Sunday 20 October 2019

Pseudotherium argentinus: A (possible) Mammaliamorph Cynodont from the Late Triassic Ischigualasto Formation of Argentina.

One of the major transformations in vertebrate evolution occurred in a series of events leading up to the origin of the Mammals. The transformation, which occurred by or before the Middle Jurassic, took place as several pulses of expansion of the relative size of the brain (encephalization) and the emergence of the uniquely mammalian neocortex.The shift in position and function of mammalian auditory ossicles were also part this transition. From their original position at the jaw joint and dual function in feeding and audition, the auditory ossicles became detached from the jaw and decoupled from feeding to take their characteristic mammalian position suspended beneath the otic capsule and functioned solely in hearing. The closest extinct relatives of the Mammals among the Cynodonts  extremely small, and it was not until the Cainozoic that the independent evolution of large body size began to are characterise various Mammal groups. Owing to their small size these fossils have proven difficult to prepare and to study in detail using conventional methods.

In a paper published in the journal PLoS One on 7 August 2019, Rachel Wallace of the Jackson School of Geosciences at the University of Texas at Austin, Ricardo Martínez of the División Paleontologia de Vertebrados at the Universidad Nacional de San Juan, and Timothy Rowe, also of the School of Geosciences at the University of Texas at Austin, describe a new species of Cynodont from the early Late Triassic Ischigualasto Formation of Valle Pintado in Ischigualasto Provincial Park, San Juan Province, Argentina.

The Ischigualasto Formation outcrops out in northwestern Argentina and forms part of the Ischigualasto-Villa Unión Basin. It comprises a sequence of fluvial channel sandstones with well-drained floodplain sandstones and mudstones. Interlayered volcanic ash layers above the base and below the top of the formation provide chronostratigraphic control and yielded ages of 231 and 226 million years old. The Ischigualasto Formation is divided into four members. The La Peña Member forms the lowest 40 m of the formation and consists of multistory channel sandstones and conglomerates covered by poorly-drained floodplain mudstones. The Cancha de Bochas member forms the next 140 m and is composed of thick, well-drained floodplain mudstones interbedded with high-sinuosity channel sandstones. The Valle de la Luna Member forms the next 470 m, and is mostly characterised by amalgamated high-sinuosity channels, abandoned channels and marsh deposits. Finally, the Quebrada de la Sal Member forms the uppermost 50 m of the Ischigualasto Formatio, and consists of tabular fluvial deposits.

 Geographic and geologic maps of the southern portion of the Ischigualasto-Villa Unión Basin. Wallace et al. (2019).

The Ischigualasto Formation is also divided from base to top into three abundance-based biozones. The Scaphonyx-Exaeretodon-Herrerasaurus biozone, which is characterised by a predominance of the Rhynchosaur Scaphonyx, the Cynodont Exaeretodon, and the Dinosaur Herrerasaurus, but also includes the majority of known fossils and the highest taxonomic diversity. The Exaeretodon biozone is characterised by low diversity and high relative abundance of the Cynodont Exaeretodon. The Jachaleria biozone is almost devoid of vertebrate fossils except for scarce specimens of the Dicynodont Jachaleria.

The specimen from which the new Cynodont species is described was found in 2006 by Ricardo Martínez during a field trip to the Ischigualasto Formation carried out by the Instituto y Museo de Ciencias Naturales of the Universidad Nacional de San Juan. It was found at the Valle Pintado locality, which is located in the upper levels of the La Peña Member and in the lower portion of the Scaphonyx-Exaeretodon-Herrerasaurus biozone, n a fossiliferous layer 40 m above the base of the Formation. This is one of the most fossiliferous horizons known in the Ischigualasto Formation, and a diverse and abundant fauna has been recovered from the same level, including several specimens of the Theropod Dinosaur Herrerasaurus, the type specimen of the basal Sauropodomorph Dinosaur Panphagia, plus various carnivorous and herbivorous Cynodonts, Rhynchosaurs, and Pseudosuchian Archosaurs.

The new species is named Pseudotherium argentinus, where 'Pseudotherium' means 'false beast' in Greek (the suffix '-therium', meaning 'beast' is commonly used to indicate Mammals in palaeontology), and 'argentinus' indicates the area where it was found. It is described from a single specimen which comprises an isolated skull that is missing the mandibles, most of the premaxillae, zygomatic arches, and quadrates. One incomplete stapes and one quadratojugal are preserved. Much of the superficial surface of the skull was exposed through manual preparation. Anatomical investigation of the interior utilise micro-computed tomography, and from these scans 3D printouts of enlarged models of the specimen were made to augment and extend observation of its surficial anatomy.

