''Thrinaxodon'' is an extinct
genus
Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
of
cynodont
The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
s, most commonly regarded by its species ''T. liorhinus'' which lived in what are now South Africa and Antarctica during the
Early Triassic
The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a un ...
. ''Thrinaxodon'' lived just after the
Permian–Triassic mass extinction event, its survival during the extinction may have been due to its burrowing habits.
Similar to other
therapsids
Therapsida is a major group of eupelycosaurian synapsids that includes mammals, their ancestors and relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including limbs that were oriented mor ...
, ''Thrinaxodon'' adopted a semi-sprawling posture, an intermediary form between the sprawling position of basal
tetrapod
Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...
s (still observed modern
Crocodilia
Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period ( Cenomanian stage) and are the closest living ...
) and the more upright posture present in current mammals.
[Blob R. 2001. Evolution of hindlimb posture in non-mammalian therapsids: biomechanical tests of paleontological hypotheses. 27(1): 14-38.] ''Thrinaxodon'' is prevalent in the fossil record in part because it was one of the few carnivores of its time, and was of a larger size than similar cynodont carnivores.
Description
![Thrinaxodon BW](https://upload.wikimedia.org/wikipedia/commons/0/02/Thrinaxodon_BW.jpg)
''Thrinaxodon'' was a small synapsid roughly the size of a fox
[ and possibly covered in hair. The dentition suggests that it was a carnivore, focusing its diet mostly on insects, small herbivores and invertebrates. Their unique secondary palate successfully separated the nasal passages from the rest of the mouth, allowing the ''Thrinaxodon'' to continue mastication without interrupting to breathe, an adaptation important for digestion.
]
Skull
The nasals of ''Thrinaxodon'' are pitted with a large number of foramina. The nasals narrow anteriorly and expand anteriorly and articulate directly with the frontals, pre-frontals and lacrimals; however, there is no interaction with the jugals or the orbitals. The maxilla of ''Thrinaxodon'' is also heavily pitted with foramina.[Estes R. 1961. Cranial anatomy of the cynodont ''Thrinaxodon'' ''liorhinus''. Museum of comparative Zoology, Harvard University, 125: 165-180.] The arrangement of foramina on the snout of ''Thrinaxodon'' resembles lizards such as ''Tupinambis
''Tupinambis'' is a lizard genus which belongs to the family Teiidae and contains eight described species. These large lizards are commonly referred to as tegus (''teiús'' in Portuguese). '' T. merianae'' (Argentine black and white tegu), '' T. ...
'' more than mammals, which bear a single large infraorbital foramen. As such, ''Thrinaxodon'' would have had non-muscular lips like those of lizards, not mobile, muscular ones like those of mammals. Without the infraorbital foramen and its associated facial flexibility, it is unlikely that ''Thrinaxodon'' would have had whiskers.[
On the skull roof of ''Thrinaxodon'', the fronto-nasal suture represents an arrow shape instead of the general transverse process seen in more primitive skull morphologies. The prefrontals, which are slightly anterior and ventral to the frontals exhibit a very small size and come in contact with the post-orbitals, frontals, nasals and lacrimals. More posteriorly on the skull, the parietals lack a sagittal crest. The cranial roof is the narrowest just posterior to the parietal foramen, which is very nearly circular in shape. The temporal crests remain quite discrete throughout the length of the skull. The temporal fenestra have been found with ossified fasciae, giving evidence of some type of a temporal muscle attachment.]
The upper jaw contains a secondary palate which separates the nasal passages from the rest of the mouth, which would have given ''Thrinaxodon'' the ability to breathe uninterrupted, even if food had been kept in its mouth. This adaptation would have allowed the ''Thrinaxodon'' to mash its food to a greater extent, decreasing the amount of time necessary for digestion. The maxillae and palatines meet medially in the upper jaw developing a midline suture. The maxillopalatine suture also includes a posterior palatine foramen. The large palatal roof component of the vomer in ''Thrinaxodon'' is just dorsal to the choana, or interior nasal passages. The pterygoid bones extend in the upper jaw and enclose small interpterygoid vacuities that are present on each side of the cultriform processes of the parasphenoids. The parasphenoid and basisphenoid are fused, except for the most anterior/dorsal end of the fused bones, in which there is a slight separation in the trabecular attachment of the basisphenoid.
