and somewhat outward in a nearly horizontal direction, is featureless externally, with a relatively smooth semicylindrical contour. On the medial aspect, however, are numerous structural features. Proximally, between the lieads, is the opening of the large longitudinal canal. Above and below this develop parallel ridges which extend back toward the opercular process arid enclose between them a grooved channel. The sharp ventral ridge is the more prominent of the two, extending far medially. These two ridges appear to lie in the same horizontal planes as ridges which, as noted in the description of the braincase ( ’37, p. 12, fig. Z ) , form the dorsal and ventral borders of a groove for the lateral head vein. The groove on the braincase is deep anteriorly and posteriorly, shallower in its middle portion ; the corresponding groove on the liyomandibular is deep in its middle portion, shallower proximally and distally. It seems certain that this area of the hyomandibular adjoined the outer surface of the vena capitis lateralis, which, running backward from the jugular canal, was a t first directed somewhat laterally but then curved more medially to follow the contour of the braincase. The dorsal ridge presumably afforded an area of attachment for muscle C,hd (levator hyoideus).

A foramen opening distally in this jugular groove is discussed below. Of interest is a narrow hut deeply excavated groove which lies just beneath the more ventral ridge. This begins abruptly with a deep incision not far beyond the head and then runs posteriorly, gradually losing in depth and broad- ening out to become lost below the opercular process. It seems obvious from its nature that this groove was associated with a ncrve rather than a blood vessel. I ts position is exactly comparable to a portion of the course of the pretrematic branch of nerve I X in such a form as Amia. This ramna passes forward along the braincase wall and then, near the head of the hyomandibular, curves sharply outward and back-

HYOMANDIBUTAR O F CROSSOPTERYGII 145

ward to pass distally along the mcdial surfacc of the hyomandibular (compare Allis, 1897, p. 686, pl. 27, fig. 32, prgl,

Dorso-laterally the hyomandibular is expanded into a prominent process, triangular in shape as seen in side view and compressed medio-laterally. Proximally it connects with the distal ends of the median ridges described above ; distally its margin descends by a sharp ridge into thc contours of thc shaft. On its lateral aspect, adjacent to the posterior niargin, is a n oval area, raised above the adjacent surface, on which no pericliondral hone was present. It seems obvious by comparison with known actinopterygians that this is an opercular process and that the area of “unfinished” bone which appears to lie beneath the proximal edgc of tlie operculum, is an area of articulation for that element.

The distal portion of the shaft is flattened and featureless medially. Laterally the longitudinal canal opens onto this surface and descends along it in a pronounced groove. The groove is bordered antero-ventrally by a ridge of moderate elevation. Postero-dorsally the proximal portion of its margin is a pronounced tubercle adjacent to the opercular articulation; more distally this margin of the grooved area is flat. The groove is subdivided into two portions, the more anterior one the larger and more deeply incised; the two become dis- tinct at about the point of emergence from the foramen.

The distal end of the bone is a hemispherical cup, lacking a perichondral layer and obviously continued in cartilage. Nothing can hence be said of the nature of possihle articular connections with the jaws or ceratolipoicl.

The proximal portion of the hyomandibular is traversed by a canal of moderate proportions with its antelmior opening 0 on the medial surface b e b e e n the two heads and its distal opening on the lateral surface a little more than half way down thc bone. The canal enters the bone with a pronounced lateral direction, apparently in line with tlie orientation of the adjacent jugular canal. It then curves more posteriorly and finally somewhat downward and laterally to i t R externttl orifice.

otc.).

146 ALFRED SHERWOOD ROMER

It is highly improbable from its course that this canal carried a stapedial (orbital) artery, as does the stapedial foramen of numerous tetrapods, or any important blood vessel of the hyoid arch. It is reasonable to assume that it is homologous with the similar canal seen in Arnia which transmitted the main trunk of the seventh nerve. The two grooves distal to the opening of the canal may have been for the mandibular and hyoid divisions of this trunk.

I n the liyoniandibular of the left side a rounded groove de- velops along the dorsal margin of the canal in its proximal portion; this appears to lead to a small foramen which opens medially in a pronounced pit proximal to the opercular process. Because of the small size of this structure relative to the thickness of the sections there is not complete proof of the presence of this foramen, and I have not been able to find a similar structure on the right hyomandibiilar. It will be noted, however, that this is a proper position for the opercular ramus of the facial nerve. Possibly its course may have been a variable one.

