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''Thliptosaurus'' (meaning "compressed lizard") is an
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of small kingoriid
dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivores that typically bore a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, t ...
from the latest
Permian period The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
of the
Karoo Basin The Karoo Supergroup is the most widespread stratigraphic unit in Africa south of the Kalahari Desert. The supergroup consists of a sequence of units, mostly of nonmarine origin, deposited between the Late Carboniferous and Early Jurassic, a per ...
in
KwaZulu-Natal KwaZulu-Natal (, also referred to as KZN) is a Provinces of South Africa, province of South Africa that was created in 1994 when the government merged the Zulu people, Zulu bantustan of KwaZulu ("Place of the Zulu" in Zulu language, Zulu) and ...
,
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
. It contains the
type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...
and only known species ''T. imperforatus''. ''Thliptosaurus'' is from the upper ''Daptocephalus'' Assemblage Zone, making it one of the youngest Permian dicynodonts known, living just prior to the
Permian mass extinction The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the sixth and last period o ...
. It also represents one of the few small bodied dicynodonts to exist at this time, when most other dicynodonts had large body sizes and many small dicynodonts (such as ''
Diictodon ''Diictodon'' (meaning "two weasel teeth") is an extinct genus of pylaecephalid dicynodont that lived during the Late Permian period, approximately 255 million years ago. Fossils have been found in the ''Cistecephalus'' Assemblage Zone of the M ...
'') had gone extinct. The unexpected discovery of ''Thliptosaurus'' in a region of the Karoo outside of the historically sampled localities suggests that it may have been part of an endemic local fauna not found in these historic sites. Such under-sampled localities may contain 'hidden diversities' of Permian faunas that are unknown from traditional samples. ''Thliptosaurus'' is also unusual for dicynodonts as it lacks a
pineal foramen A parietal eye (third eye, pineal eye) is a part of the epithalamus in some vertebrates. The eye is at the top of the head; is photoreceptive; and is associated with the pineal gland, which regulates circadian rhythmicity and hormone production ...
, suggesting that it played a much less important role in
thermoregulation Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature ...
than it did for other dicynodonts. While the name literally translates to "compressed lizard", referring to the severely crushed and flattened skull of the only known specimen, it also alludes to the Biblical '' thlipsis'', a time of tribulation and hardship preceding the
End Times Eschatology (; ) concerns expectations of the end of present age, human history, or the world itself. The end of the world or end times is predicted by several world religions (both Abrahamic and non-Abrahamic), which teach that negative world ...
, alluding to the existence of ''Thliptosaurus'' just before the mass extinction at the end of the Permian, also known as the Great Dying.


Description

''Thliptosaurus'' was a small dicynodont (skull length of only long), but it displays several characteristics indicative of maturity in dicynodonts. Only the skull is known, but like other related dicynodonts it would have been a squat, sprawling quadruped with a short tail and a large head with a
tortoise Tortoises ( ) are reptiles of the family Testudinidae of the order Testudines (Latin for "tortoise"). Like other turtles, tortoises have a shell to protect from predation and other threats. The shell in tortoises is generally hard, and like o ...
-like beak. ''Thliptosaurus'' appears completely toothless, lacking even in the distinctive tusks found in some other dicynodonts (including the closely related ''
Dicynodontoides ''Dicynodontoides'' is a genus of small to medium-bodied, herbivorous, emydopoid dicynodonts from the Late Permian. The name ''Dicynodontoides'' references its “dicynodont-like” appearance (dicynodont = two-dog-tooth) due to the caniniform t ...
''), although teeth may be obscured by the lower jaw if they are present. The post cranial skeleton is unknown, but probably resembled that of other kingoriids, such as ''
Kombuisia ''Kombuisia'' (from Afrikaans " kombuis", meaning "kitchen", from a nickname for its discover James Kitching) is an extinct genus of dicynodont from Early to Middle Triassic (Induan to Anisian) of South Africa and Antarctica. Two species are de ...
''.