The otic region is defined by the regions surrounding the temporal fenestrae. Most notable is evidence of a deep recess that is just anterior to the fenestra ovalis, containing evidence of smooth muscle interactions with the skull. Such smooth muscle interactions have been interpreted to be indicative of the tympanum and give the implications that this recess, in conjunction with the fenestra ovalis, outline the origin of the ear in ''Thrinaxodon''. This is a new synapomorphy
In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have ...
as this physiology had arisen in ''Thrinaxodon'' and had been conserved through late Cynodontia. The stapes contained a heavy cartilage plug, which was fit into the sides of the fenestra ovalis; however, only one half of the articular end of the stapes was able to cover the fenestra ovalis. The remainder of this pit opens to an "un-ossified" region which comes somewhat close to the cochlear recess, giving one the assumption that inner ear articulation occurred directly within this region.
The skull of ''Thrinaxodon'' is an important transitional fossil which supports the simplification of synapsid skulls over time. The most notable jump in bone number reduction had occurred between ''Thrinaxodon'' and ''Probainognathus'', a change so dramatic that it is most likely that the fossil record for this particular transition is incomplete. ''Thrinaxodon'' contains fewer bones in the skull than that of its pelycosaur
Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term ''mammal-like reptile'' had been used, and pelycosaur was considered an order, but this is no ...
ian ancestors.
Dentition
Data on the dentition of ''Thrinaxodon liorhinus'' was compiled by use of a micro CT scanner on a large sample of ''Thrinaxodon'' skulls, ranging between 30 and 96 mm in length. These dentition patterns are similar to that of ''Morganucodon
''Morganucodon'' (" Glamorgan tooth") is an early mammaliaform genus that lived from the Late Triassic to the Middle Jurassic. It first appeared about 205 million years ago. Unlike many other early mammaliaforms, ''Morganucodon'' is well represe ...
'', allowing one to make the assumption that these dentition patterns arose within Thrinaxodontidae and extended into the records of early Mammalia. Adult ''T. liorhinus'' assumes the dental pattern of the four incisors, one canine and six postcanines on each side of the upper jaw. This pattern is reflected in the lower jaw by a dental formula of three incisors, one canine and 7–8 postcanines on each side of the lower jaw. With this formula, one can make a small note that in general, adult Thrinaxodon contained anywhere between 44 and 46 total teeth.
Upper incisors in ''T. liorhinus'' assume a backwards directed cusp, and they go from being curved and pointed at their most distal point, and become broader and rounder as they reach their proximal insertion point into the premaxilla. The fourth upper incisor greatly resembles the form of that of a small canine; however, it is positioned too far anteriorly to be a functional canine. Lower incisors possess a very broad base, which is progressively reduced, heading distally towards the tip of the tooth. The lingual face of the lower incisors are most often concave while the labial face is often convex, and these lower incisors are oriented anteriorly, except in some cases for the third lower incisor, which can assume a more dorsoventral orientation. The incisors are, for the most part, single functional teeth encompassing a broad, cone-like morphology. The canines of ''T. liorhinus'' possess small dorsoventrally directed facets on their surfaces, by our understanding, these facets are related to occlusion. Each canine possesses a replacement canine located within the jaw, posterior to the existing canine, neither of the replacement or functional canine teeth possess any serrated margins only the small facets. It is important to note that the lower canine is directed almost vertically (dorsoventrally) while the upper canine is directed slightly anteriorly.
The upper and lower postcanines in ''T. liorhinus'' share some common features but also vary quite a fair amount in comparison to one another. The first postcanine (just posterior to the canine) is most often smaller than the other postcanines and is most often bicuspid. Including the first postcanine, if any of the other postcanines are bicuspid, then it is safe to assume that the posterior accessary cusp is present and that that tooth will not have any cingular or labial cusps. If, however, the tooth is tricuspid, then there is a chance of cingular cusps developing, if this occurs then the anterior cusp will be the first to appear and will be the most pronounced cusp. In the upper postcanines, there should be no occurrence of any teeth possessing more than 3 cusps, and there is no occurrence of any labial cusps on the upper postcanines. The majority of upper postcanines in the juvenile ''Thrinaxodon'' are bicuspid, while only one of these upper teeth are tricuspid. The upper postcanines of an intermediate (between juvenile and adult) ''Thrinaxodon'' are all tricuspid with no labial or cingular cusps. The adult upper postcanines retain the intermediate physiologies and possess only tricuspid teeth; however, it is possible for cingular cusps to develop in these adult teeth. The ultimate (posterior-most) upper canine is often the smallest of all canines in the entire jaw system. Little data is known of the juvenile and intermediate forms of the lower postcanines in ''Thrinaxodon'', but the adult lower postcanines all possess multiple (any value more than three) cusps as well as the only appearance of labial cusps. Some older specimens have been found that possess no multiple-cups lower canines, possibly a response to old age or teeth replacement.
