platypterygius australis, an australian cretaceous ichthyosaur

Platypterygius australis, an Australian Cretaceous ichthyosaur MARY WADE

Wade, Mary 198404 15: Platyptervgius uustralis, an Australian cretaceous ichthyosaur. Lethuia, Vol. 17,

Sufficient material has been assembled to restore the powerful pectoral girdle and large fore limbs of Platypterygiu~ australis (M'Coy). A reasonable approximation to the line of action of dorsal and ventral muscles which principally affected the trim of the low-set fin-blades was along the middle of the fin. Increased tension brought the fin-blades nearer horizontal (to a diving position) and varying amounts of relaxation during forward movement allowed mainly water resistance lo increase the tilt. The tlexible blade edges were crucial in this. Unequal tension on the blades would have caused turning of the animal with whatever rapidity was desired. Allowing both fin-blades to rise together to stalling point could have checked the animal abruptly as it struck prey. Large relative and absolute increases in the portions of the coracoids adjacent to the median symphysis highlight the relative development of young individuals. Positive allometric growth in this area stopped quite suddenly as the animals approached 6 m total length, leaving only general size increase. 0 fchthyosauriu, Platypterygius, Cretaceous, longipinnate. morphology, swimming abilitv.

Mary Wade, Queensland Museum, Fortitude Vullry, Queenslrrnd 4006, Awtruliu; 5th August, I Y82 (revised 1983 06 03).

LJETHAIA pp. 99-113. Oslo. ISSN 0024-116.2.

The first discovery of ichthyosaur bones in Aus- tralia was made by Mr. James Sutherland in 1865, on the headwaters of the Flinders river near O'Connell Creek. H e transmitted the piece, described as 'several vertebrae', to Professor M'Coy, for the National Museum of Victoria. M'Coy (1867) described them as Ichthyosaurus australis, but gave little data and no illustration. Meanwhile Sutherland had renewed his search at the same locality and located a flattened skull with adherent atlasiaxis, 32 cervical and thorassic vertebrae, and two pairs of larger vertebrae with single rib condyles, from the base of the tail. These last are approximately the same size as the holotype vertebrae. Though they are less crushed than the anterior vertebrae, they are crushed in the same sense, all are higher than wide. M'Coy described the holotype as '4 inches wide, 3 inches deep and I f inch long'. Sutherland (letter dated 5th November, 1866) gave his new discoveries the same field number as he had given the holotype piece, 48. He considered them 'probably the same organization' as the holotype 'five vertebral joints'. He allotted the field number 60 to other specimens 'likewise found together' at another locality. M'Coy (1869) published on the new material in such a style that it is impossible to discover that it contained two skulls. Freder- ick Chapman (1914) illustrated the more frag-

7 - Lethaia U84

mental skull, no. 60, and a fin fragment that has no collection number now, in company with the statement that I . austratis 'is typically represented by a nearly complete specimen'. This directly or indirectly led McGowan (lY72a) to re-illustrate them as the holotype of Platypterygius australis. The actual holotype has not yet been relocated but the quasi-holotype material at the National Museum of Victoria, still numbered 48, is registered: P 12989 (skull) and P 12991, P22653-4, P2265G61 (vertebral column pieces). P 12990 and P 12991 are the second skull, no. 60, and the fin fragment.

Etheridge (1888) described a snout fragment from Marathon Station as Ichthyosaurus rnura- thonensis, but McGowan (1972a) placed this in synonymy with P. australis (M'Coy) because adequate material had become available to prove them conspecific. This snout, too, is not a diagnostic fragment. It is registered F 1448. There is only one species known from the basin, so the present absence of the undiagnostic holotype causes no immediate practical problems.

Register numbers are those of the Queensland Museum, Brisbane (F551 etc.); National Muse- um of Victoria, Melbourne (P12989 etc.); and British Museum (Natural History), London (R 1664 etc.).

As Appleby (1979) foreshadowed a general

100 Mary Wade LETHAIA 17 (1984)

review of longipinnates, this preliminary paper will be confined to the morphology of the material at hand and to what it indicates about the swimming ability of the animals.

Genus Platypterygius McGowan (1972a) indicated that Platyptervgius von Huene has fore fin-blades broadened by multiple accessory digits on the leading and trailing edges of basically longipinnate fore limbs, and a pisiform bone socketed into the corner of the humerus next to the ulna. 30 th the proximal pisiform or sesamoid bones are in fact alongside the radius and ulna, so that the fore-arm is four bones wide. The sesamoid bone next to the ulna is usually the only one that is socketed into the humerus in P . australis (see below). A comparable degree of widening and strengthening has been independently achieved in some latipinnates, by the insertion of the intermedium between the radius and ulna, and the placement of a sesamoid bone as an accessory to the ulna, e.g. Brachypterygius.

In Platypterygius the so-called ‘pisiform’ bones, which developed or moved into position each side of the radius and ulna, provided bases for the extra accessory digits. Both trochanteric ridges slant posterodistally toward the sides of the ulna, as Appleby drew to my notice. The anterior humeral edge is angular. McGowan (1972a, figs. 7-10, pl. 1) supposed opposite orientation of ulna, radius and radial accessory. There are so many bones in the fin-blade which could receive the genetic description ‘pisiform’ or ‘sesamoid’ that a nomenclature based on location is employed below for the accessory sesamoid bones.