Skull

The skull of ''Thliptosaurus'' is fairly typical for dicynodonts, being broad at the back with very large
temporal fenestra Temporal fenestrae are openings in the temporal region of the skull of some amniotes, behind the orbit (eye socket). These openings have historically been used to track the evolution and affinities of reptiles. Temporal fenestrae are commonly (al ...
e, with a much shorter snout that gently tapers to a blunt point. The nostrils aren't visible on the specimen, meaning they must have been placed on the very front of the snout facing forwards, and not on the sides. The tuskless caniniform process of the
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
is prominent and sits well forward of the eyes. The front margin of the caniniform process is long and slopes gently downwards from the front, while the back margin rises steeply up to meet with the
zygomatic arch In anatomy, the zygomatic arch (colloquially known as the cheek bone), is a part of the skull formed by the zygomatic process of temporal bone, zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the ...
below the eyes. The caniniform process is similar to that of ''Dicynodontoides'', but is lacking the tusks found in that genus. No other teeth are visible on the specimen, but as much of the maxilla is covered by the lower jaw it is impossible to be sure. The
nasal bone The nasal bones are two small oblong bones, varying in size and form in different individuals; they are placed side by side at the middle and upper part of the face and by their junction, form the bridge of the upper one third of the nose. Eac ...
s sport three prominent well-pitted bosses with a rough, rugose texture. These bosses resemble those of ''Dicynodontoides'', with two flaring laterally over the front of the snout, split by the single fused
premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...
, and meeting with a third medial boss behind them. The intertemporal region, or bar, is unusually wide for a kingoriid, composed of the broad
frontal bone In the human skull, the frontal bone or sincipital bone is an unpaired bone which consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bo ...
s in the front and the flat exposed
parietal bone The parietal bones ( ) are two bones in the skull which, when joined at a fibrous joint known as a cranial suture, form the sides and roof of the neurocranium. In humans, each bone is roughly quadrilateral in form, and has two surfaces, four bord ...
s behind them, each contacting with the single preparietal (a unique bone found in dicynodonts and some other therapsids) just behind the eyes. Very unusually for a dicynodont, ''Thliptosaurus'' has no visible pineal foramen, and the suture between the parietals appears uninterrupted for their whole length. Even if the parietal foramen existed and was obscured by deformation during fossilisation, it would have to have existed as a very thin slit between the parietals that did not contact the preparietal, which itself would be a highly unusual condition for a dicynodont. However, comparisons with other kingoriids indicate that the absence of the pineal foramen is likely a genuine feature. Unfortunately, the palate is mostly obscured by the tightly occluded lower jaw, leaving only the pterygoids and the basicranium (the part of the skull underneath the
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
) readily visible. The pterygoids are relatively long and narrow, and straight for much of their length like those of ''
Compsodon ''Compsodon'' is an extinct genus of dicynodont belonging to the superfamily Emydopoidea. Fossils have been found in the Balfour Formation of South Africa and the Madumabisa Mudstone of Zambia. Description ''Compsodon helmoedi'' was original ...
'' and ''Dicynodontoides''. The two pterygoids meet at a median pterygoid plate positioned relatively far back on the skull, which also results in the rami that contact the quadrate being angled at approximately 60 degrees to the skull's long axis. The
occiput The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone lies over the occipital lobes of the ...
is poorly preserved and few details can be discerned, but the overall structure appears typical for dicynodonts, as is the basicranium.


Mandible

The
mandibular symphysis In human anatomy, the facial skeleton of the skull the external surface of the mandible is marked in the median line by a faint ridge, indicating the mandibular symphysis (Latin: ''symphysis menti'') or line of junction where the two lateral ha ...
of the
dentaries In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...
is long, creating a long, flattened shovel-like beak tip. The rest of the jaws is otherwise roughly v-shaped, as is typical for dicynodonts. The dentary has a tall plate on its sides that obscures the external mandibular fenestra, an unusual condition amongst dicynodonts. The coronoid process (a site for jaw muscle attachment) is unusually short compared to that of other kingoriids. The dentary also has a short horizontal shelf about halfway up its side, representing a greatly reduced lateral dentary shelf (a site of attachment for the external lateral adductor, an important jaw muscle in dicynodont feeding). The reflected lamina of the angular is relatively short, surrounded by a concave lateral exposure on each side. The
articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two o ...
has a curved, convex upper surface that contacts the condyles of the quadrate and extends beyond them, facilitating palinal (backwards) motion of the lower jaw, typical of dicynodonts.


