Counillonia

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Counillonia
Temporal range: Late Permian or Early Triassic
~251  Ma
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Counillonia profile.png
Life restoration of Counillonia
Scientific classification OOjs UI icon edit-ltr.svg
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Suborder: Anomodontia
Clade: Dicynodontia
Infraorder: Dicynodontoidea
Genus: Counillonia
Olivier et al., 2019
Species:
C. superoculis
Binomial name
Counillonia superoculis
Olivier et al., 2019

Counillonia is an extinct genus of dicynodont therapsid from the area of Luang Prabang in Laos, Southeast Asia that lived at around the time of the Permian-Triassic boundary and possibly dates to the earliest Early Triassic. Its type and only known species is C. superoculis. Counillonia was related to the Triassic dicynodonts such as Lystrosaurus and the Kannemeyeriiformes that survived the Permian mass extinction, but it was more closely related to the Permian genus Dicynodon than to either of these lineages. Counillonia may then possibly represent another line of dicynodonts that survived the Permian mass extinction into the Triassic period, depending on its age. The discovery of Counillonia in Laos and its unexpected evolutionary relationships hint at the less well understood geographies of dicynodont diversity across the Permo-Triassic boundary outside of well explored regions like the Karoo Basin in South Africa.

Contents

Description

Counillonia was a medium-sized dicynodont (skull length of 16 centimetres (6.3 in)) currently known only from a single skull that's missing the lower jaws. [1] However, it likely resembled other closely related dicynodonts, particularly Dicynodon itself, and so was probably a squat, sprawling quadruped with a short tail and a large head with nearly toothless jaws and a tortoise-like beak, sporting a pair of prominent tusks. [2]

Skull

The skull of Counillonia is short and relatively slender in construction, with the typically broad temporal fenestra of most dicynodonts at the back (although they are relatively reduced for a dicynodont) and a short, narrow snout that comes to a squared-off beak tip. The caniniform process of the maxilla is short and directed anteriorly so that the tusks point somewhat forward as well as down, and sits entirely in front of the eyes. The upper jaw is completely toothless apart from these two tusks. The interorbital region between the eyes is narrow, and so the large orbits are characteristically directed upwards. The nostrils are large, occupying approximately 1/3 of the snout's surface, and sit low at the front of the snout above the short and fused premaxillae. The nasal bones near the front of the snout sport a single well-developed boss that sits almost right above the nostrils and the premaxilla. This boss is bordered on each side by wide, elongate depressions that extend up on to the frontal bones behind the nasals. The boss has a roughened, rugose texture, but the surface of the rest of the snout is too weathered to determine if they were similarly rugose.

Contacts between the bones are difficult to discern, and sutures are hardly visible on the surface of the skull, mostly between the contacts of the frontals and the surrounding bones. The pineal foramen (the opening for the "third eye") is oval and lies across the parietal bones just behind the preparietal and is positioned relatively far back in the last quarter of the skull. The braincase and rear of the skull is somewhat weathered, but they appear similarly constructed to those of other dicynodonts. The secondary palate formed by the premaxilla is unusually short and reduced, and their contact with the palatine bones is not visible. The palatines themselves preserve expanded, roughly textured pads that indicate the front of the roof of the mouth was covered in keratinous horn like the beak. [1]

History of discovery

The holotype and only specimen of Counillonia was discovered in the Purple Claystone Formation of the Luang Prabang Basin in northern Laos. This sedimentary unit mostly consists of purple silty-claystones mixed with layers of conglomerates and sandstone, as well as volcaniclastic sediments. Estimated dates for the age of the formation have ranged from the Late Permian to the Late Triassic or even the earliest Jurassic period. More recently, radiometric dating using U—Pb geochronology from detrital zircon has yielded a maximum age for deposition of 251.0 ± 1.4 Ma. [3] It has been further suggested that mixing and reworking of the sediments implies that the actual depositional age of the formation is possibly even younger than this date, and so likely placing it in the Early Triassic. [1]