The radial accessory is rather large and has a characteristic, proximally tapered flask-shape (McGowan 1972a). The distal end of the humerus is not exceptionally wide, and generally only one accessory bone can fit against its end, alongside the radius or ulna. Whichever side bears the accessory facet, the radial or ulnar facet is cut

square by the smaller facet occupying the outer angle of the humerus. Thus a squarish ulnar facet on the Aptian type species P. platydactvlus (Broili 1907, PI. 13:15, 16, see also text figs. 5-8) indicates that it is the progenitor of P. australis, Albian, which had more developed facets. The presence of a radial accessory facet and related squarish radial facet on P. americanus shows that it developed from a form in which the ulnar accessory had not contacted the humerus. Both the ulnar and radial facets would have been roughly triangular, and since it appears to have been equally easy for the radial or ulnar accessory to attach to the humerus, the fore-arm should be assumed to have been four bones wide. All the species attributed to the genus still lack full description, partly because material is wanting, and partly because preparation from hard matri- ces takes a long time. For the present, the anterior and posterior sesamoid bones alongside the radius and ulna (the four-bone wide fore-arm) and the 9-10 digits will suffice to diagnose the genus Platypterygius. The position of attachment of a sesamoid bone to the humerus, or even the lack of an obvious facet, will serve to differenti- ate species.

The question of how speciation occurred in wide-ranging marine animals seems, in this example, to be answered by a probable intrinsic limitation on dispersal ability (p. 112) and the fact that the numerous P. australis inhabited an inland sea like a huge Hudson’s Bay that covered a fifth of Australia (Burger 1982). The fauna in general was very rich in fish, Inoceramus and ammonites, and indicative of a temperate climate as it lacked larger foraminifera, corals and rudis- tids. Australia was 25” nearer to Antarctica, Ir- ving (1964) calculated 56”s palaeolatitude for the richest area (Day 1969), but recent discoveries indicate it was likely to have been 6tL7O”S Embleton (pers. corn.). P. australis died, or was driven out, as a result of the filling of the basin to above sea-level at approximately the end of the Albian. Whether it ever spread from the mouth of the embayment is not clear, for Australian

Fig. 1 . Plutypterygius uustralis (M‘Coy). U A. B. Telemon specimen. FZ453, scale in A 30 cm. in B 1 0 cm. €3. The anterior 2 : teeth on the upper jaw arc on the premaxilla, the remainder arc on the maxilla. The premaxilla forks terminally around the maxillary foramen in front of the naris. The nasal-maxillary suture was damaged during acid preparation and appears as a narrow gap anterior and posterior to the naris. Jugal. lachrymal and pre-orbital extend anteriorly as three tongues of bone. The left side of the median suture is intact, the right is damaged. Thc bony floor to the depressed area in front of the pineal foramen (white pointer in B ) shows a straight median suture although the lighting was unsuccessful in showing the median frontal depression at the same time. OC. Galah Creek specimen, F551, scale 10 em. Specimen is dorsoventrally crushed, showing pineal foramen and frontal depression floored by bone. There is an anterior lateral break in the floor here but not in the Telemon specimen. Arrows indicate the nares; p = pineal foramen; d =frontal depression.

LETHAIA 17 (1984) Cretaceous ichthyosaur 101


Fig. 2. P. australis, scale bars 10 cm. OA. Boree Park specimen, F10686, right fin-blade, anterior to right, trailing edge to left, unaligned sesamoid bones at distal rear and tip. Very wide primary digits attain normal dimensions after an extra digit is intercalated between the ulnare digit and intermedial digit. 0 B. Telemon specimen, F2453. Right limb, dorsal view showing truncated base of dorsal trochanter, and ulnar accessory bone faceted to humerus. Bones in restored positions are not shown. 0 C.


ichthyosaur remains described from outside the Great Artesian Basin are very scanty (Teichert & Matheson 1944), and diagnostic bones are still lacking from material outside the basin.

The material In the Albian Toolebuc Formation and Allaru Mudstone, articulated skeletons are reasonably common. Four specimens at various stages of preparation and completeness form the basis of this study, together with the skull from Galah Creek figured by Longman (1922); this has been further prepared. There are also numbers of fragmentary specimens, including two juveniles of about 2 f and 3 m from just upstream of the Big Hole, Flinders River near Julia Creek. A shattered smaller specimen exists.

The size range of full length heads is 61 cm to 143 5 2 cm, but the larger is the Telemon specimen, and this was by no means the largest ichthyosaur of its species.

The Telemon ichthyosaur (Figs. l A , B, ?B), F2453 (Longman 1935, 1943; McGowan 1972a) is the nearest to complete, though broken in two before burial. It lacks all but one of its tail-fin vertebrae, the pelvic girdle and hind limbs, and parts of the pectoral girdle and ribs. The right fore limb was nearly complete from humerus to the start of the digits, but a contemporary photograph shows scattering had started during collection, and afterwards it was arranged as a five-fin- gered hand. Even the most recent re-assembly is not perfect; only the articulated bones are un- equivocally placed. The left humerus is excellent but the whole fin-blade is scattered and lost. This specimen was only moderately large for its species. Its length measurements here include 10 cm restored to the tip of the snout after comparison with other specimens. Head, 1.43 m; fused atlas/ axis to the end of vertebrae with double rib- condyles, 2.16 m; single headed ribs 1.37 m: total known length 4.96 m plus tail fin; estimated length 5.60 m.

The Kilterry ichthyosaur (F 12314) was larger in practically every comparable bone, and was probably about 7 m. Parts of it are on loan from

the Lord family of Kilterry, its discoverers, and parts were collected by Queensland Museum and University of Queensland workers.