Pseudotherium argentinus, a comparison of image processing methods based on utilize micro-computed tomography scans. (A) Isosurface rendering; (B) volume rendering, scattering algorithm, no digital matrix removal. Wallace et al. (2019).

Pseudotherium argentinus is considered to be a Probainognathian Cynodont (the goup that includes all Mammals, as well as some non-Mammalian groups). It has a lacrimal bone that contributes extensively to the floor of the orbit; the frontal bone has a long orbital process that contacts a short orbital process of the palatine near the floor of the orbit; the orbital process of the palatine is low and contributes little to the orbital wall; the prefrontal bone is superficially large and extends anteriorly (forwards), medial to (inside) the lacrimal, contributing to the lateral wall of the nasopharyngeal passage (nasal trumpet); a vestige of the postorbital bone is preserved behind the orbit, but lacks an ossified postorbital bar; the interpterygoid vacuities (openings in the palete) remained open throughout life; laterally flaring parasphenoid alae (ridges on the paraspenoid bone) intersect at an obtuse angle between their contacts with the petrosal promontorium (protusion on the petrosal bone); there is a longitudinal ventral process on the basisphenoid; the lambdoidal crest strongly overlaps the occipital plate; there is a large, open spaces within the spongy bone of the parietal, petrosal, squamosal, basioccipital, basisphenoid, supraoccipital, and exoccipital surrounding the braincase; the vertical margin of the petrosal (prootic) lateral flange is notched; the upper canines are long, laterally compressed and non-serrated with a ridge on both their labial and lingual surfaces; there are nine upper postcanine teeth with the first postcanine consisting of a single cusp, while blunt, indistinct cusps form the crowns of the remaining postcanines.

Pseudotherium argentinus, digitally colored 3D volume renderings of the holotype. Skull in dorsal (top), right lateral (middle), and ventral (bottom) views. Abbreviations: al, alisphenoid; alqr, quadrate ramus of alisphenoid; bo, basioccipital; ce, cavum epiptericum; eo, exoccipital; fr, frontal; if, incisive fossa; ju, jugal; la, lacrimal; mx, maxilla; na, nasal; os, orbitosphenoid; pal, palatine; par, parietal pbc, parabasisphenoid complex; pet, petrosal (= periotic); pf, prefrontal; po, postorbital; pt, pterygoid; ptqr, quadrate ramus of pterygoid; smx, septomaxilla; so, supraoccipital; sq, squamosal; st, stapes; tb, tabular; vo, vomer. Wallace et al. (2019).

A number of features suggest that the holotype was approaching full skeletal maturity at time of death. The sagittal and lamdoidal crests are well developed; the orbit is relatively small compared to other skull proportions; the prootic and opisthotic are fused to form the petrosal; and extensive fusion has occurred between the basioccipital and exoccipitals, and between the tabular, supraoccipital, and interparietal. Additionally, a short diastema between the canine and the first postcanine suggests that a tooth had been shed and not replaced. There are also irregular wear facets on all of the postcanine tooth crowns. The only suggestions of immaturity include the presence of a pair of small un-erupted replacement postcanine crowns situated at the base of the right and left fourth postcanine roots, visible in the computed tomography scans, and possibly the presence of an interpterygoid vacuity.

Because the premaxillae and lower jaw of Pseudotherium are not preserved, the form and number of incisors are unknown. The upper canines are long and curved. The canines were displaced postmortem, and the fossil is distorted on its left side, further displacing the left canine. As a result, the long roots of the canines appear to erupt through the overlying maxilla where their roots are broken and eroded. The crown morphology of the canines is distinctive, being buccolingually compressed, and with ridges running nearly the length of the crown on both labial and lingual surfaces.

Line drawings of Pseudotherium argentinus holotype. (Top) Reconstructive drawing of fossil in dorsal view. Zygomatic arches depicted with dashed lines. Major cracks in the fossil specimen were avoided in the drawing. The more complete right half of the fossil was mirrored to reconstruct the less complete left half. Drawings of fossil in its original, preserved condition, depicted in right lateral (middle) and ventral (bottom) views. Abbreviations: al, alisphenoid; alqr, quadrate ramus of alisphenoid; bo, basioccipital; ce, cavum epiptericum; eo, exoccipital; fr, frontal; if, incisive fossa; ipv: interpterygoid vacuity; ju, jugal; la, lacrimal; mx, maxilla; na, nasal; os, orbitosphenoid; pal, palatine; par, parietal pbc, parabasisphenoid complex; pet, petrosal (periotic); pf, prefrontal; po, postorbital; pt, pterygoid; ptqr, quadrate ramus of pterygoid; smx, septomaxilla; so, supraoccipital; sq, squamosal; st, stapes; tb, tabular; vo, vomer. Wallace et al. (2019).