However, the reliability of this date was contested by Jun Liu in 2020, who argued that based on biostratigraphy the Purple Claystone Formation should instead be regarded as Late Permian in age, comparing Counillonia to dicynodonts found in the 255-253 million year old Daptocephalus Assemblage Zone of South Africa. Furthermore, Liu argued that the conditions of the Permian mass extinction in equatorial regions between the palaeolatitudes where Laos was situated (such as high temperatures over 40 °C) would have been inhospitable for dicynodonts, and concluded that Counillonia instead likely pre-dates the extinction event for these reasons. [4]

The first dicynodont remains to be discovered in the Purple Claystone Formation was a single, poorly preserved partial skull discussed by French geologist Jean-Baptiste-Henri Counillon in 1896. [5] This skull was described in 1923 by another French geologist, Joseph Répelin, who named it as a new species of Dicynodon, "Dicynodon incisivum". [6] The incomplete and damaged nature of the skull made identification difficult, and it has been variously attributed to Dicynodon and Lystrosaurus due to a supposed resemblance to the latter. The specimen has since been lost, and the poor quality of the remaining illustrations of the skull are unsuitable for supporting the validity of the species, and "D. incisivum" has since been considered a nomen dubium . [1] [7] [8] As such, its relationships to other Purple Claystone dicynodonts like Counillonia remain unknown, despite the similarities the lost skull of "D. incisivum" and the type of Counillonia share with Dicynodon. [7]

More dicynodont remains were recovered by a Franco-Laotian expedition between 1993 and 2003 led by palaeontologist Philippe Taquet. Three skulls in particular were studied and briefly described in 2009 and were assigned to Dicynodon, tentatively as a new species, although this relationship was not tested and remained uncertain. [7] In 2019, the three skulls were more described in full detail and were recognised as representing two distinct new taxa, one of which, specimen LPB 1993–3, is the holotype for Counillonia. The other two skulls were assigned to another new genus, Repelinosaurus . The holotype was temporarily stored, prepared and studied at the Muséum National d'Histoire Naturelle in Paris, and is permanently housed at the Savannakhet Dinosaur Museum in Laos. [1]

LPB 1993-3 is relatively complete, although the left portion of the orbit is damaged and it is missing the stapes and quadrate bones, as well as poorly preserving the preparietal, prootic and epipterygoid bones. The top surfaces of the snout and head are also partly weathered and eroded. The genus was named in honour of the geologist Jean-Baptiste-Henri Conillon as the first person to recognise the presence of dicynodonts in Laos, while the species is from the Latin super (upward) and oculis (eyes) in reference to its upward-facing orbits. [1]

Classification

Preliminary studies of specimen LPB 1993-3 found it to be closely comparable to Dicynodon based on comparative anatomy. [7] A phylogenetic analysis was later performed when Counillonia was officially described, utilising the dataset of Angielcyzk & Kammerer (2017), [9] where Counillonia was found to be a "Dicynodon"-grade dicynodontoid forming a clade with Dicynodon and various other Dicynodon-like species. Amongst these similar species, Counillonia could be distinguished by three unique autapomorphies (derived traits): a relatively large median pterygoid plate, and a braincase with no intertuberal ridge on the basioccipital and distinct backwards-facing processes on the opisthotics. The occipital condyle that connects the skull with the spinal column is also unfused, a feature it only shares with Delectosaurus among the "Dicynodon"-grade taxa (in which they are otherwise fused). Counillonia also differs from its nearest phylogenetic and geographic relatives in various other combinations of features on the skull that further distinguish it from these other genera. Furthermore, despite not being found as particularly closely related to the existing valid species of Dicynodon, many of the genera within the "Dicynodon"-grade were formerly assigned to Dicynodon, and so the results of the phylogenetic analysis corroborate the initial identification. [1]

A simplified cladogram, an excerpt from the full analysis by Olivier and colleagues focused on the relationships of the "Dicynodon"-grade dicynodontoids, is shown below:

Dicynodontoidea

Rhachiocephalidae

Odontocyclops

Geikiidae

Basilodon

Syops

Lystrosauridae

Kannemeyeriiformes

Repelinosaurus

Shansiodontidae

Stahleckeriidae

Kannemeyeriidae

Gordonia

Dicynodon

Vivaxosaurus

Delectosaurus

Jimusaria

Euptychognathus

Sintocephalus

Counillonia

Daptocephalus

Peramodon

Turfanodon

Dinanomodon

"Dicynodon"-grade taxa

Unusually, their analysis recovered Counillonia and the other "Dicynodon"-grade taxa united as a clade sister to the Kannemeyeriiformes, and not as a grade of taxa leading up to Lystrosauridae and Kannemeyeriiformes, which are usually recovered as each other's sister taxa. In this analysis, Counillonia shares four synapomorphies with all the other members of this clade, and they all together share another two with the Kannemeyeriiformes. [1]

Another analysis performed in 2020 by Jun Liu found Counillonia to be the sister taxon to the contemporary Laotian dicynodont Repelinosaurus, together forming a clade with the newly described Chinese dicynodont Taoheodon . Liu identified a 'core-Dicynodon' clade containing these taxa, Dicynodon itself, and the Russian genera Delectosaurus and Vivaxosaurus . A simplified excerpt of the cladogram produced by Liu (2020) is shown below: [4]

Dicynodontoidea

Keyseria

Daqingshanodon

Sintocephalus

Basilodon

Syops

Euptychognathus

Lystrosaurus

Peramodon

Daptocephalus

Dinanomodon

Turfanodon

Dicynodon

Delectosaurus

Vivaxosaurus

Taoheodon

Counillonia

Repelinosaurus

Kannemeyeriiformes

"Core-Dicynodon" clade

Palaeoecology

In the Purple Clay Formation, Counillonia is currently only known to have co-existed with the basal kannemeyeriiform dicynodont Repelinosaurus and the semi-aquatic chroniosuchian tetrapod Laosuchus . [10] The only direct evidence of plants in the formation are preserved root traces in palaeosols, but a locality underlying the Purple Claystone Formation and above late Changhsingian (Late Permian) deposits preserves a rich and diverse palaeoflora. The sediments preserved indicate that the Purple Clay Formation was deposited in a braided river environment that gradually transitioned to an alluvial plain with ponds. [11] The region was volcanically active, as evidenced by the volcaniclastic rocks mixed in with the sediments of the formation. This appears to be associated with a volcanic arc that was formed as the then isolated Indochina Block where Laos was located approached the rest of the supercontinent Pangaea. [3]

Life restoration of the contemporary chroniosuchian Laosuchus. Laosuchus NT.png
Life restoration of the contemporary chroniosuchian Laosuchus .

Palaeobiogeography

The presence of typical Permian fauna like Counillonia at a time close to the Permian mass extinction may suggest that the Indochina Block, including the Laos region, may have acted as a refugium for Permian life across the Permo-Triassic boundary (similarly, plant diversities in nearby South China appear to have been relatively stable across the Permo-Triassic boundary). [1] [12] This could also be supported by the absence of "Dicynodon"-grade dicynodonts like Counillonia in other parts of the world, such as in the Karoo Basin of South Africa where they appear to have disappeared entirely. This could reflect a potential bias in the geographic sampling of Permo-Triassic dicynodonts that may hinder our understanding of how they evolved, such as potential Triassic aged "Dicynodon"-grade dicynodonts like Counillonia. [1]

Alternatively, if Counillonia is Late Permian in age, its presence in Laos would indicate that the Indochina Block was connected to the Southern and Northern China Blocks by this time. This is in contrast with previously inferred dates suggesting that these landmasses did not collide and connect with each other until the Triassic period. [4]

Related Research Articles

<span class="mw-page-title-main">Therapsid</span> Clade of tetrapods including mammals

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 more underneath the body, as opposed to the sprawling posture of many reptiles and salamanders.