The half-complete Stewart Park ichthyosaur (F3348) was an intermediate size. This has a fine proximal half of the right fore limb, but it lacks two rows of accessory digits from the ulnar side, like the holotype of P. platydactylus, it appeared at first glance to have had only eight digits [pro- gressive stages of preparation are recorded by McGowan (1972a, b) and in this paper Fig. 2C]. It was twisted to lie against the ribs, the trailing edge of the fin-blade is upward and the leading edge is downward. The characteristic flask- shaped radial accessory bone and equally characteristic, thumb-shaped ulnar accessory bone pro- vide evidence of this orientation. The ventral surface is thus the one pressed against the ribs, and the dorsal surface has been exposed. The left side of the body was removed by penecontem- poraneous erosion, so no confusion between the limbs is possible. The ridge supporting the main trochanter is exposed; it is therefore the pectora- lis ridge, as Andrews (1910) indicated. The proximal 2/3 of the humerus is flattened diagonally. As the head of the humerus is twisted a little more than 30” in this species, this flattening has crushed the posterodorsal surface of the humerus and the posterior side of the trochanter into one rumpled face, attenuating the trochanter and bending it across the anterodorsal surface of the humerus. On the diagonally opposite side, the shorter ventral trochanter is crushed and hidden by several ribs. The distal end of the humerus was spared the crushing; it carries the distal tip of the dorsal trochanter which ended a short distance from the centre of the side of the ulnar facet. The fin-blade rotated slightly on a longitudinal axis through the ulna, and the ulnar accessory bone and the radius are dislocated. The radial accessory bone had no facet. Slight bosses occur on each side lateral to the radial facet, and its tip has been bent slightly inward and distally. Fig. 2C shows, in addition, the present relation of fore limb, scapula, clavicle, ribs and vertebrae. The vertebral column lies below (depositionally above) the ribs at the top of the figure. McGowan’s orientation of this fin

Stewart Park specimen, F3348, vertebrae with articulated neural arches at top, rib cage depositionally overlies right clavicle, scapula and fore limb; humerus crushed proximally; dorsal trochanter squashed thin, stretched, and bent over lower (anterior) side of humerus; tip of radial facet, bent inwards near anterior edge, is out of sight behind boss at side of radial facet. Upper (posterior) edge of fin-blade broken off. Two radial accessory digits arose from radial accessory bone at the first carpal row but the outer accessory carpal and digits are lost. The ulnar accessory facet makes a jagged-looking edge to the humerus. There are only three ulnar accessory digits, as in the Boree Park fin-blade, A .

104 Mary Wade LETHALA 17 (1984)

is correct (McGowan 1972b). He was not correct in describing the Telemon right fore limb as viewed from the dorsal side. The trochanter he figured (McGowan 1972a, PI. 4:C) is the ventral trochanter, which is quite a well-developed knob, but not nearly as strong as the dorsal trochanter subsequently exposed in a truncated state (Fig. 2B). The Boree Park ichthyosaur ( F 10686) is largely complete but considerably disarticulated. It suffered extreme flattening parallel to the bedding plane, but appears to have been little larger than the Telemon specimen, although more mature (Johnson 1977).

A small scatter of bones from Brixton (F2299) included a beautifully preserved coracoid symphysis and right humerus (Fig. 3A-I). The humerus matches in size the Telemon specimen but the ulnar facet is particularly large, and the ulnar accessory facet has no outer side but indents the end and posterior side of the humerus.

The radial accessory of P . australis just reached the humerus in most specimens, but, in a fin fragment F3389, the humerus had fine holes in it directed toward the radial accessory, in the same way others are directed to the radius, ulna and ulnar accessory. In fin fragment F2573, which has a massive humerus, the radial accessory has a small facet on the humerus, as well as the larger facets of the radius, ulna and ulnar accessory.

The head The maxilla of Platypterygius is superficially more covered by adjacent bones than that of Mixosaurus, and less covered than any other form to which Dechaseaux (1955) drew atten- tion, o r which is adequately illustrated in the literature to hand.

In Platypterygius the maxilla carried half the upper tooth row, approximately 26 teeth. It is not so much reduced as submerged, for in all its sutures but those that are longitudinal in direction - those with the nasal - it is the lower bone in the characteristic, ichthyosaurian overlapping wedge sutures. Though it is the bone which supplied the greatest thickness around all of the external naris except the roof (supplied by the nasal bone), the premaxilla, jugal, lachrymal and prefrontal all extend over the maxilla, towards or below the naris (Fig. 1B). The large maxillary foramen in front of the naris faces approximately into the lateral groove of the premaxilla. The premaxilla forks to pass each side of the fora-

men. The upper fork braces the nasal-maxillary suture, but stops short of the naris, the lower fork extends across toward the naris or below it toward the lachrymal. They have not been observed to meet, but in a position where the deposition or erosion of 1 mm of bone may move a suture 2 or 3 cm, there are no hard and fast rules. In one example the prefrontal does cover the nasal-maxillary suture as far as the edge of the naris; in the others the maxilla is the surface bone as well as the strong surround. The differ- ences in the maxillary region between P. australis and Romer’s reconstruction (Romer 1968, Fig. 4) of P. americanus (Nace) = ‘Myoptervgius’, seem explained in terms of crushing by Romer’s own Fig. 3 drawings. The jugal is broken, and probably lapped over the maxilla and not vice versa. The two tiny bones anterior to the naris are probably not internal septo-maxilla above lachrymal but both fragments of the maxilla, and the gash extending between them from the naris to a rentrant in the rear wedge of the premaxilla is probably a break joining naris and maxillary foramen. The naris was presumably bean-shaped like P. australis.