''Thrinaxodon'' shows one of the first occurrences of replacement teeth in cynodonts. This was discerned by the presence of replacement pits, which are situated lingual to the functional tooth in the incisors and postcanines. While a replacement canine does exist, more often than not it is not erupted and the original functional canine remains.
Histology
The bone tissue of ''Thrinaxodon'' consists of fibro-lamellar bone, to a varying degree across all the separate limbs, most of which develops into parallel-fibred bone tissue towards the periphery. Each of the bones contains a large abundance of globular osteocyte lacunae which radiate a multitude of branched canaliculi. Ontogenetically early bones, of which are mostly fibro-lamellar tissue, possessed a large amount of vascular canals. These canals are oriented longitudinally within primary osteons that contain radial anastomoses. Regions consisting mostly of parallel-fibred bone tissue contain few simple vascular canals, in comparison to the nearby fibro-lamellar tissues. Parallel-fibred peripheral bone tissue are indicative that bone growth began to slow, and they bring about the assumption that this change in growth was due to the age of the specimen in question. Combine this with the greater organization of osteocyte lacunae in the periphery of adult ''T. liorhinus'', and we approach the assumption that this creature grew very quickly in order to reach adulthood at an accelerated rate. Before ''Thrinaxodon'', ontogenical patterns such as this had not been seen, establishing the idea that reaching peak size rapidly was an adaptively advantageous trait that had arisen with ''Thrinaxodon''.
Within the femur of ''Thrinaxodon'', there is no major region of the bone that is made of parallel-fibred tissues; however, there is a small ring of parallel-fibred bone within the mid-cortex. The remainder of the femur is made of fibro-lamellar tissue; however, the globular osteocyte lacunae become much more organized and the primary osteons assume less vasculature than many other bones as you begin to approach the subperiosteal surface. The femur contains very few bony trabeculae. The humerus differs from the femur in many regards, one of which being that there is a more extensive network of bony trabeculae in the humerus near the meduallary cavity of the bone. The globular osteocyte lacunae become more flattened as you get closer and closer to the midshaft of the humerus. While the vasculature is present, the humerus contains no secondary osteons. The radii and ulnae of ''Thrinaxodon'' represent roughly the same histological patterns. In contrast to the humerii and femora, the parallel-fibred region is far more distinct in the distal bones of the forelimb. The medullary cavities are surrounded by multiple layers of very poorly vascularized endosteal lamellar tissue, along with very large cavities near the medullary cavity of the metaphyses.
Discovery and naming
''Thrinaxodon'' was originally discovered in the ''Lystrosaurus Assemblage Zone
The ''Lystrosaurus'' Assemblage Zone is a tetrapod assemblage zone or biozone which correlates to the upper Adelaide and lower Tarkastad Subgroups of the Beaufort Group, a fossiliferous and geologically important geological Group of the Karoo Sup ...
'' of the Beaufort Group
The Beaufort Group is the third of the main subdivisions of the Karoo Supergroup in South Africa. It is composed of a lower Adelaide Subgroup and an upper Tarkastad Subgroup. It follows conformably after the Ecca Group and unconformably underlie ...
of South Africa. The genoholotype, BMNH R 511, was in 1887 described by Richard Owen
Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomist and paleontologist. Owen is generally considered to have been an outstanding naturalist with a remarkable gift for interpreting fossils.
Owe ...
as the plesiotype of ''Galesaurus
''Galesaurus'' (from the Greek roots for 'weasel' and 'lizard') is an extinct genus of carnivorous cynodont therapsid that lived between the Induan and the Olenekian stages of the Early Triassic in what is now South Africa. It was incorrectly cla ...
planiceps''. In 1894 it was by Harry Govier Seeley
Harry Govier Seeley (18 February 1839 – 8 January 1909) was a British paleontologist.
Early life
Seeley was born in London on 18 February 1839, the second son of Richard Hovill Seeley, a goldsmith, and his second wife Mary Govier. When his fat ...
made a separate genus with as type species
In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
''Thrinaxodon liorhinus''. Its generic name was taken from the Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
for "trident tooth", ''thrinax'' and ''odon''. The specific name is Latinised Greek for "smooth-nosed".