Ontogeny

The small size of ''Thliptosaurus'', combined with the
taphonomic distortion Taphonomy is the study of how organisms decay and become fossilized or preserved in the paleontological record. The term ''taphonomy'' (from Greek language, Greek , 'burial' and , 'law') was introduced to paleontology in 1940 by Soviet scientis ...
and
ontogenetic Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...
variability of some of its characteristic traits raised the possibility that the only known specimen of ''Thliptosaurus'' actually represented the juvenile of an unknown larger bodied animal, possibly even that of the contemporaneous kingoriid ''Dicynodontoides''. However, three traits typical of mature, fully grown dicynodonts were identified in the specimen. The snout is well ossified and fused together, with none of the irregular sutures observed in the juveniles of larger dicynodont species. There is no sign of a postfrontal bone, which is interpreted as fusing with the
postorbital bone The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some ...
during ontogeny in other dicynodonts. Thirdly, the preparietal is small relative to the size of skull, unlike the large preparietal found in juvenile dicynodonts. These traits indicate that the holotype of ''Thliptosaurus'' is not a juvenile of a larger taxon, despite its small size, and represents a mature, small bodied dicynodont.


History of discovery

The
holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...
and only known specimen of ''Thliptosaurus'', BP/1/2796, was discovered and collected by
palaeontologist Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...
James Kitching James William Kitching (6 February 1922 – 24 December 2003) was a South African vertebrate palaeontologist and regarded as one of the world’s greatest fossil finders. Career His work in the southern hemisphere, including Antarctica, led ...
in September, 1958 on an expedition at Stoffelton in western KwaZulu-Natal, near the town of Bulwer. This site records exposures of the Permian ''Daptocephalus'' Assemblage Zone (AZ) and
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
''Lystrosaurus'' Assemblage Zone across the Permo-Triassic boundary, and while of scientific importance has been relatively unexplored and under-sampled compared to more historically recognised localities in the Karoo Basin. Indeed, much of the specimens from this site were collected by Kitching himself, and many of them remain as yet unprepared and unstudied. The fossil can be recognised as belonging to the Permian ''Daptocephalus'' AZ by the light grey colour of the fossil bones and grey
siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...
matrix Matrix (: matrices or matrixes) or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the m ...
surrounding them (contrasted with the yellow bones and red matrix of Triassic fossils). The specimen was not described until 2019 by palaeontologist Christian Kammerer, who diagnosed it as its own new distinct genus of dicynodont. The holotype is held in the collections of the Evolutionary Studies Institute at the
University of the Witwatersrand The University of the Witwatersrand, Johannesburg (), commonly known as Wits University or Wits, is a multi-campus Public university, public research university situated in the northern areas of central Johannesburg, South Africa. The universit ...
,
Johannesburg Johannesburg ( , , ; Zulu language, Zulu and Xhosa language, Xhosa: eGoli ) (colloquially known as Jozi, Joburg, Jo'burg or "The City of Gold") is the most populous city in South Africa. With 5,538,596 people in the City of Johannesburg alon ...
in South Africa. The single skull of ''Thliptosaurus'' is nearly complete, but it has been severely crushed and flattened from top-to-bottom (dorsoventrally), and some broken areas of bone at the back of the skull have been restored using plaster. Details and sutures on the top of the skull are readily visible, but poor preservation and over-preparation have obliterated features on the sides and rear of the skull. The lower jaw is firmly occluded to the upper jaw, obscuring some details of the sides of the skull and many of the palate. The generic name is from the
Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...
' (Latinzed as ') and (Latinized as '), literally translated to mean "compressed lizard", referring to the flattened nature of the skull from being severely dorsoventrally compressed during fossilisation. The name also alludes to the Biblical ''thlipsis'', the name given to a period of great tribulations accompanying the End Times, as a play on the taxon's existence just before the Permian mass extinction, also known as the Great Dying. The specific name is from the
Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...
''imperforatus'', meaning "unpunctured", referring to the greatly reduced pineal foramen in the roof of the skull, if it is not entirely absent altogether.