Owen (1881) showed a deep narrow depression along the median suture between the frontal bones of Ichthyosaurus longifrons and I. ‘latifrons’ (although in ‘latifrons’ the distal edge of the nasals was just covering the frontals). This is just in front of the pineal opening through the frontals and the underlapping parietals. Long- man (1922) noted a large depression in this region of Galah Creek P. australis (F551, Fig. 1C) but could not clear it without damage to his specimen. Since acid treatment, this and the Te- lemon skull (Fig. 1B) show an elongate depression along th6 suture, floored by thin bone which is part of the frontals, though the nasals overlap the frontals a t the anterior of the depression. McGowan (1973a) discovered an ‘inter-nasal foramen’ just in advance of this in Ichthyosaurus when he acid-prepared several specimens. Al- though in internal view this foramen extends back to separate the proximal edges of the frontals, it is mostly in the nasals and wholly so dorsally.

It seems too much of a coincidence that so many somewhat similar structures should focus on this area. Also, the nostrils have never moved from the sides of the head, though the snout developed greatly. It seems likely that the centre of the head, between the front of the eyes, was pre-empted by another organ; perhaps one for


Fig. 3. P. australis. Brixton specimen, F22Y9, ~ 0 . 3 (scale 10 cm). OA-D. Coracoid symphysis. (A, ventral side; B, dorsal view: C, profile; D, symphysial face.) 13 E-I. Right humerus; in this specimen the ulnar accessory facet is displaced to the side of the humerus (see E, G, I). E. Dorsal view. 0 F. Ventral view. 0 G. Posterior (at hcad, twist of head is over 30"). 0 H. Head end. 0 I. Distal end, same orientation, except for inevitable left-right inversion.

echo-location, as in whales and dolphins (whose nostrih, on this line of reasoning, should have moved first, and were not limited by external development of the pineal organ). The frontals are relatively wide, reaching midway across the front of the temporal fenestrae behind the na-

sals. Their rear suture with the parietals is tapered but differs from all other sutures in having an interdigitated overlap of each bone on the other, though mostly frontal over parietal. The parietal foramen passes through the overlap of this suture, and so provides a reason why immov-

106 Mary Wade LETHAIA 17 (1984) -- . I

,/‘

A

Fig. 4. Platypterygius coracoids. 0 A. Juvenile P. australrs F12317. OB. Large juvenile, P . plarydacfylus holotype, after Broili. 0 C. Adult P. ausrralis F 10686; the shoulder supports have been foreshortened by the crushing that flattened thc bone, and the anterior notch is reduced, possibly as much as 50%. All three specimens were flattened. Stipple indicates once-vertical facet surfaces.

ability was needed, if more reason is needed than that it is the roof of the cranium. The unwelded sutures around the nasals, in contrast, suggest that the skull was somewhat kinetic. Frequent breakage shows it was the weakest area in Pla- typterygius skulls.

The supraoccipital has not been freed of ma- trix, but a narrow arch such as the one Baptano- don had is visible behind and under the parietals of the Telemon specimen (Fig. 1B).

The Galah Creek skull is nearly complete and relatively deeply etched. It is the only one in which the parasphenoid process is visible. This is laterally flattened and widens only slightly a t its junction with the basisphenoid. Twenty-three cm length is now exposed. Assuming it to extend almost to the change in taper of the snout, possibly as much as 10 cm are yet to be exposed, but the teeth fell out before burial, and the limestone between the lower jaws is packed with loose ichthyosaur teeth which would have to be sacri- ficed, to observe its full length.

Coracoid size and shape The larger of the two juvenile specimens (F 12317) consists of a broken humerus, scapula, articulated coracoids, eight articulated vertebrae complete with neural arches, and scraps of ribs. The coracoids seem remarkably small for the size of the other bones, but this is due to a difference in their proportions from adult coracoids: their glenoid and scapular facets are naturally in pro-

portion to their humerus and scapula. These are just half the size of their Telemon specimen equivalents. The symphyseal portion is very narrow compared with adult proportions (Fig. 4). The vertebrae are squashed oval with the long axis (height) 5.4 crn, while Telemon vertebrae in this region are 9 cm high and a little wider. The only anterior Boree Park vertebra is 8.8 cm high and wide but its original position is unknown. An external mould of a 61 cm head (F12313) was associated with a fore limb the humerus of which is a little smaller than the broken humerus of F 12317. Compared to Telemon’s 5.6 m length, 3 m seems reasonable for the juvenile owner of the coracoids, F 12317, and 2.5 m for F 12313, head mould, fore limb and moulds of vertebrae.

The distance from coracoid symphysis to the base of the anterior notch is only 1.7 cm in the more complete juvenile coracoid; 1.6 cm in its less complete partner. The distance from the symphysis to the angle between the glenoid and scapular facets is 5.7 cm. The respective measurements of Boree Park coracoid are 1 7 2 1 cm but the glenoid and scapular facets are foreshortened by crushing. The incomplete fragmental but uncrushed Telemon coracoids consist of one glenoid facet and the other symphyseal sector. They fitted just within the Boree Park coracoid outlines, but the notch is not preserved. The Kilterry ( F 12314) coracoid is 23 cm wide but the portion bearing the notch is not in any boulder yet etched. Its varying thickness suggests the proportions resembled Boree Park coracoid.

The P. platydactylus coracoid (Broili 1907: 149, Fig. 4) was just over twice the total width of F 12317, and its anterior notch was just over half its breadth. Broili estimated the animal was over 5 m long, and, given the same tail proportions as the present reconstruction of the 5.60 m Tele- mon specimen, it would have been about 5.20 m; the Boree Park specimen was about 6 m.

