Pyrotherium

Extinct genus of mammals

Pyrotherium ('fire beast') is an extinct genus of South American ungulate, of the order Pyrotheria, that lived in what is now Argentina and Bolivia, during the Late Oligocene. It was named Pyrotherium because the first specimens were excavated from an ancient volcanic ash deposit. Fossils of the genus have been found in the Deseado and Sarmiento Formations of Argentina and the Salla Formation of Bolivia. So far, two valid species have been described, Pyrotherium romeroi, which lived in what it is today Argentina and P. macfaddeni from Bolivia, at the end of Oligocene. P. romeroi in particular is the most recent known pyrothere in the fossil record and best known for its fossil remains, which although incomplete are the best preserved in the entire order, indicating that they are also the largest, with an estimated body length from but it is not related to the current elephants (proboscideans); the resemblance is so great that when studying the fossil remains, it was attributed in the past a relationship with elephants, although the true relationship of this herbivore is still controversial today. these strata are now known as part of the Deseadan mammal age (SALMA) in the area of the Deseado estuary, although there is the doubt whether the holotype of Pyrotherium romeroi really comes from Neuquén, it being possible that the remains actually came from Chubut. Ameghino considered that these areas corresponded to older terrains, from the Paleocene subsequent studies have shown that they actually come from the Oligocene, and in fact the Pyrotherium fossils have reached become the guide fossil of the late Oligocene. Because the remains of this animal originally appeared in the volcanic ash beds of the Deseado Formation, they gave rise to the name of the genus, which means "fire beast". Ameghino named several species from the Deseado area such as P. sorondoi based on partial remains, mainly teeth, but later studies indicated that they are part of a single species. and later they were classified as a different and smaller species, P. macfaddeni, whose species name is in honor of paleontologist Bruce J. MacFadden. but it is possible that they correspond to some different genus with which it is closely related, but not yet described.

The skull of Pyrotherium romeroi was long and narrow, made up of massive bones. It reaches in length from its front teeth to its occipital condyle, and has an elongated, relatively narrow snout seen from above, with retracted nostrils, a large nasal opening located between the eye sockets in the middle of the front bone, in parallel to the back of the skull, with thick bone walls for muscle support; inside there are cavities filled with air. The occipital region, in particular the condyles, was particularly high, as a consequence of the flexion of the posterior part of the skull with respect to the plane of the base, which formed an obtuse angle with that of the palatine bone; in this and other characteristics, Pyrotherium resembled proboscideans. There is a small ridge that emerges from the premaxilla and reaches the nasal bone, which appears to be broken and surrounded by a rough texture, which could be the result of erosion. How large it may have been is unknown, as it may have been only a prominence similar to that seen in the narial process of the notoungulates and rodents, or even almost a ridge; this ridge is not known in other mammals, but perhaps it served as a holding point for the muscles of a possible proboscis or trunk. The brain cavity (neurocranium) is damaged and surrounded by spongy bone tissue; Loomis considered that it indicated that in life P. romeroi had a small brain, about long and wide. A pyramidal and semilunate have also been found. Also included is an astragalus and calcaneus, and a femur. The shape of the distal joint allowed the tibia to move backwards widely, which compensated for the lack of flexibility in the foot joint. The tibia was much shorter than the femur, and the fibula was very close to the tibia, except in the central part. The astragalus was strongly flattened, very simple in appearance, and neckless, with a slightly hinted tibial trochlea and a facet of the navicular located directly below the trochlea. The tarsus of Pyrotherium was characteristic: the calcaneus tubercle was compressed dorsoventrally, as was the trochlea of the astragalus; in addition, it presents an extreme reduction in the contact between the heel and the cuboid. Loomis 1914). Loomis 1921 However, the mixture of characteristics of the animal is such that it has led to comparing and relating it at different times with other groups, such as the marsupial diprotodontids (Lydekker 1893; Loomis 1921), the amblipodan pantodonts (Zittel 1893), perissodactyls (Ameghino, 1888), the notoungulates (Osborn 1910; Loomis 1914; the xenungulates (Simpson 1945; Cifelli 1983; Lucas 1986, 1993), and the dinoceratans via their supposed relationship with xenungulates (Lucas 1986, 1993); in some studies, the complete study of the tarsus of Pyrotherium fails to support a relationship with xenungulatans, instead the derived characteristics of Pyrotherium were not observed other than in other mammals examined except for the embrithopod Arsinoitherium from the Paleogene of Africa. If this is due to a common ancestor, or to the unusual mode of locomotion used by these animals (graviportal and plantigrade) remains a mystery to be seen. However, Gaudry (1909) himself established that Pyrotherium was sufficiently different from any other group of large mammals that it should have its own order, with no clear relation to other mammals. The most recent analysis published out, like the work of Billet in 2010, suggests that pyrotheres such as Pyrotherium are a group of specialized notoungulates, related to Notostylops, although this is still a controversial idea. This analysis also made it possible to infer the chewing patterns of Pyrotherium. This would be dominated by the so called phase 1, in which the mandible is tilted and directed mesially, while the cutting ridges of the molars were compressing the food bolus. Then a phase 2 was developed, in which the jaw moved laterally; this move seems to have been less significant. This type of chewing and molars resembles that observed in some other mammals, such as the Macropus kangaroos, the perissodactyl Lophiodon, the marsupial Diprotodon and the proboscidean Deinotherium, but in these animals their enamel (and molar lophs) wear out quickly into adulthood, leaving a flat surface for grinding, whereas in Pyrotherium the lophs are much more resistant and can be clearly seen even in elderly individuals, in whom the worn molars still have sharp ridges. A similar condition is only seen in embrithopods such as Arsinoitherium, which also has vertically arranged enamel and in Namatherium, which closely resembles Pyrotherium in this respect by having enamel and highly inclined enamel facets. With a weight of in P. macfaddeni to in P. romeroi based on estimates of its molars, and in P. macfadeni at for P. romeroi with base in equations derived from the head body ratios, Pyrotherium was among the largest native mammals in South America. Its bones are extremely dense, even more than in other large meridiungulates such as the notoungulate toxodonts and astrapotheres, which implies an extreme specialization towards graviportality; X ray microtomography analysis of the bone density of its humerus and femur indicate that its medullary area was particularly compact, almost comparable to the pachyostosis of aquatic or semiaquatic mammals, with thick trabeculae and very small intratrabecular cavities, although they resemble externally the bones of proboscideans or rhinos, which would help it better absorb the impact energy on the bones. this would contradict the hypothesis that they were semiaquatic animals, similar to hippopotamuses, while the remains of astrapotheres (another group of large, tusked native ungulates) are in fact found in areas associated with bodies of water, which would imply that they would live in humid environments and were able to spend some time in the water. Pyrotherium would have used its incisors and trunk in order to collect food such as leaves and branches of the trees, in a similar way to black rhinos and African forest elephants.