THE CHOSSOPTERPGIAN ITYOMANDIBTJLAR COMPARED WITH THllT O F OTHER FISHES

The three salient structural features of the crossopterygian hyomandibular, as here described, are (1) an opercular process, (2) a canal for the facial nerve and (3) a doitble articulation with the braincase.

Simp a n opercular process is present in typical actinopterygians, it may be reasonably assumed that the condition in ‘‘ Mcgalichthys” is that which was present in the primitire bony fishes, perhaps from the beginning of the development of a hyoidean type of operculum. No corresponding process is (quite naturally) known in C1iondricth;ves.

A canal piercing the hyomandibular €01. the main trunk of the facial nerve is found among the higher members of the Actinopterygii. Polyodon, Polypterns and some sturgeons lack this canal, and various theories have been suggested to account for the yaried relations of the nerve to the lipo-

HYOMANDIBTJLAR O F CROSSOPTERYGlI 147

mandibular (compare Goodrich, ’30, pp. 416420). The fact that it is now found to be present in Crossopterygii a s well as in the early fossil Actinopterpgii suggests strongly that it was present in the ancestral bony fisli and that its absence is due to loss of tlic canal through reduction in size of the hyomandibular or a shift in relative position of the nerve. In the sturgeon the canal, when present, is very short and in Yolyodon the iiervc emerges dorsal to the shaft of the hyo- mandih lar , suggesting that (as the shape of the bone indi- cates) there has been a reduction in the extent of the liyomaridibular dorsally, thus freeing the nerve. This reduction of the bone is presumabhly associated with reduction of the opercular apparatus and consequently of that portion of the hyomaiidibular which supported the operculum. The iierve has not “eaten its way into the cartilage” to attain the typical actinopteryg-ian condition, but has on the contrary “eaten i ts way out7’ in degenerate forms. However, the assumption of the primitive presence of the canal does not afford any lnetter explanation of the peculiar situation seen in Polypterus than any other theory.

The most interesting feature of the “Illcgalichthys” hyomandibular is the presence of a double articulation with the otic capsule (fig. 2). In all fish hitherto known the h p mandibular has but a single cranial attachment; and this attachment has strongly contrasting relationships (compare Goodrich, ’30, pp. 419423: de Beer., ’37, pp. 410-416). In sharks the athwliment is a ventral one, below the area traversed by the vena capitis lateralis arid other elements in the “cranio- quadrate passage” of Goodrich. In known bony fishes the attachment is by a head dorsal to these elements. To recoiicile these two structural situations, various theories have been proposed. It has been suggested (1) that the hpomandibulars in sharks and bony fishes are noii-homologous as a whole or i n pa r t ; ( 2 ) that the head of the bone has migrated from one position to the other along the rim of the “jugular canal” through which the lateral head vein and other structures frequently pans f o i ~ a r d from the otic into the temporal region;

148 ALFRED SHEEWOOD ROMER

or (3) that the hyomandihular was primitively two-headed, the bony fishes retaiiiiiig the dorsal head, the sharks tlie ventral one. The discovery in “Megaliclithys” of a double- headed bone strongly suggests that the last theory, advocated especially by cle Beer, is the correct one. The theory of migra- tioii along the margin of the jugular canal is by no means debarred, however. It will be rioted that the “ blegalichthps” element has a continuous span of bone between tlie two heads

7

A

c

\

/ PPAR

C vii M J

Fig. 2 Diagrams to show the relations of the hyomandihular t o other structures in various fish groups. On the right the l ipmandibular is shown in position. On thc left, the hyomandibular is rcmoned; i ts articular areas are shaded and dorsal or ventral attachments not prcseilt indicated in broken line. A , cros sopterygian condition ; E, gcneralized shark condition ; C”, typical actiiiopterygian condition; CT, cliorda tympani; OA, orbital artcry; PPAR, pacocci pita1 01

p r o t i c process to which dorsal head attaches; STA, stapedlal a r te ry ; VCL, venn capitis lateralis; VII H, liyoid rainus of facial nerve; V11 X1, mandibular ramus oC facial nerve.

HYOMANUiBULbR OF CROSSOPTBRYGII 149

so that while functioiially dicliocephalous it is not far from a technically lioloceplialous condition (the ribs of man>- primitive tetrapods are structurally analogons) .