Fig. 4 shows the three flattened but nearly complete Platypterygius coracoids in outline. They represent three lengths of animal and form a corresponding morphologic series in which the width of the sector adjacent to the median symphysis increases more rapidly than overall length, and the size of the anterior notch is practically at a stand-still. P. platydactylus (after Broili) has a slightly larger absolute size of its notch. This may prove to be a species character, or prove to lie within the range of individual variation of P. australis. Combined with its intermediate length, the intermediate coracoid proportions of P. pla-

LETHALA 17 (1984) Cretaceous ichthyosaur 107

tydactylus strongly indicate that the holotype was a large juvenile.

Baby Platypterygius would have had to be born at a minute size if they had had the adult coracoid shape. The fossil ichthyosaurs enclosing embryos have a high proportion of large embryos for the size of the mother (see Hofmann 1958). That is in keeping with the assumption that they represent birth fatalities. Even though the sam- ple is assumed to be prejudiced, it does not seem that normal young were particularly small; so once differing shapes of young and adult coracoids are established for a group, an extreme change of proportions in the coracoids during growth is a useful tool for determining the relative age of pre-adult individuals. It is not yet observed in groups with small adult size, but differing allometric patterns are known in at least three larger-size ichthyosaur genera.

The length of about 6 m for young adults of Platypterygius to some extent confirms the obser- vations of Johnson (1977), who gave evidence of larger amounts of cartilage in younger adult material than older. The 5.60 m Telemon specimen had rougher cartilage-bearing surfaces than any other specimen we have, and its phalanges are relatively small. Before obtaining Johnson’s paper, I had described the proximal trailing edge of its fin as having more cartilage around the bones than any other. The ‘fine sandpaper’ texture she described on small limbs seems in P. australis to be due to a thin film of lamellar bone over spongiosa; the lamellar layer thickens with age and size and the texture (partly transparency) is lost.

Fore limb morphology and function The radial and ulnar accessories, like the humerus, have thin lamellar bone all over their outside surfaces. The outer phalanges, oval to pisiform in shape, have rough vertical edges and raised rims. Two large, very thin bones were distal to the (absent) radial accessory in a photo taken during collection of the Telemon specimen right fore limb. They have rough upper and lower surfaces and all edges rough. They seem to have been completely buried in cartilage, though their arrangement cannot be confirmed from the col- lecting photograph. The equivalent bones in Stewart Park right fore limb are more regular, thicker, and smooth-surfaced like normal phalanges. As every worker seems to note, limbs differ in minor details. The largest preserved fin-

blade, that of Boree Park, F10686, is the only known specimen with an extra digit inserted part way along the blade. It is also broken off through, or just below, the wrist (Fig. 2A) and all the upper section is missing. The left fore limb is compressed side on and its humerus is flattened too. A flat coracoid and scapula may be the left side of the girdle, as the right humerus is missing, but as they are squashed to a maximum of 2 cm thick (about 1/5 normal) and flat instead of curved, which side is up is a mystery. The spongiosa, which presumably was filled with mud, is crushed to featureless bone and mud powder, as has probably been the case with every ichthyosaur body bone described as ‘massive’ in internal texture. The most satisfactory way to interpret the right fin-blade seems to be to count a transect seven rows of phalanges from the shat- tering and displacement of (and near) its proximal edge. The digits recurve from the stronger edge (Fig. 2A), and a strong leading edge is a hydrodynamic requirement, so the assumption is made that that is the leading edge: the total is three radial accessory digits, three primary digits and four ulnar accessory digits. The primary digit presumed to arise from the ulnare divides in two shortly below this, and despite two attempts to re-unite, stays as two digits. A scale reconstruction without it resulted in a sag in the leading edge, so it is possible that the structure of this fin-blade is not abnormal. The divarication oc- curs beyond the break off of any known partial blade. As all fin-blades do, F10686 thins outward from a low plateau formed by the three primary digits and first radial accessory digit. This ‘plateau’ area is only 3-3 f cm thick, so the edges and tips are very thin indeed; the trailing edge thins even more than the leading edge where, as far as F10686 shows, the last (third) row of phalanges is equal in size to the third row on the trailing edge. The roughened vertical edges of all the oval and round bones suggest that they were cartilage-surrounded in life, that is, that both sides of the fin-blade terminated in cartilage flanges as did the hind fin-blade figured by Owen (1881:28, Fig. 5). As all digits taper toward their tips, the bony rear edge of the fin- blade would curve if a mass of tightly packed small sesamoid bones, not arranged in digits, did not occur there. They seem to have kept the bony trailing edge practically straight. The leading edge, after widening for a few phalanges outward from the wrist, curves gracefully back, cutting off digit after digit until all are passed by

108 Mary Wade LETHAIA 17 (1YX.l)

Fig. 5. P. uuslralis. Restored after Tclemon specimen and others. Length over all 5.6 m. Drawn by Rohcrt Allen, Queenslatid Museum

and the tip is formed by a single small sesamoid bone - and an unknown amount of cartilage. The huge fin-blades (the broken length of the blade bones is 85 cm) were as thin as those of a large shark, though stiffer because they included bones.

The median coracoid symphysis is deep and slightly curved so that it cannot slip (Fig. 3A). The bone becomes thinner and longer away from the symphysis, and thickens again by extension upward and downward to make a diamond shaped glenoid facet with its long axis horizontal and short axis vertical. It is deeply to very deeply sculptured to carry cartilage, and apparently this cupped the head of the humerus, in its cartilage sheath, in a straight-out position almost at right angles to the coracoid symphysis. The anterior of the glenoid facet of the Kilterry coracoid is 23 cm from the symphysis and the posterior 22 cm.

