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Full text of "An Ichthyosaurian Skull from Queensland, Plates XV-XVI"

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(Plates XV and XVI and Text-figures 1 and 2.) 

The remains whieh are the subjeet of this paper were found at Galah Creek, 
about twelve miles from Hughenden, in the Rolling Downs formation (Lower 
Cretaeeous) of Western Queensland, and were eolleeted, forwarded, and kindly 
donated to the Queensland Museum by Mr. S. Dunn and Mr. William Elliott in 
May. 1914. It is my pleasant duty heartily to thank these gentlemen for their 
enthusiastie work in seeuring this large and valuable specimen for our eollections. 

MATERIAL.— As will be seen from the profile view, illustrated in Plate XV.. 
this large skull is in six pieces. The extreme end of the rostrum is missing, but, 
judging from the structure of the anterior part preserved, only a small portion 
would be needed to eomplete the skull. Gilmore! has pointed out how frequently 
the extreme anterior segment is missing in Ichthyosaurs. and how fraetures are 
caused by the eraeking of specimens when enclosed in an elongate eoneretionary 

The skull is massive, with a maximum length (mandibular) of 1,026 mm., 
and a maximum width (articular area of mandible) of 395 mm. It is evident that 
great changes have taken plaee since it eame to rest. As a result of tremendous 
vertieal pressure, the whole of the teeth, with the exeeption of broken roots, have 
heen foreed from the continuous dental grooves, eharaeteristie of /chthyosaurus, 
and the premaxillary and mandibular rami are now in juxtaposition. Fortunately, 
many of the teeth have been preserved. mostly as fragments, on the lateral and 
lower surfaces of the jaw. In the posterior part of the skull there are still greater 
evidences of changes under intense pressure. On the left-hand side the orbit has 
been crushed down and its original eontours are not distinguishable. As a result 
of this lateral torsion, the mandible has been somewhat displaced to the right. The 
supratemporal fosse are preserved in fairly symmetrieal condition. Great diffieulty 
has been experienced in studying some of the component parts. The distortion of the 
skull is aecompanied by a very close investiture of the remains by a fine hard lime- 
stone matrix, which in plaees is almost indistinguishable from the aetual fossil. 
The matrix involving Cratochelone berneyi2 deseribed by the author in 1915 from 
the same distriet, was very similar in texture. "This investing material evidently 
penetrated the skull after the decay of eartilage, eementing the disrupted elements 

! C. W. Gilmore, Mem. Carnegie Mus., Pitts., II, 1905, p. 80. 
* H. A. Longman, Mem. Qld. Mus., III, 1915, p. 25. 



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Face page 247. 


On the upper part of the skull the elongate premaxille can he traced back 
for a distance of 615 mm. to the narial openings. Between these two bones in the 
superior surface is a well-marked symphysial groove. The nasals are exposed from 
beneath the premaxille at about the anterior third of the length of the skull, and 
at first are on a lower plane than the hemispherical superior borders of the diverging 
premaxillee, forming a triangular recess. Further back there is a secondary 
triangular depression, the borders of which are parallel to the anterior recess, but 
this is entirely internasal. Lateral divisions of the nasals extend outwards beyond 
the frontals towards the superior border of the orbits. Sutures with post frontals 
cannot be traced. 

The external narial openings can be seen on both sides, but they are distorted. 
A semicircular raised border is present behind the openings. Incomplete maxillee 
are present, but the sutures between them arid the lachrymal bones and the jugals 
cannot be positively traeed on either side. 

In the region of the frontals a remarkable rectangular raised process was 
present in the undisturbed fossil. On careful development this proved to be mainly 
matrix closely investing a troughlike depression, with raised lateral borders. as 
may be seen in Plate XV. At first this was thought to be a veritable raised bony 
border surrounding the pineal foramen, and suggesting an unusual development of 
"the third eye," but the true foramen is apparently situated in a more posterior 
position. This closely adpressed structure consists of two parallel bars. thinly 
joined anteriorly; the bars are 90 length and are symmetrically disposed 
at a distance of 10 mm. from the median line. If this structure is actually in sita, 
which seems unlikely, it would demand generic recognition. The frontal bones 
evidently do not extend far beyond the area of this structure. In view of the partial 
disorganization of the specimen, possibly associated with an attack from other 
predaeeous or scavenging monsters of the period, it is suggested that this eurious 
structure represents an inverted cranial element. It cannot, however, be allocated 
with any of the bones, the contours of which are so clearly demonstrated by Sollas’ 
classic sections, and possibly represents hyobranchial elements. And it is. of course, 
possible that further material will demonstrate characters which will warrant the 
establishment of a new genus fcr this large Australian Ichthyosaur. The prominent 
ridges, which are present in the parietal region and on the nasal bones. appear 
to be distinctive features. From the occipital border of the parietal region a convex 
median ridge extends anteriorly, and this is accentuated by the presence on each 
side of elongate valleys, the lateral sides of which curve upwards to form the 
borders of the supratemporal fossz. At the anterior termination of the median 
ridge there is a cavity which could not be traced into the internal tables of the 
skull, but which probably represents a disrupted pineal foramen. This is nearly 
in line with the anterior borders of the supratemporal fosse. and is thus in the 
usual position for the foramen. 