Classification

''Thliptosaurus'' was found to be a member of the clade
Emydopoidea Emydopoidea is a group of Late Permian dicynodont therapsids. It includes the small-bodied '' Emydops'', '' Myosaurus'', and kingoriids, and the burrowing cistecephalids. Phylogeny Below is a cladogram modified from Macungo et al. (2022) show ...
in the family Kingoriidae, closely related to the genera ''Kombuisia'' and ''Dicynodontoides''. ''Thliptosaurus'' shares with other kingoriids a dentary plate that occludes and obscures the mandibular fenestra, an arcing anterior process of the lacrimal that contacts the nasal, cutting off the maxilla from contacting the
prefrontal bone The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern and ...
, and an absence of the postfrontal bone (also shared with other
kistecephalia Kistecephalia is a clade of dicynodont therapsids. The group was first named in 1894, and was reinstated as a clade in 2009. Kistecephalia is a stem-based taxon defined as all taxa more closely related to '' Cistecephalus microrhinus'' than '' E ...
ns). It shares an extremely reduced pineal foramen with the kingoriid ''Kombuisia frerensis'' and
cistecephalid Cistecephalidae is an extinct family (biology), family of dicynodont therapsids from the Late Permian of South Africa, India and Zambia. It includes the genera ''Cistecephalus'', ''Cistecephaloides'', and ''Kawingasaurus''. Cistecephalids are tho ...
''
Kawingasaurus ''Kawingasaurus'' is an extinct genus of dicynodont therapsid from the Late Permian Usili Formation of Tanzania. It is a member of the family Cistecephalidae, and like other cistecephalids it is thought to have been fossorial. It is a member o ...
''. However, it is distinguished from these genera by having a moderately broad intertemporal bar, intermediate in width between that of ''Kombuisia'' and ''Kawingasaurus''. The intertemporal bar also has broad, flat exposures of the parietals, a primitive trait for dicynodonts but that has been lost in other kingoriids. A
phylogenetic analysis In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical data ...
using the latest iteration of the data matrix from Kammerer (2018),. itself modified from Angielcyzk & Kammerer (2017), agreed with this interpretation. A simplified excerpt of the consensus tree from this analysis, focused in on the relationships of Emydopoidea, is shown in the
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
below: Kammerer noted that the recovery of ''
Myosaurus ''Myosaurus'' (Greek for “mice-like lizard,” with mys- meaning mouse, and -sauros meaning lizard) is a genus of dicynodont synapsids.Hammer, W. R., & Cosgriff, J. W. 1981. Myosaurus gracilis, an anomodont reptile from the Lower Triassic of An ...
'' as the
sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
to Kingoriidae was unusual, as it is usually recovered as a close relative of ''
Cistecephalus ''Cistecephalus'' is an extinct genus of dicynodont therapsid from the Late Permian of southern Africa (South Africa and Zambia). It was a small, specialised, burrowing dicynodont, possibly with habits similar to a modern mole. The head was fla ...
''. However, the support for this relationship was low, and so he did not regard this result with much significance. Similarly, the analysis recovered an 'expansive Endothiodontia', including several taxa not normally recovered in this clade, such as ''
Pristerodon ''Pristerodon'' is an extinct genus of dicynodont therapsid from the Late Permian of South Africa, Zambia and India. Paleobiology Brain and senses ''Pristerodon'' were among the earliest land animals able to hear airborne sound as opposed t ...
'' and
Endothiodontidae Endothiodontia is a clade of dicynodont therapsids that includes the family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relation ...
, as successive outgroups to Emdyopoidea, but the support for this was also low. Otherwise, the analysis recovered a generally similar topology to other recent analyses of this data matrix.


Taxonomic distinction

The small, crushed nature of the sole specimen of ''Thliptosaurus'' raised the possibility that it may belong to an existing genus, and the seemingly unique features are just taphonomic. The skull of ''Thliptosaurus'' was compared to similarly sized and crushed specimens of the other kingoriids ''Kombuisia'' and ''Dicynodontoides'', the latter of which co-existed with ''Thliptosaurus''. However, even when crushed flat, ''Thliptosaurus'' could be distinguished from the other two kingoriids by the greater width of the intertemporal bar, and the parietals remained obscured postorbitals in the other taxa while they are exposed in ''Thliptosaurus''. The pineal foramen was also still readily identifiable in the other kingoriid specimens, as would be expected for dorsoventral crushing. This implies that the characteristic broad temporal bar and absent pineal foramen of ''Thliptosaurus'' were not simply the result of taphonomic distortion, and are indeed genuine diagnostic characteristics of a distinct kingoriid taxon.


