Permian anteosaurus were active and dexterous predators, judging by the structure of the vestibular apparatus

For a long time, it was believed that the Permian therapsids – the direct ancestors of modern mammals – were overweight, slow and clumsy creatures that could hardly move on their semi-erect limbs and were able to provide themselves with oxygen only due to its increased content in the atmosphere, and not an effective respiratory system. But a recent tomographic study of the skull of one of the largest carnivorous dinocephals – the South African anteosaurus (Anteosaurus magnificus) – showed that during life this six hundred kilogram monster was quite mobile and could actively pursue its prey, as modern carnivorous mammals do.

Dinocephals dominated the Earth in the middle of the Permian period (270–260 million years ago), representing a large and diverse group of four-legged animals that, judging by the available finds, have mastered almost the entire Pangea. The ancestral form of dinocephals is most likely biarmosuchs, primitive predatory therapsids found in the Urals, while all later groups of these animals most likely originated in the south, and only then some of them migrated to north. It is assumed that the estemmenosuchus may also belong to primitive dinocephals, but this is inaccurate, because some researchers bring Estemmenosuchus closer to gorgonops. Traditionally, representatives of two infraorders are referred to as dinocephals: mainly herbivorous tapinocephals (Tapinocephalia) and carnivorous anteosaurs (Anteosauria).

Tapinocephals were mainly represented by large forms (for example, the famous moschops reached 2.7 m in length and weighed one and a half tons, and the tapinocephalus that gave the name to the whole group was even larger) with a very characteristic appearance: a short tail, long forelimbs (due to of this, the shoulders of the dinocephalus were located significantly above the sacrum) and a massive head with pronounced pachyostosis of the frontal bones. The exact purpose of these bony thickenings is still unclear: researchers suggest that they helped with intraspecific clashes (see E. Snively, A. Cox, 2007. Structural mechanics of Pachycephalosaur crania permitted head-butting behavior), but domestic paleontologists more often adhere to the version that pachiostous growths of dinocephalic bones, pierced by large blood vessels and not protected by any horn formations, served for thermoregulation and helped these giant animals not to overheat in the sun (M. Ivakhnenko, 2008. Cranial morphology and evolution of Permian Dinomorpha (Eotherapsida) of eastern Europe).

Two large dinocephals - the carnivorous anteosaurus (left) and the herbivorous moschops (right). Image © John Sibbick
Two large dinocephals – the carnivorous anteosaurus (left) and the herbivorous moschops (right). Image © John Sibbick

Anteosaurs are considered more primitive animals than tapinocephals, since both in their way of life and in general features of the structure of the skeleton, they are more similar to primitive dinocephals than to their herbivorous contemporaries. They had thinner skull bones (see C. Kammerer, 2009. Systematics of the Anteosauria (Therapsida: Dinocephalia), L. Boonstra, 1969. The fauna of the Tapinocephalus zone (Beaufort beds of the Karoo)), and pachystous thickening , presumably, developed to absorb the load created by the powerful adductor muscles during biting, so that for them “butting” between males seems even less likely. A group of scientists from South Africa, Sweden and Great Britain tried to understand the role of these bone formations in anteosaurs using modern methods. They performed X-ray microcomputer tomography of the skull of a young anteosaur BP / 1/7074, which literally allowed a look inside its head. Previously, such studies were carried out only for herbivorous tapinocephals (since the structure of their evolutionarily advanced skulls was of more interest to scientists), while the neuroanatomy of carnivorous dinocephals remained practically unexplored.

The first interesting feature revealed by tomography of the skull of an anteosaur was the structure of its inner ear: in comparison with the moschops, the anteosaur had much longer and more curved semicircular canals, which suggests a well-developed sense of balance, characteristic of a fairly mobile animal. In general, the size of the inner ear of an anteosaur, in relation to body size, is larger than that of any other known Permian therapsid, both herbivorous and carnivorous: according to the researchers, this means that the anteosaurus could quickly move its eyes, head and neck, which means it could deftly track and overtake your prey, ahead of other carnivores. But since the skull BP / 1/7074 belongs to a young animal, it is impossible to say with certainty whether similar proportions were preserved in the structure of the inner ear upon reaching maturity: unfortunately, developmental studies of therapsids are extremely rare. The only data available to date is a study of the structure of two skulls of cynosaurs of different ages (Cynosaurus), cynodont terapsids closest to mammals: judging by the relative size and structure of the inner ear, at least part of the Permian therapsids, the inner ear reached anatomical maturity at a relatively early age, as is typical for modern mammals (J. Benoit et al., 2017. The bony labyrinth of late Permian Biarmosuchia: palaeobiology and diversity in non-mammalian Therapsida).