? W. J. Sollas, Phil. Trans. Roy. Soc., B, 203, 1916, pp. 66-126. 


In the temporal region the anterior horn of the squamosal extends to the 
middle of the large oval fosse, articulating with the postorbital. The fosse are 
fairly symmetrical. approximately 120 mm. in length. with a breadth of 75 mm. 

On the right-hand side the orbit is well preserved. except for its posterior 
border. The cavity has been largely set free from the cement-like matrix, which here 
contained molluscan fragments. It was carefully excavated in the hope that the 
characteristic sclerotic plates, possibly driven inwards, might be exposed, but 
these have entirely disappeared. It is evident that the orbit was the characteristic 
oval. Its vertical diameter at the periphery is 110 mm. Beneath the orbit, portions 
of the jugal can be seen, but the full extent of the zygomatic arch with its sutures 
cannot be outlined. The jugal appears first as a raised process near the midline of 
the anterior border of the orbit and then curves down to form its lower edge. 

A large supratemporal bone is present on the left-hand side, and its superior 
margin junctions with the lateral border of the squamoso-postorbital arcade. Much 
controversy has taken place over the “ additional temporal bone,” as S. W. Williston 
called it in the Ichthyosaurs, and the author has followed the nomenclature of 
Lydekker,? Sollas (loc. cit.) Gilmore, and Andrews in calling it supratemporal. 
Williston considered the inner bone of this “ Diapsid”? group the tabular and the 
outer the squamosal,> but in view of Watson's demonstrations it is surely better 
to reserve the name “squamosal ” for the more constant element. Perhaps the latero- 
temporal or ~ sclerodermal plate,” as Owen called it, is really a separated division 
of the quadratojugal. 

The quadratojugal is present, and its posterior portion is visible in the 
occipital region, where it forms the inferolateral border of the vacuity presented by 
the curved shaft of the quadrate. Its sutures with the supratemporal are obscure. 

Basr-OcciPrrAL.— The stout symmetrically convex condyle extends back- 
wards beyond the pterygoids for a distance of 33 mm.; the tranverse diameter is. 
74 mm., vertical diameter 64. 

Basis CRANIL— The suture between the basioecipital and the basisphenoid 
can be traced, giving a length of 60 mm. to the former bone. The basisphenoid 
is about 75 mm. in length, and forms with the posterior element a rectangular 
rostrum for the support of the pterygoids. The basisphenoid has a visible width 
of about 60. whilst the basioccipital is about 40 mm, Near the posterior margin 
of the basisphenoid the opening of the single canal for the carotic arteries can be 
clearly seen ; this foramen is circular and has a diameter of 10 mm. In the median 
line of the interpterygoid vacuities the splint-like parasphenoid may be seen, but this 
has been only partially freed from the matrix, compared with which it is very friable. 
This hone evidently increases in thickness towards its upper surface and is trian- 
gular in section. It can be traced anteriorly for a distance of 220 mm., where it is 

1R. Lydekker, Catal. Foss. Rept. Brit. Mus., Part II, 1889, p. 3. 

58. W. Williston, Phylogeny and Classification of Reptiles (Journ. Geo XXV), 1917, 
p. 416. 

6D. M. S. Watson, Ann. Mag. Nat. Hist. (8) XIV, 1914, pp. 84-95. 









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lost in matrix. The pterygoids provide the greater part of the base of the posterior 
moiety of the skull. On the left-hand side the pterygoid is in juxtaposition with 
and somewhat overlaps. owing to displacement, the basioccipital and basisphencid. 
The right pterygoid’ has been tilted below the plane of the flanges presented by the 
axial bones. From the basiszeipital to the lateral border the pterygoid attains a 
maximum breadth of 110 mm. 