In existing selachians tlie hyomanclibulai- trunk of the facial iicrve emerges on to the outer surface of the hyomandibular without either piercing the hone or passiiig backward beiieath it. This situation could be reached bj- the assumption that the shark originally possessed the crossopterygiaii structure

L

VII n A

B VII M=CT

Fig. 2a Diagrams t o show the development of the tetrapod stapes from the crossoptery-gian iipm:tndihular. A, a primitive reptile (OpIiiacodon). B, em- bryonic mammal. For cxplanation see figure 2.

but has lost the dorsal head and the rpg-ion of the shaft adjacent to it, thus opening the canal dorsally and “freeing” the nerve. As against the general lendcncy to assume that a sha rk condition should he regardcd as that of a primitive griathostornc, it should be remeinbered that as fa r as our present pleontologi- cal knowledge goes, the bony fislics, partienlarlp the Clrossopte- rygii, appear to be a much older group than the Elasmo- br arichii.

150 ALFRED SHERWOOD IlOlLlEH

In all major respects the “Megalichthys ” hyomandibular thus appears to be of a type from which that of all other jawed fishes could be derived and is, therefore, in all proba- bility primitive.

T H E HYOMANDIBULAR COMPARED WITH THE TETRAPOD STBPES

Certain aspects of the evolution of the stapes from a hgo- mandibular of the “ Megalichthys ’ ’ type liave been discussed by Eaton (’39) on the basis of data furnished by the present writer. Since, however, the ‘‘ Negalichthys ” hyomandibular had not then been adequately reconstructed and since recent work in this laboratory has afYorded new data on the stapes of early land vertebrates, some of the points concerned may be worthy of further consideration (figs. 2, C; 2 a).

A primitive type of stapes which appears to be cliaracteristic of that expected in a primitive reptile and especially in a primitive reptilian ancestor of the mammals is that seen in such a pelycosaur as Ophiacodon (Romer and Price, ’40). This appears to be directly comparable in inany regards to the hyomandibular of “R/legalichthys. ” In both we are dealing with a massive rod-like structure with a cartilaginous distal end ( a feature which rules out of discussion the nature of the distal articulations). In both the proximal end is bifurcated, arid dorsal and ventral attachments to the braincase are present. In both cases the dorsal attachments are to seemingly homologous areas of the otic capsule (parotic or paroccipital process). In both cases the vena capitis lateralis appears to have passed backward through a gap between the two heads (there is no “jugular canal” in the tetrapod, so that the vein was superficial throughout). In both cases the ventral head articulates with ail area external to the saccular region of t,he ear; it, seems certain that the ventral head of the hpomandibular is the homologue of the footplate of the stapes.

Three points of distinction may he noted between the ‘ ‘ Megaliclithys ” hyomandibular and the primitive reptilian stapes : (1) the prominent opercular process of the fish is not present in the reptile; (2) the nerve canal of the fish is

HYOMASDIBULAR OF CROSSOPTERYGII 151

absent in the reptile ; (3) the footplate of tlie stapes contains an arterial carial for which there is no fish homologue. How- ever, these differences may be reasonablv accounted for in relation to the functional changes which have taken place between fish and reptile evolutionary stages.

With the loss of the operculum in land fornis, the opercular process becomes superfluous in its original function. Whether it has disappeared or whether it has migrated distally to become a tympanic attaclinierit (as advocated by Eaton) is a point which cannot be settled on avitilable fossil evidence.

I f this process be reduced or lost, it might be expected that (as was noted to be tlie case in fish with the operculum reduced) the canal for the hpomandibular nerve would be lost leaving that nerve (including the chorda tympani) to curve outward around the posterior border of the bone.

The development of a foramen for the stapedial artery is readily explained on topographic grounds. This artery, the fish orbital artery, as discussed in connection with the braincase in “I\legalichthys” appears to have passed upward from the veiitral surface of the skull just behind the ventral head of the hyomandibular a i d turned forward just above tlie head to enter the jugular canal. It is morphologically difficult to shift this structure more posteriorly or dorsally, and any expansion of the ventral articular area into a footplate would tend to surround the artery and cause the formation of a stapedial foramen.

We need not coiicerii ourselves liere with tlie highly special- ized auditory apparatus of existing amphibian orders ; the fossil Labyrinthodontia, however, as a presumably primitive group must he considered. ITnfortunately our knowledge of the stapes in early members of the group is quite limited. In such forms the stapes is essentially rod-like and lacks the pronounced dorsal process seen in both the crossopterygian and primitive reptile. But the process is present, although reduced, in such a form as Edops (now being studied in this laboratory). The reduction is due to the fact that the paroccipital process, to which the dorsal process of the stapes at-

152 ALFRED SHERWOOD KOMEE

taches, extends far laterally parallel and close to the shaft of the stapes. Since the two structures are closely apposed, an elongate dorsal process is unnecessary and, indeed, has no room ill wliich to develop.