Palaeobiology


Loss of the pineal foramen

The seemingly total absence of the pineal foramen in ''Thliptosaurus'' is very unusual for dicynodonts, and it has otherwise only been recorded in the related kingoriid ''Kombuisia frerensis'' and cistecephalid ''Kawingasaurus'' (although its presence or absence is rarely intraspecifically variable in some other dicynodonts). The pineal foramen, supporting a
pineal gland The pineal gland (also known as the pineal body or epiphysis cerebri) is a small endocrine gland in the brain of most vertebrates. It produces melatonin, a serotonin-derived hormone, which modulates sleep, sleep patterns following the diurnal c ...
and pineal "third" eye is suggested to have played a role in thermoregulation for dicynodonts, allowing them to register changes in light levels throughout the day and yearly seasonal changes that would allow them respond and modify thermoregulatory behaviours, such as basking, as is recorded in modern
tuatara The tuatara (''Sphenodon punctatus'') is a species of reptile endemic to New Zealand. Despite its close resemblance to lizards, it is actually the only extant member of a distinct lineage, the previously highly diverse order Rhynchocephal ...
s and some
lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...
s. The absence of the pineal complex in ''Thliptosaurus'' then suggests it was playing a less important role in thermoregulation, if any at all, although the reasons for this are unclear. The independent loss of the pineal foramen in advanced
therocephalia Therocephalia is an extinct clade of therapsids (mammals and their close extinct relatives) from the Permian and Triassic periods. The therocephalians ("beast-heads") are named after their large skulls, which, along with the structure of their te ...
ns and
cynodont Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 Megaannum, mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extin ...
s across the Permo-Triassic boundary has been suggested to be a consequence of developing
endothermic An endothermic process is a chemical or physical process that absorbs heat from its surroundings. In terms of thermodynamics, it is a thermodynamic process with an increase in the enthalpy (or internal energy ) of the system.Oxtoby, D. W; Gillis, ...
(i.e. warm blooded) metabolisms, something inferred not to have occurred in the contemporary dicynodonts or
gorgonopsia Gorgonopsia (from the Greek Gorgon, a mythological beast, and 'aspect') is an extinct clade of Saber-toothed predator, sabre-toothed therapsids from the Middle Permian, Middle to the Upper Permian, roughly between 270 and 252 million years ago. ...
ns. The independent loss of the pineal foramen in ''Thliptosaurus'' and other kistecephalian dicynodonts may support an alternative hypothesis for the loss of the pineal foramen in therapsids. It's possible that changing global environmental conditions at the end of the Permian, such as global warming and the movement of the continents towards the equator, meant that less control was needed over thermoregulation in therapsids like ''Thliptosaurus'', decreasing the need for the pineal foramen until it was ultimately lost. This is similar to some modern tropical lizards, which have much reduced pineal eyes as they do not need to regulate their body temperature as well as lizards in more seasonal and temperate environments. These changes to therapsid thermoregulation influenced by the environment may then be more important to the acquisition of endothermy in mammal evolution than sometimes presumed.