Owing to the presence of superimposed hyoid rods, and a brecciated mass 
of broken teeth and matrix containing associated fossils, the central portion of the 
lower surface of the fossil is obscured. The extent of the interpterygoid vacuities 
cannot be gauged, but, judging from the converging inner margins of the bones, 
the contours in this area are similar to the skull of Ichthyosaurus longifrons as 
figured by Owen.” The palatine elements appear to be displaced and are not visible 
in the same plane. 

Posrerion REGION.—'Tlhe contours of the sunerior border of the oecipital 
region. formed by the parietals and processes frem the squamosals, are quite 
continuous, being convex in the median area and then sloping to lateral concavities. 
Viewed from above, the postero-external borders of the squamosal are seen to 
curve symmetrically backwards. and, although tae oceiput is somewhat disrupted, 
the contours are quite elegant. The inner process of the squamosal unites in an 
oblique suture with the lateral arm of the parietal near the median line of the 
supratemporal fossa. 

A large quadrate is present on each side. but owing to mandibular pressure 
these bones have been forced somewhat out of position. 

Above the region of the foramen magnum, only small fragments are visible 
in the matrix of elements which correspond to the superior occipitalia, which nave 
apparently been forced inwards. Possibly these are paired extensions of the 
supraoceipital which form part of the lateral borders of the foramen magnum. 

The opisthotics are in place on each side. and junction with the basioceipital. 
the ~ stapes,” and the squamosal. 


Next to the quadrate, the largest bone in the occipital region is the element 
called “stapes” by Sollas and Andrews. This acts as a strong lateral buttress of 
the basioceipital, and lies above the posterior flange of the pterygoid. It has an 
expanded facet for junction with the basioecipital, with an adjoining superior 
surface for association with the opisthotic. Cope,® who was the first to name this 
bone, did so with diffidence, and figured it as distinctly separated from the basi- 
occipital, whereas modern authors rightly show it as a buttress supporting the 
rostrum of the condyle. Owen? named it the paroccipital, but apparently only 

* Owen, Liassic Reptilia, Mon. Pal. Soc., 1881, Pl. XXV. 
* Cope, Proc. Amer. Assn. Ad. Se., 1871, p. 199, fig. 2. 
» Owen, Mon. loc er, pr 94. 


dealt with two pairs of occipital elements below the supraoccipital. C. W. Andrews 
notes that this bone (stapes) “seems to have lost its auditory function, !9 and it 
is obvious that this so-called stapes cannot be associated with the fenestra ovalis, 
as Cope supposed. The stapes is ususlly regarded as the homologue of the hyomandi- 
bulare of fishes, and a large stapes is recorded for the Cotylosauria. Case figures the 
stapes of Dimetrodon, a bone which in this and allied Permian reptiles is regarded 
by Broom*? as the tympanic. The writer is unable to find, however, a parallei in 
literature to the interpretation of this buttress bone of the Ichthyosaurs as a stapes. 

The columella auris of modern reptiles, the proximal end of which is 
presumably homologous with the stapes, is always placed antero-laterally to the 
basioccipital, and is quite distinct in position from this buttress bone. 

The writer suggests that these lower lateral elements in the occipital region, 
the so-ealled stapes, should be interpreted as inferior divisions of the exoccipitals. 
That the upper elements are true exoccipitals seems to be demonstrated by the 
position of the foramen for the post-auditory nerves. as clearly shown in Andrews’s 
illustrations (loc. cif.), and also by their relations to the foramen magnum. The 
unusual extension of the intermediate lateral occipitalia, the opisthoties, to the 
basioecipital, to which they also act as buttress bones, has probably brought about 
a division of the exoccipitals into upper and lower portions. 

These lower lateral elements may thus be interpretated as inferior divisions 
of the exoccipitals. This change in nomenclature, giving the exoccipitals a ventral 
extension, appears to be generally supported by the position of the occipital elements 
in the Permian Tetrapoda studied by von Huene”? and by R. Broom, and in the 
Stegocephalia illustrated by C. Wiman.* It is in consonance with the general 
arrangement of the bones in modern reptiles, where the exoccipitals are usually 
the lower lateral elements in juxtaposition with the basioccipital, the opisthoties 
uniting with them antero-superiorly in adult life (distinet in Chelonians); these 
relationships of the two elements are shown by Parker's studies of the development 
of the skull in the snake and the lizard.!9 ` It is not at variance with Howes and 
Swinnerton's interpretation of the development of the skull of Sphenodon.!* It agrees 
also with the positions given by Kingsley in his diagram of the schematic vertebrate 
skull.5 Huxley wrote that but for its large size he would have regarded the 
adjoining bone, now generally accepted as the opisthotie, as the stapes.!? 