Fig. 6 shows sections through a humerus (p. 104, F2573). They demonstrate the narrowly rounded anterior edge, broadly rounded posteri- or edge, and the lateral bulges produced by trochanters which trend distally toward the middle of the ulnar facet. Since the posterior side of the fin-blade was the wider (Fig. 3A), this trend was toward the middle of the fin-blade. The ventral trochanter is broadly triangular. The base of the triangle was the edge of the cartilage-bearing cap

of the humerus. This cap dips further down the bone at the other three corners. Even in a diving position the limbs must have retained a slight upward setting or they would have been uncon- trollable at speed. With lift diminished, the corn- bination of downward drive from the tail and heavy-boned head would have allowed the animal to dive at any angle. The effect of the dense head bones is underlined by the hydrodynamic behaviour of ichthyosaur corpses such as Boree Park F 10686, which hit the sea-floor vertically. The snout rammed 60 cm into shelly limey mud and anchored it.

In this very slightly up-tilted diving position, relaxation of tension would have allowed the slight uplift under the fin-blades to tilt up the leading edge cartilages (if they were not already tilted). This would have increased resistance and helped to rotate the limbs back to the angle that brought them into balance with the new tension. Differing tension would have turned the animal toward the side with more drag (Fig. 5) while the dorsal fin (McGowan 1973b) tended to stop rolling and veering.

The animals would have opened their mouths economically when taking prey: a wide gape takes extra time to open and close, and may let prey escape; also aquatic animals without gill openings try to keep the gape small during move-

LETHAIA 17 (1984)

ment because mouths are unsatisfactory as brakes. The fore limbs of P. australis were actually available as brakes, which would work more abruptly the faster the animal moved, being lift- ed by the resistance of the water. It could have stall-braked as it struck, or, if it wished to swal- low in air, have converted its movement to an upward sweep.

Cretaceous ichthyosaur 109

Restoration It is still necessary to restore the tail-fin size from Broili's data on P. platydactylus, as only scattered tail vertebrae are known. Broili (1907) recorded the mounted dimensions on p. 139 'Die Lange des Skeletts . . . betragt 4.74 m'. The whole animal, he concluded (Broili 1907: 139, 159), was over 5 m. He also concluded that a t most about 20 vertebrae were missing at the top of his string of tail-fin vertebrae. An approxi- mate estimate of the length missing there can be obtained by constructing the best possible tan- gents to the upper and lower surfaces of the tail- fin vertebrae on his plate 12, and cutting off the resultant cone at the diameter of the last of the body vertebrae. This distance would accommo- date another 20 vertebrae no larger than the largest five preserved, so it seems likely that about 18 vertebrae were actually missing in the gap. Measurement suggests 67 cm for these parts of the fin. At the tail end his Fig. 3 shows a cotton-reel shaped terminal vertebra that hints the absence of one to a few more. Together, it seems the most likely estimate of length for the tail-fin vertebrae was 70 cm or a little more. The tail height (about 1.20 m for a 50" downturn) is small for a 5 m beast, as Broili (1907) and McGowan (1972b) both said of their smaller esti- mates. Appleby (1979:937) has shown that the rib-condyles rise toward the tail bend in latipinnates; and though there is less evidence in Pla- typterygius - the preservation is inadequate - they do rise to an unknown extent. This indicates stronger hypaxial musculature here too.

The head of the Telemon specimen was 1/4 of the total length, and, despite a good deal of very finely porous spongiosa, had much more lamellar bone than the body bones. Potential air-holding space seems small, particularly when the tongue is considered. Although all the body bones are almost completely made of coarse spongiosa, the large coracoids, humeri and fins provided another anterior concentration of weight, low in the body. The centre of gravity must have been

Fig. 6. P . uwtrulir. Lydia Downs F2573, left humerus, antero- poster0 plane in dashed lines. Transverse section at AB through the tips of the cartilage cap, CD at waist, EF at distal expansion. Hachurcd band crossing AB, projection of proximal crest, bisected by longitudinal axis of head.

well forward, close below the centre of buoyancy located between the lungs, approximately at maximum body width.

The restoration (Fig. 5 ) is scaled to a length of 5.60 m along the mid line. Basic dimensions are those of the young adult Telemon specimen (p. 103). The curve of the ribs, compounded with their high-set pectoral vertebral apophyses, indicates a width of about 80 cm a short distance below the backbone; rib length (Stewart Park and Boree Park specimens) suggests a body height of 80 cm above the coracoids. Coracoid- humerus width approached 90 cm at the proximal end of the fin-blades (Kilterry and Telemon specimens). The occipital condyle has its articu- lar surface perpendicular to the length of the skull (Galah Creek specimen) but the basioccipi- tal was most likely rotated a little by the dorsoventral squashing. The skull widths are 45 cm at the widest (Telemon specimen, rear of lower jaw and jugal arch), narrowing slightly upward to the rim of the temporal fossa. The animal doubled its height and breadth from the back of the head to the pectoral region, reached its broadest not far behind the pectoral fins, and tapered to the tail. The shape restored to these dimensions is streamlined at every level, and is adequate to the needs of an excellent swimmer and diver.

