Memoir on poekilopleuron bucklandii

FORELIMB.

§. XXVth. Shoulder.

This bone comes from the Mouen block and is situated below the ribs, thus it belongs very certainly to our animal; however it would be tempting to believe that it is strange when its dimensions are compared to those of the femur and the mass of the animal, which can easily be appreciated from the collection of other bones.

It is a nearly general fact that the forelimbs of crocodilian and lacertilian reptiles are shorter and weaker than the hind limbs; in several species the difference is very pronounced. But our fossil reptiles from the environs of Caen display a much stronger disproportion still between these limbs: Poekilopleuron, Steneosaurus from Quilly, and Teleosaurus furnishing the proof. These latter above all have excessively small forelimbs; the two pairs of limbs differ from one another perhaps more than in gerbils and kangaroos.

{82} To return to our Poekilopleuron, this difference in the length and strength of the limbs should give it a unique form and influence its habits. The weakness of the forequarters, as much as the great strength of the hindquarters, confirms the remark suggested by the study of the vertebrae and ribs, namely that Poekilopleuron could have sprung with a prodigious force like a spring that relaxes itself; but this could not be on a solid plane, the forelimbs having been too weak to resist the shock produced by so heavy a mass when it fell: all this energy was employed for rapid and vigorous swimming in a sea that the atmospheric circumstances at that time may have rendered subject to frequent and violent storms, or for making jumps to its surface when it was tranquil.

The humerus is from the left side: its superior end or head had a straight oval shape and was continuous with the deltoid crest, which was extended down to near the middle of the bone and made a considerable projection in front. The nearly cylindrical body of the bone was widened transversely near the base to form the inferior end. Posteriorly there exists a wide, slightly deep, triangular depression representing the olecranon fossa; anteriorly there are depressions and rugosities for attachment of the ligamentous capsule. The articular surface (fig. 4) is separated into two parts by a slightly deep furrow; the external, corresponding to the radius, is nearly square with a rounded border; the internal, straighter anteroposteriorly and longer transversely, has its edges equally rounded.

One can see by this description and by figures 1, 2, 3, 4, from plate VII, that the humerus of Poekilopleuron differs from those of crocodiles and lizards: it is more collected in its forms; its deltoid crest is more developed and extended farther onto the body of the bone; its inferior end is wider transversely and less straight anteroposteriorly than in crocodiles, but its articular pulley is less pronounced and above all less directed anteriorly than in lizards; by the vagueness of its configuration it most resembles that of crocodiles: together these are characters that again announce an intermediate type between the two families.

{83} The humerus of Teleosaurus is much more slender; its very flat head is continuous with the slightly pronounced and very short deltoid crest; the inferior end is strongly straight and slightly wider than the body of the bone; the articular pulley is a little more marked⁽¹⁾. I only know the superior end of the humerus of Steneosaurus from Quilly; it resembles well that of the same bone of Teleosaurus, but it shows relatively greater proportions.

Thus, according to the relationships of its forelimb, Poekilopleuron is more similar to living crocodiles and lizards than to the crocodilians of its epoch.

{84} I stopped with the result presented in fig. 5, where the radius, a, is placed relative to the ulna, b, against one other as viewed from their anterior face, which corresponds to the palm of the manus; I took the largest bone for the ulna, and regarded the more voluminous end as superior, guided in these determinations by that shown by the forearm bones of crocodiles and lizards. In the fossil, these two bones are, as the humerus, more compact in size and more robust than in their living analogs.

The superior end of the ulna (fig. 10, b) is triangular, a little depressed and much larger than the inferior (fig. 11, b), which is subtriangular with a rounded border; there is no salient olecranon as shown in monitors; in this Poekilopleuron resembles crocodiles. Fig. 8 shows the ulna from its posterior face, and Fig. 9 from its internal.

The two ends of the radius are almost equally voluminous: the superior, fig. 10, a, is triangular and flattened; the inferior, fig. 11, a, is nearly oval, directed slightly forwards, flat on the side of the palmar face of the bone, and rounded on the dorsal side. The body of the bone is cylindrical; near its middle, its internal border shows a salient, compressed tuberosity that resembles the bicipital tuberosity of certain mammals. Lizards and crocodiles have nothing similar; as one knows, they do not have the biceps muscle proper, but one or several flexors, variable according to the genus and species. No doubt the radial tuberosity of Poekilopleuron would give attachment to a forearm flexor: was this a biceps?… I acknowledge that the presence of this tuberosity led me to decide to place it on the inside, and as a result to situate it above the end a, fig. 10; I could have not improbably turned the bone end for end to put the tuberosity on the outside. However it is wise to note that the face of the radius (fig. 5, a) shows, in the part that I regard as inferior, two superficial furrows that seem to signal the presence of digital flexor tendons and could confirm the position that I have adopted. Fig. 6 presents the radius from its posterior face; fig. 7 from its internal edge.

I have a very little thing relative to the phalanges, but this little thing is interesting in that it fixes our ideas on the separation of the digits, and indicates equally the principal differences of these digits from those of the pes.

Two mutilated pieces belong to these bones: the first (fig. 16, 17) is an anterior part of the ungual phalanx. It was whole; unfortunately the blow that chanced to discover it reduced its posterior part to powder, but the imprint remained; thus I can give the form of the entire phalanx. It is compressed and strongly arched; it differs from those of the pes by these two characters, as well as by its great disproportion in size. The form of this phalanx suggests hooked and even acute unguals; but how would they have preserved their point if they were used to support a mass as heavy as that of their owner on the ground? One would be led to believe that they were retractable as those of geckos; it is evident that they could not serve to support the animal, but served to seize or hold prey.

The other phalanx (fig. 18) that I attribute to the manus, because of its small size, is in very poor condition; its two ends are broken and lost, only a portion of the shaft of the body remains so the character of a phalanx can hardly be recognized there.