''Thrinaxodon'' was initially believed to be isolated to that region. Other fossils in South Africa were recovered from the Normandien and Katberg Formation
The Katberg Formation is a geological formation that is found in the Beaufort Group, a major geological group that forms part of the greater Karoo Supergroup in South Africa. The Katberg Formation is the lowermost geological formation of the Tark ...
s. It had not been until 1977 that additional fossils of ''Thrinaxodon'' had been discovered in the Fremouw Formation
The Fremouw Formation is a Triassic-age rock formation in the Transantarctic Mountains of Antarctica. It contains the oldest known fossils of tetrapods from Antarctica, including synapsids, reptiles and amphibians. Fossilized trees have also been ...
of Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
. Upon its discovery there, numerous experiments were done to confirm whether or not they had found a new species of Thrinaxodontidae, or if they had found another area which ''T. liorhinus'' called home. The first experiment was to evaluate the average number of pre-sacral vertebrae in the Antarctic vs African ''Thrinaxodon''. The data actually showed a slight difference between the two, in that the African ''T. liorhinus'' contained 26 presacrals, while the Antarctic ''Thrinaxodon'' had 27 pre-sacrals. In comparison to other cynodonts, 27 pre-sacrals appeared to be the norm throughout this sub-section of the fossil record. The next step was to evaluate the size of the skull in the two different discovery groups, and in this study they found no difference between the two, the first indication that they may in fact be of the same species. The ribs were the final physiology to be cross-examined, and while they portrayed slight differences in the expanded ribs, against one another, the most important synapomorphy remained consistent between the two, and that was that the intercostal plates overlapped with one another. These evaluations led to the conclusion that they had not found a new species of Thrinaxodontidae, but yet they had found that ''Thrinaxodon'' occupied two different geographical regions, which today are separated by an immense expanse of ocean. This discovery was one of many to support the idea of a connected land mass, and that during the early Triassic, Africa and Antarctica must have been linked in some way, shape or form.
Classification
''Thrinaxodon'' belongs to the clade Epicynodontia
Epicynodontia is a clade of cynodont therapsids that includes most cynodonts, such as galesaurids, thrinaxodontids, and Eucynodontia (including mammals). It was erected as a stem-based taxon by Hopson and Kitching (2001) and defined as the mo ...
, a subdivision of the greater clade Cynodontia. Cynodontia eventually led to the evolution of ''Morganucodon'' and all other mammalia. Cynodontia belongs to the clade Therapsida, which was the first major clade along the line of the Synapsida. Synapsida represents one of two major splitting points, under the clade Amniota, which also split into Sauropsida, the larger clade containing today's reptiles, birds and Crocodilia. ''Thrinaxodon'' represents a fossil transitional in morphology on the road to humans and other extant mammals.
Paleobiology
Ontogeny
There appear to be nine cranial features that successfully separate ''Thrinaxodon'' into four ontogenetic stages. The paper denotes that in general, the ''Thrinaxodon'' skull increased in size isometrically, except for four regions, one of which being the optic region. Much of the data assumes that the length of the sagittal crest increased at a greater rate in relation to the rest of the skull. The posterior sagittal crest to appear in an earlier ontogenetic stage than the more anterior crest had, and in conjunction with the dorsal deposition of bone, a unified sagittal crest had developed rather than having a single suture span the entire length of the skull.
The bone histology of ''Thrinaxodon'' indicates that it most likely had very rapid bone growth during juvenile development, and much slower development throughout adulthood, giving rise to the idea that ''Thrinaxodon'' reached peak size very early in its life.[Botha J. Chinsamy A. 2005. Growth patterns of ''Thrinaxodon'' ''liorhinus'', a non-mammalian cynodont from the lower Triassic of South Africa. Paleontology. 48(2): 385-394.]
Posture
The posture of ''Thrinaxodon'' is an interesting subject, because it represents a transition between the sprawling behavior of the more lizard-like pelycosaurs and the more upright behavior found in modern, and many extinct, Mammalia. In cynodonts such as ''Thrinaxodon'', the distal femoral condyle articulates with the acetabulum in a way that permits the hindlimb to present itself at a 45-degree angle to the rest of the system. This is a large difference in comparison to the distal femoral condyle of pelycosaurs, which permits the femur to be parallel with the ground, forcing them to assume a sprawling-like posture. More interesting is that there is an adaptation that has only been observed within Thrinaxodontidae, which allows them to assume upright posture, similar to that of early Mammalia, within their burrows. These changes in posture are supported by the physiological changes in the torso of ''Thrinaxodon''. Such changes as the first appearance of a segmented rib compartment, in which ''Thrinaxodon'' expresses both thoracic and lumbar vertebrae. The thoracic segment of the vertebrae contain ribs with large intercostal plates that most likely assisted with either protection or supporting the main frame of the back. This newly developed arrangement allowed for the appropriate space for a diaphragm, however, without proper soft tissue records, the presence of a diaphragm is purely speculative.