The further development of the stapes toward the mam- maliaii condition includes as a major change the loss from the bone of the dorsal process; in agreement wit11 current theory x7c have indicated this structure in our diagram (fig. 2a,B) its forming the cranial attachment f o r the hyoid apparatus.

TIIE TERM CHOANICHTHYES

It ~ 7 a s long fashionable to subdivide the bony fishes into two major groups consisting on the one hand of the Dipnoi ant3 on the other of the Teleostomi, including Crossoptcrygii and hctinopterygii. More recent work, however, has tended to show that the Crossopterygii and Actinopterygii are far removcd from one another and that on tlic other halid crossopterygian-dipnoan relationships are extremely close, the t ~ 7 o forming a natural group of relatives of the primitive tetrapods. The validity of this conclusion was strongly im- pressed upon the writer through the studp of the braincase. I know of no term available to use for this latter group and suggested that it be designated the subclass Choanichthpcs, the name referring to the presence of internal nostrils in these fishes as a point of diagnostic importance. Dr. C. L. Hnbbs, however, has since called my attention to the fact that in 1919 he used the term hmphibioidei to ~ 0 ~ 7 e r this same group. This term thus has priority. However, taxonomic priority does not govern by law thc names of supergeneric groups, and the writer confesses to a preference for his own term on the grounds of syllabic brevity, a t least.

XOTES ON THE BRAINCARE

While my earlier paper was in press Riive-Soderberg published an account of the external features of the braincase of Rhizodopsis. Despite his lack of access to internal struc-

llYORlAKD1BLTLA411 OF CROSSOPTERYGTI 153

tures this work was a vast improvement upon earlier published accounts and only in a relatively few instances need his inter- pretations be disputed. My material was admittedly imperfect in the superficial portions of the nasal region. Knowledge of this area has been suppleniented by the publication of an excellent account of the nasal and ethmoid regton of Russian examples of Eustlienopteron by Jawik ( '37). No further data of importance have since been published. In 1939, however, Sterisio and Jarvik published a general critique of recent work on fossil fish skull structure which brings out certain points on which disagreement still exists regarding the CY'OS-

sopterygian braincase.

Fig. 3 Iliagram of the left lateral surface of a portion of the brainease of " Megalichthps" to show the position of certain foramina discussed in the text

1. Tlie nature of the openings from the braincase in the trigeminal region is disputed (fig. 3) . These openings include ( A ) a gap between anterior and posterior segments of thc braincase, (B), (C) two more posterior openings in the lateral wall, aiid (D) a more ventral opening, from the floor of the braincase into the channel occupied by the vena capitis lateralis. I have interpreted them as being occupied respec- tively by ( A ) the profundus, (B) mandihular and maxillary divisions of the trigeminal, (C) the lateralis component of the facial and (D) the vena cerebralis medialis. Sterisiii and Jarvik, however, mo11ld have the profundus occupy canal B and transfer maxillary and mandibular branches of the tri- gcmirius to D. This, however, appears to be an improbable, if not impossible, solution, for canal D runs out from a fa r more ventral position which would be difficult for visceral fibers


from the dorsal portion of the medulla t,o reach; and, still worse, it opens externally not directly on the surface but into tlie bottom of a deep trough occupied by the lateral head vein. This is an unnecessarily circuitous and morphologically improbable route for an important nerve trunk to follow.

2. More posteriorly, in the “occipital” region there occur (in addition to hypoglossal openings) three foramina leading from braincase to surface : (E) an antero-ventral foramen passing through the internal ear cavity and opening close to the vena capitis lat eralis, which both SZve-Soderberg and the writer believe (but with some doubt) to have transmitted the glossopharyrigeal nerve ; (F) a more posterior and dorsal foramen which seems unquestionably to be for the vagus; (G) a still more dorsal foramen leading out from a point nearly opposite the dorsal surface of the medulla. This last I believe to be an opening f o r posterior lateralis components which are generally associated with nerves IX and X. This interpretation is questioned by Stensio and Jarvilr. They helieve that opening G was for a posterior cerebral (“jugu- l a r ” ) vein “nach dem Verhalten bei den Fischen im allge- meinen,” and that lateral line coniponents passed in “normal” fashion through foramina E and F .