<span class="mw-page-title-main">Dicynodont</span> Extinct clade of therapsids

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, typically toothless beak, unique amongst all synapsids. Dicynodonts first appeared in Southern Pangaea during the mid-Permian, ca. 270–260 million years ago, and became globally distributed and the dominant herbivorous animals in the Late Permian, ca. 260–252 Mya. They were devastated by the end-Permian Extinction that wiped out most other therapsids ca. 252 Mya. They rebounded during the Triassic but died out towards the end of that period. They were the most successful and diverse of the non-mammalian therapsids, with over 70 genera known, varying from rat-sized burrowers to elephant-sized browsers.

<span class="mw-page-title-main">Anomodont</span> Suborder of stem-mammals

Anomodontia is an extinct group of non-mammalian therapsids from the Permian and Triassic periods. By far the most speciose group are the dicynodonts, a clade of beaked, tusked herbivores. Anomodonts were very diverse during the Middle Permian, including primitive forms like Anomocephalus and Patranomodon and groups like Venyukovioidea and Dromasauria. Dicynodonts became the most successful and abundant of all herbivores in the Late Permian, filling ecological niches ranging from large browsers down to small burrowers. Few dicynodont families survived the Permian–Triassic extinction event, but one lineage (Kannemeyeriiformes) evolved into large, stocky forms that became dominant terrestrial herbivores right until the Late Triassic, when changing conditions caused them to decline, finally going extinct during the Triassic–Jurassic extinction event.

<i>Daptocephalus</i> Assemblage Zone

The Daptocephalus Assemblage Zone is a tetrapod assemblage zone or biozone found in the Adelaide Subgroup of the Beaufort Group, a majorly fossiliferous and geologically important geological Group of the Karoo Supergroup in South Africa. This biozone has outcrops located in the upper Teekloof Formation west of 24°E, the majority of the Balfour Formation east of 24°E, and the Normandien Formation in the north. It has numerous localities which are spread out from Colesberg in the Northern Cape, Graaff-Reniet to Mthatha in the Eastern Cape, and from Bloemfontein to Harrismith in the Free State. The Daptocephalus Assemblage Zone is one of eight biozones found in the Beaufort Group and is considered Late Permian (Lopingian) in age. Its contact with the overlying Lystrosaurus Assemblage Zone marks the Permian-Triassic boundary.

<i>Wadiasaurus</i> Extinct genus of dicynodonts

Wadiasaurus is an extinct genus of dicynodont from the family Kannemeyeria, that lived in herds from the early to Middle Triassic. Substantial fossorial evidence of W. indicus was recovered from Yerrapalli Formation of the Pranhita-Godavari valley, India, and it is so far the only Kannemeyeriid known for certain from India. The Kannemeyeriiformes underwent a significant diversification during the middle Triassic, with roughly 40 known species distributed worldwide. All Kannemeyeriiformes were medium to large bodied, graviportal herbivores with relatively erect posture and gait. Wadiasaurus indicus is currently the only known species of Wadiasaurus.

<i>Moghreberia</i> Extinct genus of dicynodonts

Moghreberia is an extinct genus of dicynodont predicted to have lived only in the mid-Triassic, primarily during the early middle Carnian and found only in the Angara Basin of Morocco. Moghreberia belonged to the Stahleckeriidae family, a group of anomodont therapsids and is most commonly known by its species Moghreberia nmachouensis. Its name is derived from the Arabic phrase al-Maghrib al-Aqsa meaning “the far west”, a term used by Arabic scholars to refer to the approximate region of Morocco, the area in which this animal’s fossil was first discovered. The extinction of many dicynodonts has been attributed to pressures of the Carnian Pluvial Episode, which occurred around 234-232 Ma and generated major ecological and climate changes for years to come.