Pseudotherium and some related taxa of interest display a special kind of transitional mammalian characters. These are features such as the complex pattern of pterygopalatine troughs and ridges around the choana or the bifurcation of the paroccipital process, that are seen in the earliest fossil members of crown Mammalia, but that are subsequently so entirely transformed that nothing quite like them is found in extant Mammals. 

Wallace et al. conclude that Pseudotherium should lie within Mammaliamorpha, but also that it lacks a number of features that have been considered diagnostic of Mammaliamorpha in other analyses. Such features include several diagnostic derived character states that are present in the Tritylodontidae (a Cynodont group closly related to the Mammaliaformes) and basal Mammaliaformes, but which are lacking in Pseudotherium. For example, Pseudotherium retains vestigial prefrontal and postorbital bones, which are entirely absent within Mammaliamorpha. In the palate, Pseudotherium lacks the anterior extension of the ventral pterygoid keel onto the vomer, as is seen in Tritylodontids, Morganucodon (an early Mammaliaforme which is well known from a large number of specimens), and other mammaliaforms. Pseudotherium lacks fully divided postcanine tooth roots, another condition generally considered diagnostic of Mammaliamorpha. Additionally, Pseudotherium has an ossified medial orbital wall (as in Mammaliamorphs), but this wall fails to extend posteriorly to enclose the orbital fissure behind the orbit. The orbital fissure in basal Mammaliamorpha is almost completely closed by the orbitosphenoid and alisphenoid. Pseudotherium also lacks a floor beneath the cavum epiptericum (which held the trigeminal ganglion), which is at least partially present in Tritylodontids, and fully present in Mammaliaformes.

Despite this Pseudotherium shares a number of derived character states widely recognized as diagnostic of the Mammaliamorpha. The presence of such features in Pseudotherium may indicate that these character states are more widely distributed than previously believed, that they may be homoplastic (gained or lost independently in separate lineages over the course of evolution), or that their distribution is equivocal because of incompleteness of some of the other relevant taxa. In several
cases, these features can only be identified with certainty from computed tomography scans. Probably the most significant resemblance Pseudotherium shares with Mammaliamorphs is in its cranial endocast in which the cerebral hemisphers form tall, elongated domes separated by a deep interhemispheric sulcus. Pseudotherium shares with Mammaliamorpha ossification of the orbital wall (anterior portion of the orbital fissure), in which sheets of bone from the frontal and palatine join to provide a solid orbital wall (although it fails to fully close the orbital fissure behind the orbit). The Tritheledontids preserve a more plesiomorphic condition (condition closer to the ancestral state) in which both the orbital wall and orbital fissure remain broadly open. Pseudotherium also shares with Mammaliamorphs the loss of an intact postorbital arch that separates the orbit from the temporal fenestra (although Pseudotherium retains a vestigial postorbital bone).

As in mammaliamorphs, Pseudotheriuim has a secondary palate that extends to the back of the tooth row. The arrangement of bones surrounding the choana takes on a distinct configuration in which parabasisphenoid and pterygoid no longer form a single continuous ventral parasagittal ridge, and instead form parallel parasagittal ridges (pterygopalatine ridges) separated by a shallow trough which may mark the passage of the auditory (eustacean) tube from the nasopharynx to the middle ear. Broad parasphenoid alae are also present in Pseudotherium and in basal Mammaliamorphs. The condition of these characters in Tritheledontids has not been reported, but should be observable in computed tomography scans.

The discovery of Pseudotherium argentinus underscores the diversity of small Cynodonts in the Mid- to Late Triassic and highlights the acquisition of growing numbers of mammalian features as a distinctive feature of this radiation. Although the phylogenetic position of Pseudotherium is not fully resolved, it shares with the other ‘taxa of interest’ a number of novelties that link it closely to the origin and early diversification of Mammaliamorpha. Current evidence suggests that the evolution of endothermy, lactation, parental care, prolonged activity, and the beginnings of encephalization were the products of this segment of history, and that it played out in miniaturized Cynodonts.The current uncertainty on phylogenetic relationships among those taxa that have been referred to as Tritheledontids and Brasilodontids is based in part on incompleteness, and also on differing strategies for sampling taxa for analysis. It seems clear that resolving this phylogenetic ambiguity will more precisely elucidate the sequence of events culminating in the origin of Mammalia, and that computed tomography may be a key technology in providing character evidence for this phylogeny.

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