WhiIe one cannot be too positive, the writer is not inclined to accept their conclusions. Their statement that the posterior jugular vein passes out through a separate foramen in fishes in general does not appear to be in agreement with such data as are familiar to me. As far as I can discover the vena cerebralis posterior invariably accompanies tlic vagus in living bony fishes; in the Chondrichthyes alone are there instarices in which the vein emerges by a foramen separate from but close to tlie vagus. On the other hand the posterior lateral line nerve frequently abandons the glossopliaryngcal entirely (compare Norris arid Hughes, ’20, pp. 356-358) and in many cases both in Choridrichthycs and bony fishes the dorsal portion, a t least, of the N. lateralis metoticus emerges by a separate dorsal foramen. I n a number of “placoderms” Stcnsio has described a dorsal foramen which may possibly

HYOMANDIBULAR O F CROSSOPTERYGII 155

be identical with G of “ RIegalichthys.” He has assumed that this carries the vein. This, however, appears to be by no means certain, and the function of this foramen deserves reinvestigation. Modern coiicepts of the lateral line system, as ably set forth, for example, by Haller (’34, pp. 658-670), indicate that the lateralis fibers form an essentially inde- pendent group, subdivided into pro- arid metotic portions. The N. lateralis metoticus is essentially a unit; the fact that its rami are generally associated with nerves I X and X is due to topography rather than to any real relationship. With this view my interpretation of the foramina is in complete accord. We appear to have in “Megalichthys” and perhaps in placo- dernis a generalized and perhaps primitive condition in which K. lateralis metoticus emerges through a foramen of its own.

TOPOGRAPHY O F THE OTIC XEGION

In my earlier account I expressed considerable doubt as to the identification of certain structural features in the posterior part of the braincase. This doubt was in part due to our inadequate knowledge of this region in primitive amphibians. Studies in this laboratory on the braincases of Eryops and Edops have in great measure remedied this deficiency.

Whether the parotic process of “Megalichthys” was properly comparable with the paroccipital process of early tetrapods seemed doubtful because of the considerable differences in position. It appears, however, in the light of amphibian evidence, that the two arc actually homologous, for the essential relationships are the same. The process has shifted posteriorly and extended far laterally, in correlation with the development of a compact occipital surface in a trans- verse plane; this is not improbably associated with the development of motility of the head on the body, such as must have occurred with the iiiitiation of tetrapod locomotion.

A pronounced supraotic fossa is present in “Megalich.thys, ” covered dorsally not only by dermal elements but by a film of bone from the braincase as well. This fossa is compared

156 ALFRED SHRRWOOD ROMER

by Save-Xoderbcrgli and by Stensio and Jarvik to the “fossa Bridgei” of actinopterpgians, occupied by occipital musculature. 1 had earlier considered this comparison, but rejected i t because the relatively narrow posterior opening made it seem improbable that the “Megalichthys ” fossa mas occupied by musculature. Further consideration, however, leads me to believe that the comparison is a proper one; it may be noted that the homologoils area in primitive amphibians appears also to have been occupied by occipital musculature (Olson, ’36, fig. 3, etc.).

The posttemporal fenestra of l o ~ e r tetrapods does not a t first sight appear to be particularly comparable to the supraotic region of the crossopterygian. But in the amphibian Edops the posterior portion of the fenestra is a deep pocket, opcn posteriorly and roofed, as in “Xfegalichthys” by a film of hone connected with the braincase. The posttemporal fenestra of a tetrapod corresponds to the crossopterygian supraotic fossa plus the fenestra connecting it anteriorly with the temporal region.

CORRELATION O F BRAINC4SE P A T T E R 1 WITH DERMAL ROOF INTERPRETATION

Study of the crossopterygian braincase brings out a marked contrast in proportions between this group and tetrapods (’37, fig. 16, etc.). I n crossopterygians the otico-occipital 01- para- chordal moiety (roughly posterior to pituitary and pineal) is long; the anterior, trabecular, part relatively short. In amphibians tlie posterior region is relatively extremely short, and the anterior portion has been greatly elongated. This contrast in proportions correlates admirably with Westoll’s recent interpretation of the evolution of the elements of the dermal slrull roof (fig. 4).