<i>Myosaurus</i> Extinct genus of dicynodont from the lower Triassic

Myosaurus is a genus of Anomodontia in the order Therapsida. They are also classified as Dicynodontia, which is a subclade of Anomodontia. The Mysosaurus was a small, herbivorous reptile that existed around the early Triassic period. All of the fossils found of this species were found in Antarctica and South Africa. Compared to other fossils found from species that existed during this time, the Myosaurus is not common in the fossil record. This is due to a shortage of discovered fossils that possess characteristics unique to the Myosaurus. Notably, under 130 fossil fragments have been found that have been classified as Myosauridae, and almost all have been skulls. These skulls can be classified as Myosaurus because this species, unlike other dicynodonts, do not possess tusks or postfrontal teeth. The only species identified in the family Myosauridae is the Myosaurus Gracilis, or M. Gracilis. It should be recognized that the Myosaurus is almost always referred to as the M. Gracilis in scientific research.

<i>Biseridens</i> Extinct genus of therapsids

Biseridens is an extinct genus of anomodont therapsid, and one of the most basal anomodont genera known. Originally known from a partial skull misidentified as an eotitanosuchian in 1997, another well-preserved skull was found in the Qingtoushan Formation in the Qilian Mountains of Gansu, China, in 2009 that clarified its relationships to anomodonts, such as the dicynodonts.

<i>Dicynodontoides</i> Extinct genus of dicynodonts

Dicynodontoides is a genus of small to medium-bodied, herbivorous, emydopoid dicynodonts from the Late Permian. The name Dicynodontoides references its “dicynodont-like” appearance due to the caniniform tusks featured by most members of this infraorder. Kingoria, a junior synonym, has been used more widely in the literature than the more obscure Dicynodontoides, which is similar-sounding to another distantly related genus of dicynodont, Dicynodon. Two species are recognized: D. recurvidens from South Africa, and D. nowacki from Tanzania.

<i>Odontocyclops</i> Extinct genus of dicynodonts

Odontocyclops is an extinct genus of Dicynodonts that lived in the Late Permian. Dicynodonts are believed to be the first major assemblage of terrestrial herbivores. Fossils of Odontocyclops have been found in the Karoo Basin of South Africa and the Luangwa Valley of Zambia. The phylogenetic classification of Odontocyclops has been long under debate, but most current research places them as their own genus of Dicynodonts and being very closely related to Rhachiocephalus and Oudenodon.

<i>Tetragonias</i> Extinct genus of dicynodonts

Tetragonias is an extinct genus of dicynodont from the Anisian Manda Beds of Tanzania. With tetra meaning “four,” and goni meaning “angle,” the name references the square shape of the Tetragonias skull when viewed dorsally. Not to be confused with the plant Tetragonia,Tetragonias were dicynodont anomodonts discovered in the late 1960s by paleontologist A. R. I. Cruickshank in the Manda Formation. Only the type species, T. njalilus, has been recognized.

<i>Kombuisia</i> Extinct genus of dicynodonts

Kombuisia is a genus of dicynodont from Early to Middle Triassic of South Africa and Antarctica. Two species were described for the genus: Kombuisia frerensis (type) and Kombuisia antarctica.

<span class="mw-page-title-main">Venyukovioidea</span> Extinct infraorder of therapsids

Venyukovioidea is an infraorder of anomodont therapsids related to dicynodonts from the Permian of Russia. They have also been known as 'Venjukovioidea', as well as by the similar names 'Venyukoviamorpha' or 'Venjukoviamorpha' in literature. This in part owes to a misspelling by Russian palaeontologist Ivan Efremov in 1940 when he mistakenly spelt Venyukovia, the namesake of the group, with a 'j' instead of a 'y', which permeated through subsequent therapsid literature before the mistake was caught and corrected. The order Ulemicia has also been coined for a similar taxonomic concept in Russian scientific literature, which notably excludes Suminia and Parasuminia.