Burrowing
''Thrinaxodon'' has been identified as a burrowing cynodont by numerous discoveries in preserved burrow hollows. There is evidence that the burrows are in fact built by the ''Thrinaxodon'' to live in them, and they do not simply inhabit leftover burrows by other creatures. Due to the evolution of a segmented vertebral column into thoracic, lumbar and sacral vertebrae, ''Thrinaxodon'' was able to achieve flexibilities that permitted it to comfortably rest within smaller burrows, which may have led to habits such as aestivation or torpor. This evolution of a segmented rib cage suggests that this may have been the first instance of a diaphragm in the synapsid fossil record; however, without the proper soft tissue impressions this is nothing more than an assumption.[Brink A. Note on a new skeleton of ''Thrinaxodon liorhinus''. Abstract. 15-22.]
The earliest discovery of a burrowing ''Thrinaxodon'' places the specimen found around 251 million years ago, a time frame surrounding the Permian–Triassic extinction event
The Permian–Triassic (P–T, P–Tr) extinction event, also known as the Latest Permian extinction event, the End-Permian Extinction and colloquially as the Great Dying, formed the boundary between the Permian and Triassic geologic periods, as ...
. Much of these fossils had been found in the flood plains of South Africa, in the Karoo Basin
The Karoo Supergroup is the most widespread stratigraphy, stratigraphic unit in Africa south of the Kalahari Desert. The group (stratigraphy), supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Pennsylvan ...
. This behavior had been seen at a relatively low occurrence in the pre-Cenozoic, dominated by therapsids, early-Triassic cynodonts and some early Mammalia. ''Thrinaxodon'' was in fact the first burrowing cynodont that has been found, showing similar behavioral patterns to that of Trirachodon. The first burrowing vertebrate on record was the dicynodont synapsid ''Diictodon
''Diictodon'' is an extinct genus of pylaecephalid dicynodont. These mammal-like synapsids lived during the Late Permian period, approximately 255 million years ago. Fossils have been found in the ''Cistecephalus'' Assemblage Zone of the Mad ...
'', and it is possible that these burrowing patterns had passed on to the future cynodonts due to the adaptive advantage of burrowing during the extinction. The burrow of ''Thrinaxodon'' consists of two laterally sloping halves, a pattern that has only been observed in burrowing non-mammalian Cynodontia. The changes in vertebral/rib anatomy that arose in ''Thrinaxodon'' permit the animals to a greater range of flexibility, and the ability to place their snout underneath their hindlimbs, an adaptive response to small living quarters, in order to preserve warmth and/or for aestivation purposes.
A ''Thrinaxodon'' burrow contained an injured temnospondyl, ''Broomistega
''Broomistega'' is an extinct genus of temnospondyl amphibian in the family Rhinesuchidae. It is known from one species, ''Broomistega putterilli'', which was renamed in 2000 from ''Lydekkerina putterilli'' Broom 1930. Fossils are known from the ...
''. The burrow was scanned using a synchrotron
A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed p ...
, a tool used to observe the contents of the burrows in this experiment, and not damage the intact specimens. The synchrotron revealed an injured rhinesuchid, ''Broomistega'' ''putterilli'', showing signs of broken or damaged limbs and two skull perforations, most likely inflicted by the canines of another carnivore. The distance between the perforations was measured in relation to the distance between the canines of the ''Thrinaxodon'' in question, and no such relation was found. Therefore, we may assume that the temnospondyl found refuge in within the burrow after a traumatic experience and the ''T. liorhinus'' allowed it to stay in its burrow until they both ultimately met their respective deaths. Interspecific shelter sharing is a rare anomaly within the fossil record, this ''T. liorhinus'' shows one of the first occurrences of this type of behavior in the fossil record, and, however, we are currently unsure if the temnospondyl inhabited the burrow before or after the death of the nesting ''Thrinaxodon''.
See also
* List of therapsids
This list of therapsids is an attempt to create a comprehensive listing of all genera that have ever been included in the Therapsida excluding mammals and purely vernacular terms. The list includes all commonly accepted genera, but also genera tha ...
References
Further reading
* 69.
* 202–203.
{{Taxonbar, from=Q310940
Prehistoric cynodont genera
Early Triassic synapsids of Africa
Triassic South Africa
Fossils of South Africa
Prehistoric vertebrates of Antarctica
Triassic Antarctica
Fossils of Antarctica
Transitional fossils
Articles containing video clips
Fossil taxa described in 1894
Taxa named by Harry Seeley