The crossopterygian skull shows, with some variations, a dermal bone pattern which ought, a priori, to be readily comparable with that of early land forms. But a major stumbling block existed. I n the center of the posterior half of the skull are a pair of elements (stippled in fig. 4 A) which mere almost

HYOMANDIBULAR O F CROSSOPTERY(:II 157

univcrsally identified as the ietrapod parietals. Howel-er, the pineal opening, found invariably in earlp lard forms between the parietals, here is present hetween a pair of smaller anterior elements u s id ly thou@ to be the frontals (hatched in the

A B C

D w W

E F Fig.4 Diagrams to show the cyolutlonary trend of the skull roof pattern from

fish to reptile and its relation to thc change i n proportions of underlying structures. A, a typical crossopterygian, Ostcolepis. B, a n amphibian like crossopterpgian, Elpistostege. C, an archaic Dmouian amphibian, Ichthyostega. D, a generalized Carboniferous amphibian, Palaeogyrinus. E, a cotylosaur, Romeria. F, a primitive uianimal like reptile, Dimrtrodon. The postparietal (dermal supraoccipital) is stlpplrd thioughout, the p'trietal hatched. The main elements are labelled in A and U t o sliow the Iiewer interpretation of the crossopterygian pattern. I n A, U and F tho position of the pituitary is indicated by a cross. I n B thc marginal portions of the skull are unknown but have been restored t o facilitate comparisons. In A the extrascapulars, once thought t o be part of the s k ~ l l pattern, are indicated in dotted outline. I n E the postparietals are absent in the only known specimen but may be restored with confidence. A after Sgve-S6derbergh and Westoll; €3 after Westoll; C after Save-Soderbergh; D after Watson; E after Price.


figure). To account for this discrepancy long seemed impossible. Such theories as were proposed were none too satisfactory, and in one recent ease involved a most fantastic series of improbable assumptions. Some years ago I suggested (’36, p. 253) a minor modification of views then current in pointing out that the fish “extra-scapulars” (customarily homologized with the tetrapod postparietals and tabulars) were not properly part of the skull pattern; but I could contribute nothing toward a solution of the major problem involved.

A bright ray of light was shed into this darkness by a brilliant suggestion of Westoll ( ’36, p. 166; ’40, pp. 71-73). This was to the effect that the major premises upon which earlier theories had been based were incorrect; that there had been no shifting of the pineal opening or mysterious “re- shuffling” of elements ; that thc supposed parietals were actually the tetrapod postparietals ; and the supposed frontals were actually the parietals. In the shift from fish to amphibian the postparietals, originally long, have become much shortened (relatively, at least) ; the parietals have in consequence become relatively more posteriorly placed, and the anterior part of the skull has become greatly elongated (fig. 4, L4, C). This theory of TVestoll’s (and this theory alone) is in perfect accord with the facts of brainease evolution. The postparietals of Westoll’s interpretation run the entire length of the posterior half of the braincase, which undergoes grcat relative shortcn- ing in tetrapods ; the anterior portion of the braincase, bearing the remaining elements, becomes greatly elongated to occupy most of the skull length. I n A, D and F of figure 4, 1 have marked the position of the pituitary with a cross, to indicate the posterior shifting of this diagnostic endocranial structure. As will be seen, the relative shift is closely comparable to that assumed by Westoll for the dermal elements.

Westoll’s theory received further confirmation by his discovery (1938) of the transitional type named by him Elpisto- stege, in which the relative shortening of the otico-occipital region is already begun (fig. 4, B). I t is further confirmed by consideration of the still later history of the skull roof. I n

HYOI1IAKDIBULAR OF CROSSOPTERYGII 159

primitive reptiles generally (fig. 4, E, F) the postparietals are still further reduced. Later they may be pushed down onto the occipital surface, and by the end of the Triassic they had disappeared entirely from almost all land types. We are witnessing in reptiles the end of a process of reduction begun bp their fish ancestors. Hand in hand with this is a crowding of the parietals to the posterior elid of the braincase. In a typical crossopterygian the parietals and postparietals to- gether occupy about 80% of the length of the skull roof. Tn the successive forms figured this percentage becomes reduced to 62, 44, 36, 24 and 11%. This shortening of the posterior part of the braincase and related dermal elements seems to be associated with concurrent changes in the enclosed nervous system which have changed the essentially straight brain- stem of “Megalichthys” (Romer, ’37, fig. 10) into the highly flexed (and consequently much shorter) structure seen in tetrapods. If the embryological relations be considered, it is reasonable to assumc that the primary change was that of the nervous system, and that the skeletal changes were second- ary. The underlying funetional “causes ” for these morphological changes are well worthy o€ further study and consideration.

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