Daqingshanodon is an extinct genus of dicynodont therapsid from the Late Permian of Inner Mongolia, China. The type species D. limbus was described in 1989 from a single skull found in the Naobaogou Formation. Daqingshanodon belongs to a group of dicynodonts called cryptodonts. It is the smallest known cryptodont, and the only one known from China. Like other cryptodonts, it has a pair of rounded nasal bosses above its nostrils and a ridge of bone on the upper jaw called the postcaniniform process. Daqingshanodon has a pair of elongated, recurved tusks extending from its beak-like snout. It is distinguished from other dicynodonts by the presence of a distinct ridge running along the side of the skull from below the eye socket to the area around the tusks. The skull of Daqingshanodon is less than 10 centimetres (3.9 in) long, yet this specimen is thought to have been an adult on the basis of its well-developed nasal bosses.

<i>Jimusaria</i> Extinct genus of dicynodonts

Jimusaria is an extinct genus of dicynodont therapsid from the Late Permian (Changhsingian) Guodikeng Formation of China. The type species J. sinkianensis was originally named as a species of Dicynodon, the first from Asia, but was given its own genus in 1963 before being sunk back into Dicynodon in 1988. The genus was resurrected in 2011 by palaeontologist Christian Kammerer in a taxonomic revision of the genus Dicynodon. Jimusaria was a mid-sized dicynodont, and was similar in appearance to the South African Dicynodon, but differed from it in features such as its narrower snout.

<i>Turfanodon</i> Extinct genus of dicynodonts

Turfanodon is an extinct genus of dicynodont therapsid from the Late Permian Sunan, Guodikeng, and Naobaogou Formations of China. The holotype of T. bogdaensis was discovered between 1963-1964 and was originally named in 1973 by A. Sun with the type species Turfanodon bogdaensis, Turfanodon was reclassified as a junior synonym of the related Dicynodon in 1988 by G. M. King. T. bogdaensis remained a species of Dicynodon for over two decades before the genus was reinstated in 2011 in a revision of the taxonomy of Dicynodon by palaeontologist Christian Kammerer. A second species from Inner Mongolia, T. jiufengensis, was named in 2021 by palaeontologist Jun Liu from a nearly complete skeleton and other referred bones. Turfanodon was a relatively large dicynodont, and similar in appearance to the related Daptocephalus from South Africa.

<i>Laosuchus</i> Extinct genus of tetrapods

Laosuchus is an extinct genus of chroniosuchian known from the Permian-Triassic boundary of Asia. Two species have been named.

<i>Thliptosaurus</i> Extinct genus of dicynodonts

Thliptosaurus is an extinct genus of small kingoriid dicynodont from the latest Permian period of the Karoo Basin in KwaZulu-Natal, South Africa. It contains the type 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. 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 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, suggesting that it played a much less important role in thermoregulation than it did for other dicynodonts.

<i>Repelinosaurus</i> Extinct genus of dicynodonts

Repelinosaurus is an extinct genus of dicynodont from the Purple Claystone Formation of Luang Prabang in Laos, Southeast Asia that lived at around the time of the Permian-Triassic boundary and possibly dates to the earliest Early Triassic. Its type and only known species is R. robustus. Repelinosaurus was originally described as the earliest known kannemeyeriiform dicynodont, supporting the idea of a more rapid radiation of the Triassic kannemeyeriiform dicynodonts during the Early Triassic following the Permian mass extinction. However, it may alternatively be more closely related to the Permian Dicynodon. The discovery of a potential early kannemeyeriiform in an understudied locality like Laos highlights the importance of such places in dicynodont research, which has been largely focused on historically important localities such as the Karoo Basin of South Africa.

Taoheodon is an extinct genus of dicynodont therapsid from the Sunjiagou Formation in the Shanxi province of China, dated to the Wuchiapingian age of the Late Permian. Its type and only known species is T. baizhijuni. Taoheodon was a close relative of the well known Dicynodon, and may represent a biogeographical link between the South African Dicynodon and similar dicynodonts found in Laos.

References

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