The structure of the occipital bone of the antheosaurus (highlighted in yellow) and the bony labyrinth, including the inner ear (highlighted in purple). Anterior semicircular canal - anterior semicircular canal, common crus - common leg, jugular foramen - jugular foramen, floccular fossa - floccular fossa, exposed vestibule - vestibule of the ear, internal auditory meatus - internal auditory canal, fenestra vestibule - window of the vestibule (opposite ear). Drawing from the discussed article in Acta Palaeontologica Polonica
The structure of the occipital bone of the antheosaurus (highlighted in yellow) and the bony labyrinth, including the inner ear (highlighted in purple). Anterior semicircular canal – anterior semicircular canal, common crus – common leg, jugular foramen – jugular foramen, floccular fossa – floccular fossa, exposed vestibule – vestibule of the ear, internal auditory meatus – internal auditory canal, fenestra vestibule – window of the vestibule (opposite ear). Drawing from the discussed article in Acta Palaeontologica Polonica

The hypothesis of a fairly mobile lifestyle is confirmed by the presence of a deep and wide floccular fossa, in which a piece (flocculus) of the cerebellum was located during life – the most ancient part of this section of the brain, which is the most important part of the vestibular apparatus. For comparison, in Moschops this fossa was much smaller and not so deep so that during life the Moschops was definitely slower than its predatory relative, whose lifestyle now seems to be not at all as sedentary as it was previously described by some paleontologists (see, for example, G. King, 1988. Encyclopedia of Paleoherpetology и M. Ivakhnenko, 2008. Cranial morphology and evolution of Permian Dinomorpha (Eotherapsida) of eastern Europe).

In addition to the structure of the vestibular apparatus, the researchers also paid attention to the orientation of the anteosaur’s cerebral cavity in relation to the rest of the skull. In anteosaurus, the long axis of the cerebral cavity is practically parallel to the axis of the skull (a conditional line from the occipital condyle to the tip of the muzzle), while in the Moschops, with the horizontal orientation of the cerebral cavity, the tip of the muzzle is directed steeply downward. In addition, this orientation can be confirmed by comparing the angle between the plane of the lateral semicircular canal and the long axis of the skull: in anteosaurus, this angle is 25°, and in Moschops, 65°. The authors suggest that in the course of evolution, tapinocephalics underwent a significant reorganization of the initial structure of the skull, which was probably caused by adaptation to the use of the head as a weapon or to a specific way of feeding. In modern mammals, this reorientation of the skull relative to the long axis of the cerebral cavity is called clinorinchia (see  Primate Cranial Diversity) and is usually interpreted as an adaptation to herbivorousness and butting in ungulates, and in primates as a consequence of brain enlargement. Since tapinocephalic brains are relatively small, it is assumed that the tilt of their skull was formed for the same reasons as in modern sheep or goats, while nothing like this is observed in anteosaurs. Perhaps, if predatory dinocephals organized inter-male battles, then they behaved rather not like modern red deer, but like musk deer, which use their enlarged upper canines both for demonstration and as a weapon during conflicts.

The location of the brain cavity of anteosaurus (A) and moschops (B) in relation to the main skull. Pineal (parietal) foramen - parietal eye, occipital condyle - occipital condyle, fighting surface of the skull - pachyostosis of the skull, according to the authors' assumption, used in butting, floccular fossa - floccular fossa. Drawing from the discussed article in Acta Palaeontologica Polonica
The location of the brain cavity of anteosaurus (A) and moschops (B) in relation to the main skull. Pineal (parietal) foramen – parietal eye, occipital condyle – occipital condyle, fighting surface of the skull – pachyostosis of the skull, according to the authors’ assumption, used in butting, floccular fossa – floccular fossa. Drawing from the discussed article in Acta Palaeontologica Polonica

All these considerations, taking into account the data of previous studies, allowed the authors of the discussed work to suggest that in the course of evolution, primitive dinocephals, in which pachiostous thickening of the skull mainly served to strengthen bones and reduce the load on bites, gradually began to transform into more progressive tapinocephals, in which pachiostosis is impressive. expanded (in some species, the thickness of the cranium in the forehead area reached 30 centimeters, see M. Day et al., 2015. Youngest dinocephalian fossils extend the Tapinocephalus Zone, Karoo Basin, South Africa) and began to perform social functions.

Thus, new data (the position of the brain cavity of the anteosaur relative to the skull, the structure of its inner ear and the size of the floccular fossa) confirm that the anteosaurs were not clumsy amphibiotic predators, but were distinguished by significant mobility and agility by the standards of Permian therapids – which, given the latest data from isotope analysis their bones (K. Rey et al., 2020. Oxygen isotopes and ecological inferences of Permian (Guadalupian) tetrapods from the main Karoo Basin of South Africa), allows these animals to be presented as analogs of modern bears, and not crocodiles, as was previously thought. Also, the study of the internal anatomy of the skull of the anteosaurus speaks in favor of the hypothesis that while tapinocephals were herd animals with a fairly developed social behavior (which is indirectly confirmed by the findings of tapinocephal groups of 5-12 individuals, B. Rubidge et al., 2019. The postcranial skeleton of the basal tapinocephalid dinocephalian Tapinocaninus pamelae (Synapsida: Therapsida) from the South African Karoo Supergroup), anteosaurs were distinguished by a more primitive skull structure and, most likely, led a lifestyle of solitary predators, as is typical for many modern carnivorous mammals.

Source: J. Benoit, A. Kruger, S. Jirah, V. Fernandez, B. S. Rubidge. Palaeoneurology and palaeobiology of the dinocephalian therapsid Anteosaurus magnificus // Acta Palaeontologica Polonica. 2021. DOI: 10.4202/app.00800.2020.

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Alexandr Ivanov earned his Licentiate Engineer in Systems and Computer Engineering from the Free International University of Moldova. Since 2013, Alexandr has been working as a freelance web programmer.
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