osteobones:This magnificent beast is Inostrancevia, the last and largest of the gorgonopsids. Inostr

osteobones:This magnificent beast is Inostrancevia, the last and largest of the gorgonopsids. Inostrancevia was indeed very large, the skeleton shown here is about the size of bear, with a skull of close to 60cm. I don’t think any weight estimates for Inostrancevia have been published in the last 20 years but if I had to put my money down, I would say that it is somewhere over 300kg. This is a very rough estimate so don’t take it too seriously. Inostrancevia was discovered in Russia back in 1922. Like the rest of gorgonopsia, which are a basal group of therapsids, it lived in the Permian, near the end around 260-254mya. Researching for this post I found that there was very little information on Inostrancevia, and what little there was, was written in Russian. Being as forthcoming as they are I couldn’t get too much info on this beast despite it being known from two complete skeletons. So much of what I write will be general and applies to most of gorgonopsia and I shall throw my own observations in as well.Okay let’s start with the exciting stuff, that amazing looking head of his. Although Inostrancevia was a more derived “mammal like reptile” than dimetrodon, it still had not developed a truly mammalian jaw musculature.  The jaw was closed by three main abductor muscles. Two acted much as our temporalis muscle does today. One attached at the temporal fenestra to the posterior of the lower jaw, it pulled straight upwards. The other of these two proto temporalis muscles attached to the squamosal at the posterior part of the cranium to the coronoid process. This muscle pulled the lower jaw upwards and backwards. Interestingly the gorgonopsids are some of the earliest annimals to possess a coronoid process (the process sticking out dorsally of the dentaries in mammals), which allowed for a faster jaw closing movement by acting as a lever arm. The third muscle acted as a proto massiter muscle, attaching from the interior of the zygomatic arch to the posterior of the mandible. It helped add power and stability to the bite. One interesting feature in the gorgonopsid skull is a moveable quadrate. The quadrate is a smallish bone at the posterior of the skull that is part of the jaw joint. In gorgonopsids the quadrate was not fixed, and is able to slide and rotate to allow the lower jaw to open wider, which is almost certainly and adaptation to having enlarged canine teeth. So now let’s talk about those teeth, I will start with the stars of Inostrancevia’s dentition the enlarged canines. These huge teeth, for obvious reasons, have drawn comparisons to saber-toothed cats like smilodon. While understandable, there is one feature that separates them,  gorgonopsian canines are quite thick and robust while smilodon’s are laterally compressed and more blade like. While this might not seem like a big difference it has large behavioural implications, in particular Inostrancevia would have been able to better hold on to prey with its mouth as its teeth were less likely to break from lateral stress. This idea of holding on to prey with its jaws is supported by its very robust mandible, in contrast smilodon actually has a fairly thin mandible. Inostrancevia and other gorgonopsians also had quite pronounced incisors. All the incisors are large, pointed, and serrated making them ideal for griping and tearing.Inostrancevia had very robust fore limbs, as can be seen in the above pictures. Its humerus in particular is very large with very well developed muscle attachment sites. Both radius and ulna are well developed indicating large lower arm muscles. The scapula is well developed for a therapsid although lacks some of the more advanced features that mammals have such as divisions made by a spine. The femur is long and thick, although somewhat flattened (note the extreme flatness of the limb elements in the photos are likely due to taphonomic reasons and they would not have been so thin in life). The tibia and fibula are both well developed and about equal in size indicating strong lower leg muscles. The long upper limb elements as well as the well-developed fibula and radius indicate that Inostrancevia was not a particularly efficient runner and could probably only run in short bursts. The stance of gorgonopsids is an interesting topic; while they most likely walked with their legs sprawled out like a lizard they had the ability to sustain a more erect stance, much like a crocodile’s “high walk”. This stance would require less lateral body movement, and rely more on limb based propulsion. This would have been a much more energy efficient method of obtaining higher speeds, and this idea would be carried on to their relatives the mammals, which have a completely erect posture. Another feature of the skeleton supporting this more erect running stance is the relatively inflexible spine, which would have limited the side to side movement lizards require for movement. Interestingly, as can be seen in the pictures, synapsid dorsal vertebrae started to differentiate into thoracic and lumbar sections towards the end of the Permian.The skeletal features of Inostrancevia paint a picture of a powerful carnivore. How gorgonopsids hunted and used those huge canines is still somewhat of a mystery. Their large, powerful forelimbs suggest gorgonopsids could have possibly subdued prey with them, much like modern cats. The large canines were obviously a key part in killing their prey, as through time the canines got proportionately larger. Perhaps they were the ecological equivalent of cats, stalking and ambushing prey, subduing them with powerful forelimbs and then finishing it off with a bite to the throat. Although the robust nature of the canines as well as the large incisors could indicate that gorgonopsids used their mouth and dentition to subdue prey. I think much more research into the biology and functional morphology of these creatures need to be done as there is shockingly little information out there.References Ivakhnenko, M. F. (2001). “Tetrapods from the East European Placket-Late Paleozoic Natural Territorial Complex”. Proceedings of the Paleontological Institute of the Russian Academy of Sciences (in Russian) 283: 1–200Gebauer, E.V.I. (2007 Phylogeny and evolution of the Gorgonopsia with a special reference to the skull and skeleton of GPIT/RE/7113 (‘Aelurognathus?’ parringtoni) (Ph.D. thesis). Tübingen: Eberhard-Karls Universität Tübingen. pp. 1–316.Kemp, T. (1969). On the Functional Morphology of the Gorgonopsid Skull. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences (1934-1990), (801), 1-83. -- source link
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