Palaeoecology


Late Permian small dicynodonts

''Thliptosaurus'' was one of the few small-bodied dicynodonts (i.e. those with a skull length >15 cm) present in the latest Permian just before the Permian mass extinction, as they had suffered a major decline in diversity after their success in the earlier
Middle Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek ...
to early-
Late Permian Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudonym used by Dave Groh ...
. Small dicynodonts went from being over three times as numerous as all other
tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...
s combined in the ''Tropidostoma'' Assemblage Zone (primarily in the form of ''Diictodon'') to down to only 19.2% of total tetrapods in the lower ''Daptocephalus'' Assemblage Zone (AZ), and then to just 4% by the upper ''Daptocephalus'' AZ when ''Thliptosaurus'' had appeared. At the same time, larger species of dicynodonts were on the increase, occupying up to 64% of all tetrapods in the upper ''Daptocephalus'' AZ. The newly evolved ''
Lystrosaurus ''Lystrosaurus'' (; 'shovel lizard'; proper Ancient Greek is ''lístron'' ‘tool for leveling or smoothing, shovel, spade, hoe’) is an extinct genus of herbivorous dicynodont therapsids from the late Permian and Early Triassic epochs (arou ...
'' in particular was especially abundant at 34% of the total. The presence of a previously unknown small dicynodont in the Upper ''Daptocephalus'' AZ then is surprising. Despite their decline, small dicynodonts still survived the Permian mass extinction, as evidenced by the kingoriid ''Kombuisia'' and myosaurid ''Myosaurus'' from
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean (also known as the Antarctic Ocean), it contains the geographic South Pole. ...
and
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
. Both were present during the
Induan The Induan is the first age of the Early Triassic epoch in the geologic timescale, or the lowest stage of the Lower Triassic series in chronostratigraphy. It spans the time between 251.9 Ma and 249.9 Ma (million years ago). The Induan is so ...
of 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 251.9 Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which ...
, but while ''Myosaurus'' perished shortly after ''Kombuisia'' persisted into the earliest
Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...
of the
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epoch (geology), epochs of the Triassic period (geology), period or the middle of three series (stratigraphy), series in which the Triassic system (stratigraphy), system is di ...
. The ancestor-descendant relationships of these small dicynodonts to those of the Permian is unclear, but they do not appear to be directly related to any known Permian species. It has been suggested that the ancestors of the small Triassic dicynodonts (as well as the successful Triassic
Kannemeyeriiformes Kannemeyeriiformes is a group of large-bodied Triassic dicynodonts. As a clade, Kannemeyeriiformes has been defined to include the species '' Kannemeyeria simocephalus'' and all dicynodonts more closely related to it than to the species '' Lystro ...
) must have been present outside of the well-sampled historical localities of the Karoo Basin. The discovery of ''Thliptosaurus'' may support this idea, as it could indicate that local, endemic second-order faunas existed amongst the more widespread well known faunas of the ''Daptocephalus'' AZ. More crucially, it suggests that such 'missing diversities' may be present even in the under-sampled localities of otherwise historically important regions like the Karoo Basin.


KwaZulu-Natal palaeoenvironment

The environment of the upper ''Daptocephalus'' Assemblage Zone has been interpreted as well-drained floodplains, with a lower water table than the preceding wetter lower ''Daptocephalus'' AZ. This may have reduced the inferred marginal river and low undergrowth habitats that small dicynodonts are thought to have preferred, leading to their decline. Favourable habitats for small dicynodonts must have still existed, including in KwaZulu-Natal, as evidenced by ''Thliptosaurus''. In KwaZulu-Natal, ''Thliptosaurus'' co-existed with other more widespread fauna typical of the ''Daptocephalus'' AZ, namely the dicynodonts '' Daptocephalus'' itself and ''Lystrosaurus''. It also at least co-existed with the smaller dicynodont '' Kwazulusaurus'', which may represent another local endemic dicynodont species from KwaZulu-Natal (however, it's possibly a synonym of ''Lystrosaurus''), as well as the large predatory therocephalian ''
Moschorhinus ''Moschorhinus'' is an extinct genus of therocephalian synapsid in the family Akidnognathidae with only one species: ''M. kitchingi'', which has been found in the Late Permian to Early Triassic of the South African Karoo Supergroup. It was a larg ...
''. Other therapsids known from elsewhere in the latest upper ''Daptocephalus'' AZ include the larger dicynodonts ''
Oudenodon ''Oudenodon'' is an extinct genus of dicynodont. It was common throughout southern Africa during the Late Permian. Several species of ''Oudenodon'' are known. Both ''O. bainii'', the type species, and ''O. grandis'' are known from South Africa ...
'', '' Pelanomodon'' and '' Dinanomodon'', the therocephalian ''
Promoschorhynchus ''Promoschorhynchus'' is a genus of akidnognathid therocephalians from the Late Permian and Early Triassic of South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Afri ...
'' and remains of gorgonopsians.


References

{{Taxonbar, from=Q63100948 Emydopoidea Changhsingian genus first appearances Changhsingian genus extinctions Lopingian synapsids of Africa Permian South Africa Fossils of South Africa Fossil taxa described in 2019