19€, W. Andrews, Marine Rept. Oxford Clay, Brit. Mus., 1910, p. 11. 
uE. C. Case, Bull. Amer. Mus. Nat. Hist., XXVIII, 1910, p. 190. - 
1? R. Broom, Bull. Amer. Mus. Nat. Hist XXVIII, 1910, p. 223. 

13 von Huene, Bull. Amer. Mus. Nat. Hist., NN XII, 1913, pp. 315-386. 
4 R. Broom, Bull. Amer. Mus. Nat. -Hist. XX XII, 1913, p. 563, ete. 
15 €, Wiman, Bull. Geol. Inst. Upsala, XIII, 1915, Pt. 1. 

16 W, K. Parker, Phil. Trans. Roy. Soc., Vols. 169 and 170, 1878-79. 
17 Howes and Swinnerton, Trans. Zool. Soe., XVI, 1901. 

18 Kingsley, Outlines Comp. Anat. Vert., 2nd edit., p. 74. 

19 Huxley, Anatomy of Vertebrated Animals, 1871, p. 211. 



The opisthotie or paroceipital very rarely appears to meet the basi- 
occipital below the exoceipitals in other reptiles, although the relations between 
these elements are variable. The writer has diffidenee in using terms that are 
not accepted by leading authorities, but the occipital region of the Ichthyosaurs 
provides material for several interpretations, and the use of exoccipital for the 
lower element seems to solve the difficulty of à most anomalous * stapes.” 

In the modern cetaceans the stapes is frequently reduced to a small conical 
plug. and. judging from analogy, the auditory functions of the Ichthyosaurs would 
not have been greatly utilised. Possibly the real stapes is the “long slender 
process ” demonstrated in Section 494 of the very fine series in Sollas’ great work 
(loc. cit.). 

The stapes is often missing in fossils. The elaborate studies by D. M. $. 
Watson of the position of the fenestra ovalis in Therapsids, Seymouria, etc. 
(P.Z.S., 1914, and 1919) have an important bearing here, but the posterior 
aspect of the occipitalia in our specimen presents no evidence on this point. 

The massive architecture of the occipital region was evidently associated 
with the attachment of powerful nuchal muscles. Perhaps a specialist will one 
day. work out details of the probable musculature of the Ichthyosaurs on similar 
lines to the recent studies by Gregory and Camp on Cynognathus.?9 

Lower Jaw.—On the left hand side the lower jaw is practically complete,. 
except for the missing anterior segment and a small portion of the angulare. The 
dentary is very elongated and is no less than 875 mm. in maximum length. Parallel 
with the alveolar border, and situated about 20 mm. below it, is a groove which 
is shallow anteriorly, but then deepens, giving the characteristic conjoined gun- 
barrel effect of the Ichthyosaurian rostrum. The posterior process of the dentary, 
which overlaps the surangulare, runs back to below the mid-region of the supra- 
temporal fossa. Here the semi-spherical contours (in section) of the upper rod, or 
gun-barrel-like process, sink into the same plane as the surangulare and angulare. 

The left angulare is not quite complete at its posterior end, and here its 
outer contours have been disturhed. It is a longer and more massive bone than the 
surangulare, but just at the termination of the dentary the two posterior elements * 
are of equal depth. In this region the arrangement of the bones is very similar to 
the outer view given by C. W. Andrews for Ophthalmosaurus in Text-fig. 20 (loc. cit.), 
except that the angulare is distinetly extended to form the posterior portion of the 
mandible. Strong depressor muscles were evidently attached here, working with 
short leverage in association with the powerful levators placed in front of the 
articulation. Sollas points out that the levator muscles originating in the temporal 
region, inserted on the lower jaw, acting as levers of the third order, were “admirably 
adapted for snapping ; and the Ichthyosanrus, from all that we know of it, must have 
obtained its food by seizing fish “upon the wing. " 

20 Gregory and Camp, Bull. Amer. Mus. Nat. Hist. XXXVIIL 1918, pp. 447-563. 



Fig. 1.—Skull of IcnTHYOSAURUS AUSTRALIS. Posterior view. Approximately one-third 
natural size. 

Fig. 2.—Anterior section of rostrum, IcuTIHYOSAURUS AUSTRALIS, showing disrupted teeth. 

: Approximately one-half natural size. 
Face page 253.