The Deseado Formation of Patagonia

by Frederic Brewster Loomis

Premolar 2 is three-rooted, with a triangular crown on which are three mamma-like tubercles, the larger one in front, and two behind. As the crown is worn, these unite into a flat, grinding surface, surrounding a central pit which opens behind. Premolars 3 and 4 and all the molars are large, four-rooted, quadrilateral teeth, each with two transverse crests running clear across the crown, and with a small cingulum across the anterior margin. Before they are worn, the top of each crest is tuberculated, and the cingulum is crenulated. In wearing, the anterior face of each crest is ground down; so that instead of the crown wearing to a level surface, it retains throughout life two oblique grinding surfaces.

The lower dentition is more reduced than the upper. When in position, the tips of the two lower tushes diverge, so as to come in contact with the tips of the second upper tushes, from which I conclude that the lower tush is the second, rather than the first incisor, the latter having been lost when the second became enlarged as was the case in elephants. This lower tush has the same oval cross section, enamel on the front face only, and beveled tip as the corresponding upper incisor; but, in the same individual, is somewhat longer and slenderer. When isolated, however, it is difficult to tell whether one is handling a small upper tush or a larger lower one.

The remaining incisors, the canine, the first and the second premolars are wanting, and their place is taken by a small diastema. The lower premolars and the molars are similar to those of the upper jaw, except the cingulum is on the posterior margin, and the wear is on the posterior face of each transverse crest.

The skull is very long and narrow, with wide and deep zygomatic arches. The nasal opening is moved back from the front of the snout to just opposite the orbit, leaving a long, narrow, but heavy snout, made up mostly of the premaxillae, on the anterior end of which is an oval boss, which must have served as an attachment for muscles. With the tushes developed so as to bite against each other, as in a gnawing animal, I can not see any possibility for the development of a pendant proboscis, but think that the snout must have been developed more like that of a pig, but probably to a greater degree. The premaxillae are long and heavy, and prolonged backward to contain the roots of the great tushes; but these bones are not developed on the palatal side of the snout at all. The maxillae are also massive, carrying the premolars and molars, and extending forward to the bases of the tushes. They have developed downward so as to carry the plane of the palate far below the plane of the basicranium, and causing the upward bend in the basicranial axis, which is so characteristic of elephants. This bend leaves the occipital condyles a full foot above the plane of the teeth. The maxilla extends upward so as to bound the major part of anterior margin of the nasal opening, and of the orbit, which latter opening is small and directed forward. The zygomatic process is large and makes a considerable portion of the arch. The jugal is a broad flat bone making up most of the zygomatic arch and extending back so as to take a small part in making the glenoid fossa, as in elephants.

The top of the brain case was crushed in before the burial of the animal, the anterior part being present, and about 40 mm. below its normal position, but the parietal region having been loose, exposed the brain cavity, the ear chamber and some of the cellular vacuities. The nasal bones are long and light in build, and are pushed back so that they lie between the postorbital processes of the frontals. The frontals were united medianly, and prolonged on either side of the nasals to make the postorbital processes. The back margin of the frontals is broken away. The parietals are lost, but it is apparent that there was a short sagittal crest. From the middle, high lambdoidal crests extend to either side, and become continuous with the upper margins of the zygomatic arches. The posterior face of the skull slopes back from the lambdoidal crests for a considerable distance, down to the moderate sized foramen magnum.

The squamosum is a large bone, with the lambdoidal crest and the extension of the zygomatic arch on its upper surface. It carries the major part of the glenoid fossa. Behind the auditory meatus is a large post-tympanic portion which extends down and unites with the pre-tympanic portion, completely inclosing the opening of the ear and crowding the tympanic from being exposed on the side of the skull. There is a very short paroccipital process, and this posterior portion of the squamosum is the part which resembles that of elephants, hyracoids and, to some extent, Toxodontia. There is, however, no cavity in the squamosum as in toxodonts generally. The tympanic bulla is small, but little swollen, and hollow. It is quite exactly like that of probocideans. The basioccipital is fused to the exoccipitals. The occipital condyles are very high above the plane of the teeth, are set wide apart, and are cylindrical bosses which would not allow a free movement of the head laterally, but only in the up and down direction. This last is again a feature of the elephants. The pterygoid bone is greatly enlarged to compensate for the bend in the basicranial axis, and the pterygoids, together with the alisphenoids, make broad plates bounding either side of the posterior nasal chamber, exactly as in Palaeomastodon. The palatal bones are slender in front, and broaden toward the rear, again, as in elephants.

On the interior of the brain case is the cavity for the brain which indicates that this organ was of diminutive size, measuring about 150 mm. in length by 50 mm. in width at the widest part. It indicates a brain with very small cerebral hemispheres, which, however, had a swollen posterior margin, a larger cerebellum, and a wide medulla oblongata. The impression which I obtained of this brain is strikingly like that given for Palaeomastodon. On either side of the brain cavity are a couple of vacuities, apparently for lightening the weight of the skull. At the inner end of the auditory meatus is a large ear chamber, divided into a smaller anterior or cochlear portion, and into a larger posterior ear chamber proper.

Fig. 109. Palatal views of the basicranial region of A. Pyrotherium, and B. Palaeomastodon, for comparisons; alc., alisphenoidal canal; als., alisphenoid bone; Bsp., basisphenoid bone; eu., Eustachian canal; f.l.m., foramen lacerum medium; f.l.p., foramen lacerum posterior; f.o., foramen ovale; i.c.c., foramen for the internal common carotid; mx., maxilla; oc.f., occipital foramen; pal., palatine bone; p.p.f., post-palatal foramen; pt., pterygoid bone; pt.s., and p.ty.sq., post-tympanic process of squamosum; sq., squamosum; st.m.f., stylomastoid foramen.

In [figure 109], I have placed a diagram of the base of the skull of Pyrotherium, along beside that of Palaeomastodon, for comparison of the basicranial foramena. The skull of Palaeomastodon is somewhat more elongated, especially in the posterior part. In both, there are two antorbital foramena; the post-palatal foramena of Pyrotherium are a trifle further back, but this palatal region in both is of the same type which is peculiar to elephants and Pyrotherium. In Pyrotherium the condylar foramen is separate, while in elephants it is fused in with the foramen lacerum posterior. This latter foramen in both cases is situated just back of the tympanic, and in Pyrotherium is of considerably larger size than in Palaeomastodon. The foramen lacerum medium is in front of the tympanic and in Pyrotherium appears considerably larger, mostly because it is under the margin of the tympanic in Palaeomastodon. The foramen for the internal common carotid in Palaeomastodon pierces the tympanic bone just to the inside of the middle line, while in Pyrotherium it is on the outer margin of the tympanic. The Eustachian canal is on the external border of the tympanic in both cases, but in Pyrotherium it is further back. The foramen ovale of Palaeomastodon is in the posterior part of the alisphenoid bone, but with the shorter alisphenoid of Pyrotherium, this foramen is pushed back to the posterior margin of the bone. In both cases, the alisphenoidal canal starts under the base of the fused alisphenoid and pterygoid, and opens into the orbit. The stylomastoid foramen of Pyrotherium is situated further out than in the case of Palaeomastodon. The fusion of the post-tympanic portion of the squamosum is, in Palaeomastodon, much further advanced than in Pyrotherium, so that the passage to the ear is not apparent in the basal view of the former, but makes a considerable notch on the under side of the skull of Pyrotherium.