There are many things that affect underwater


speed. P. australis had excellent streamlining: the skin texture to accompany it can scarcely have caused unusual drag. The amplitude of swing and the rate of beat are roughly inverse, and as the length of the tail including fin is about 2 m, it is likely the amplitude was high and the rate merely average. As far as can be seen the tail was moderately small. Its proportions are really unknown, for P. platydactylus was itself restored in this area and the angle of tail bend at rest is not known. After Appleby (1979), the bulk of the hypaxial muscles in comparison with the epaxial muscles indicates the ability to vary the tail bend at will. The lower lobe is provided with most or all the musculature. The ichthyosaur tail fin has been described as if it had had an inert upper lobe (McGowan 1973b) presumably because dolphin tail fins are layered collagen but it can also reasonably be restored with some musculature. It was developed as an improvement to a functioning muscular tail fin from between paired lateral muscles, and control of its motility must always have been a high priority requirement. All told, this is probably not an example of a small tail fin indicating a poor swimmer, but of the right size being big enough.

The negative pitch generated by the tail fin would not alone have been enough to support the heavy head end from the tail region, because Platypterygius backbones were relatively supple (p. 111), even in a dorsoventral direction. The widespread limbs gave more adequate support to the long body and heavy front end.

The question of whether the fore limbs were narrowly (McGowan 1972b) or broadly attached to the body cannot be answered objectively. The bony pectoral fin-blade was about 3~ longer than McGowan assumed, and wider. A broad cartilage on the whole trailing edge seems likely, and would also be the most likely filling for the irregular edge of radial accessory digit 3 (other than small pisiform bones which were not there to be observed in F10686). The sides of the humerus and radial and ulnar accessories were free of cartilage, but, if we consider the carbon film around the bony fore limbs of Stenoptery- gius, that becomes noticeably paler and thinner toward the arm, and in this form too, these bones were smooth-surfaced and laterally free of cartilage. The shape of the bony fin-tips, fingers separate in Stenopterygius and together in Platyptery- gius, suggests a pointed fin-blade such as Owen (1881, PI. 285) attributed to ‘Ichthyosaurus communis?’. The longipinnate R 4550 (Andrews 1924)

had one ulnar accessory digit, and, like R1664, broad attachment and ‘fringing fins’ around the bones. Narrow attachment must be assumed to be a primitive (generalized reptilian) character that possibly persisted in the Mixosauroidea and perhaps even in unspecialized longipinnates. Pla- typterygius is specialized in parallel with the broad-based latipinnates, and, like the other longipinnates with accessory digits, appears to have had cartilaginous fin-edges. The balance is in favour of a broad contact with the body.

The trim of the limbs must have been critical for well-directed movement at speed (Fig. 5 ) . Paddling with the known fin-blades would upset the trim far more than any possible advantage if the animal was driving forward with its tail; on the other hand few swimmers of any kind fail to try out every possibility for movement if they are idling. Fish that propel themselves over distance or at speed with their tails alone will habitually hold station and paddle short distances with pectoral fins instead; it is not aquarium-induced behaviour. Watching through a glass bucket at an open-sea reef where many kinds of fish were customarily fed, I was surprised how universal is the change from tail movement to fin movement and vice versa. It is reminiscent of the well- known change a cuttlefish makes from jet pro- pulsion to fin movement, and may well explain McGowan’s observation of Amazonian dolphins paddling with their narrow necked fore limbs (McGowan 197%). Broad or narrow-based, the Platypterygius fore limbs probably came into use if it wished to stop and look, or laze at the surface; indeed, any time it wanted to keep sta- tionary, e.g. in sleep, balance if not elevation must have been maintained by fin movement, the tail was inescapably propulsive.

The muscle casts Andrews (1924) described and figured from near the tip of an ichthyosaur fin- blade R 4550 were transverse bars of connective tissue with oblique structure between. It seems they could have aided in stiffening the fin-tip to stop it drooping, or activated the tip in slow paddling. They may, of course, have been much more widespread on the fin-blade, but bones blot out the musculature. According to Andrews less distal muscles in R 1664 bend over the posterior margin, or meet the anterior margin nearly at right angles.

The detailed work of Hofmann (1958, fig. 10) has clearly illustrated the way ichthyosaur fin- blades became imbricated when deposited at a slight angle and it happens on any irregular sur-

LETHAIA 17 (1984) Cretaceous ichthyosaur 1 11

face of deposition. Unfortunately, his work was not available to Oemichen (1938) when that spe- cialist in fluid flow and vertical flight discovered ichthyosaur fore limbs which were articulated through the carpal rows and then imbricated. His ingenious explanation of how they operated is null but the movements and fluid flow indicated in his Figs. 5 & 6 can be transferred to the flexible cartilages of the horizontally attached fore (and presumably hind) limbs when Platy- pterygius was holding station or moving slowly by limbs only. Oemichen restored Ichthyosaurus ‘burgundiae’ as inflexible in a dorsoventral direction, supported half out of the water by constant- ly moving, subvertical limbs. The apophyses of the Platypterygius vertebral column were not complex enough to stop vertical bending, and the amphicoelous centra would have allowed bending in any direction. In Platypferygius at rest at the surface, even if the lungs extended to near the rear of the body cavity, the rear 2 m would have sagged comfortably in the water. The short neck was all that would bend anterior to the lungs, but that was quite enough to lay the snout on the water, or in it if the ichthyosaur wished to submerge the eyes and watch for danger or prey. Density must have played the greatest part in the height at which it floated.

In the absence of closely related reptiles, the comparative densities of other supple swimmers must be sought. Cott (1961) found that the specific gravity of nine Nile crocodiles between 1 and 2.3 m varied little from their average of 1.080. If stones are available (according to his calculations the nine had none) free-ranging animals commence swallowing them as ballast at 0.45 m length, and on average, carry about 170 ballast. By 2.5 m all specimens of hundreds sam- pled had ballast stones lodged in the stomach, bringing their centres of gravity forward and down. Cott recorded the relative clumsiness of small, unballasted individuals. Colbert ( 1962) measured the specific gravity of a baby alligator and found it 0.89, so it is possible that baby crocodiles have to contend with low specific gravity as well as n o ballast. McGowan (1973b) pointed out that small vertebrates, being less ossified, are often light. His examples, birds and mammals, all received parental feeding, so his additional emphasis on their fat content may not apply. Cott (1961) noted ‘Immediately before a dive the naris are closed and the animal’ (crocodile) ‘sinks - presumably by contracting the tho- rax and abdomen and so increasing specific grav-

ity’. Whether this is the mechanism or they expel a little air first, they certainly can sink like stones released at the surface. Similar shrinkage, but induced partly by external water pressure, has been invoked as one of the causes of high density in submerged sea snakes (Heatwole & Seymour 1975). These also lose size (and weight) by outward diffusion of COz through the skin, greatly in excess of oxygen taken in (Seymour & Web- ster 1975). Various sea snakes can rid themselves of 33 to 94% of the C02 exchanged into their blood for oxygen, and the total gas loss leaves them relatively denser. Figures for the reduced volume of air in the lungs of submerged snakes (Graham et al. 1975) are available only for Pela- mis platurus, but Heatwole & Seymour (1975) record that all kinds of sea snake can vary their specific gravity sufficiently to float on the surface or lie on the bottom without having to cling to, or brace against, fixed objects.

If Stenopterygius had carried ballast stones into the lower Jurassic limestones at Holzmaden we would know it. Their large coracoid-humerus complex was less dense than crocodile ballast stones but better placed, and Cott (1961) noted the improved diving ability of ballasted animals. Whatever the density of extreme juveniles, older ichthyosaurs were probably as dense as sea-water or a little denser. Comparison favours slight negative density. The large fin-blades would have had to flutter only very slightly in sleep to keep the animals level and up; near enough to the surface to elevate and breathe in sleep, but not necessarily above the surface, exposed to the dessication and chill factor of any wind.

If sea snakes are taken as a model, the volume to surface ratio is much higher in the ichthyosaurs, and cutaneous diffusion could not have played a great part in density changes. Any loss of C02 and N2 from the blood must have been a help when submerged, however, and the whole skin was probably permeable to gases while the sea snakes are partly scaly.

The body space was totally encased in ribs, and the presence of abdominal ribs must have allowed stringent control of breathing, whether it was involuntary as in sleep or the more vigorous exhale and inhale of breathing while they broke surface during swimming. A diaphragm is the mammalian structure, which cetaceans and pin- nepeds have utilized wonderfully, but its absence is no reason to dub the ichthyosaurs inefficient breathers. The cetaceans made a more certain relative improvement in ease of breathing when


they got their nostrils to the top of the head. The ichthyosaurs, even Platypterygius which had its nostrils higher relative to the eyes than many others, had to rise higher in the water than any cetacean of the same size. In Platypterygius the difference Cas an additional 5 cm. Schools of dolphins often keep up movements like this for hours on end, interspersed with more vigorous games and rest periods. I t is clearly not tiring to healthy animals but it must have slightly retarded the potential speed.

Both models, sea snakes and crocodiles, can drowse on shallow sea-floors as easily as at the surface. Perhaps ichthyosaurs had this ability also. The ventral surface and low spreading limbs would have made them stable.

Conclusions It was early noted that ichthyosaur tail-fins generated negative pitch, and suggested that the limbs, supposed to be anything from vertical to horizontal in attachment, also gave lift. From these facts came the belief that the animals were more or less ‘tied’ to the surface, and could dive only poorly. The scientific folklore was quanti- fied by Oemichen (1938) and widely accepted in spite of the evidence of streamlined bodies, and gut contents of fish and cephalopods, themselves good swimmers.

The uncrushed coracoids of P. australis show very lowset fore limbs. These were under the control of powerful dorsal and ventral muscles attached to large trochanters. These adjusted the trim for diving, swimming or soaring in the water. The large, well-armed jaws indicate good fishing capability.

Consideration of the abilities of other supple, strongly aquatic reptiles (crocodiles and sea snakes) suggests the possibility that Platypterygius would also have had a density little greater than sea water, perhaps variable to just lighter than sea water.

The full encasement of its body cavity in ribs probably allowed quick compression and expansion so that a fairly short period was all that was needed to exhale and inhale. Probably, if it was quiescent after diving to the sea floor (at shelf depths) it would have been able to rest there like modern aquatic reptiles. It should not be en- dowed retrospectively with a dolphin’s need for air because of its dolphin-like shape. It was a capable, fully marine reptile.

As a reptile it did not share the heritage which allowed cetaceans to develop their ability to sense and avoid or endure bad weather, and it may not have developed as suitable an alterna- tive. Ichthyosaurs generally are associated with shallow water deposits and faunas. Platyptery- gius is dispersed around the world, but allo- patrically speciated, at least in the Albian. Cur- rently only one species of sea snake is pelagic, and sea-going crocodiles are restricted to shallow shelf depths by the need to rest on the sea floor. Platypterygius is likely to have had the necessity to rest on the sea floor when oceanic gales made rest at or near the surface untenable. With the sea floor out of reach, oceanic crossings would have been chancy and probably rare.

Acknowledgements. - I am indebted to C. McGowan and R. Molnar for literature which would otherwise have been difficult to obtain, and to R. Molnar and R. A. Thulborn for discussion on anatomy and helpful criticism of the manuscript. At a later stage I had the benefit of discussion with R. M. Appleby.

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platypterygius australis, an australian cretaceous ichthyosaur

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