Simorhinella

Simorhinella Temporal range: Middle Permian (late Capitanian) 262–260  Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Holotype specimen of Simorhinella (NHMUK 49422), a partial skull and jaws of a young juvenile, viewed from below
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Therapsida
Clade:	† Therocephalia
Family:	† Lycosuchidae
Genus:	† Simorhinella Broom, 1915
Species:	†S. baini
Binomial name
†Simorhinella baini Broom, 1915

Simorhinella (meaning "little pug-nose" in Greek) is an extinct genus of therocephalian therapsids from the Guadalupian (middle Permian) of South Africa. It is includes only a single species, Simorhinella baini, named by South African paleontologist Robert Broom in 1915. Broom named Simorhinella on the basis of a single small fossil from the British Museum of Natural History collected in 1878 that includes the skull and jaws from the eye sockets forward of a young juvenile. ^[1] The skull is unusual in that it has an extremely short and broad snout, unlike the longer and narrower snouts of most other early therocephalians. Because of the skull's distinctiveness, the classification of Simorhinella within Therocephalia was unclear during the 20th and early 21st centuries. ^[2] In 2014, the skull of a much larger therocephalian was described and identified as an adult of Simorhinella by Fernando Abdala and colleagues based on a unique combination of shared features, including a distinctive bony crest on the vomer of the palate found in both specimens. From its anatomy, they proposed that Simorhinella was closely related to the basal therocephalian Lycosuchus and so placed it in the family Lycosuchidae, though its precise evolutionary relationships remain untested. ^[3]

History of discovery

The holotype specimen of Simorhinella, NHMUK (formerly BM) 49422, was discovered by road engineer and geologist Thomas Bain some time in the late 19th century, and was acquired by the British Museum (later the Natural History Museum, London) in 1878. It was collected at the Weltevreden farm in the Prince Albert district of Western Cape Province in South Africa. ^[3] The specimen remained undescribed until Scottish-South African palaeontologist Robert Broom examined the British Museum's collection of carnivorous therapsids in 1915, wherein he named the specimen Simorhinella baini. Broom's description of the specimen was brief, but he correctly identified it as belonging to a young juvenile therocephalian. ^[1]

NHMUK 49422 comprises the front half of a skull and lower jaws up to and including the eye sockets. The specimen is weathered and its surfaces are cracked, such that Broom struggled to distinguish the sutures of individual bones. Nonetheless, Broom highlighted the unusual breadth and shortness of the snout, being broader across the eyes than it is long. Broom allied Simorhinella with other small therocephalians such as Ictidognathus and Scaloposaurus , which at the time were classified under the now defunct grouping Scaloposauria—an artificial collection of small therocephalians often named from juvenile specimens, like Simorhinella. ^[1]

The juvenile holotype of Simorhinella in viewed from above (eyes to the left), note the breadth of the snout

Little was written about Simorhinella in the decades following its description. This was in part because the specimen was incomplete and missing key parts of the skull (such as the intertemporal region) that would aid in identifying its relationship to other therocephalians, e.g. as Lieuwe Dirk Boonstra noted in 1954. ^[4] Indeed, Boonstra would later regard Simorhinella as a nomen dubium in his 1969 review of the Tapinocephalus Assemblage Zone fauna due to its juvenile nature. ^[5] The dissolution of Scaloposauria during this time and uncertainty over the validity of its genera led to further complications, but some authors, namely Christiane Mendrez in 1975, maintained the genus was distinct and upheld Simorhinella a valid taxon. ^[6] This uncertainty over its classification persisted into the early 21st century, even with advancements in understanding therocephalian systematics. ^[2]

In 2014, Fernando Abdala and colleagues described a large skull belonging to a lycosuchid therocephalian that was collected from the upper Tapinocephalus AZ, catalogued as BP/1/5592. They identified BP/1/5592 as an adult specimen of Simorhinella, despite the marked size difference—its snout measuring 182 millimetres (7.2 in) in length compared to 21 millimetres (0.83 in) in the holotype. Nonetheless, Abdala and colleagues identified a unique suite of characteristics that the two specimens share suggesting they are adult and juvenile of the same taxon, and thus determining that Simorhinella was a lycosuchid. BP/1/5592 was discovered by John Nyaphuli and Bruce S. Rubidge near farm Rheboksfontein 74, in the Victoria West district of Northern Cape Province, and is housed in the Evolutionary Studies Institute of the University of the Witwatersrand in Johannesburg, South Africa. It comprises a weathered skull missing the lower jaws and most of its teeth and a few bones from the postcranial skeleton. The skull of BP/1/5592 has suffered some dorso-ventral (top to bottom) compression, displacing some of the dorsal skull backwards relative to the base. ^[3]

Although both specimens come from widely separated localities, they were both discovered in regions of the southwestern part of the Karoo Basin that expose strata (rock layers) belonging to the upper Tapinocephalus Assemblage zone (AZ) of the Abrahamskraal Formation, one of several formations that make up the Beaufort Group in the Karoo Basin. ^[3] In 2020, the upper strata of the Tapinocephalus AZ (encompassing the stratigraphic range of Simorhinella) was formally defined as the Diictodon - Styracocephalus Subzone, which has been constrained to date between 262 and 260 million years ago during the late Capitanian stage of the Guadalupian (or middle Permian). ^[7]

Description

Simorhinella is a large therocephalian, with a skull approximately ~37 centimetres (15 in) long. This makes Simorhinella one of the largest known lycosuchids, and indeed among the largest of any therocephalians. It is only surpassed in dimensions by SAM-PK-9005 (holotype of the dubious lycosuchid Scymnosaurus major ), a partial lycosuchid skull with a snout length of 226 millimetres (8.9 in) compared to the 182 millimetres (7.2 in) long snout of Simorhinella. ^[3]

Skull and jaws

The skull of Simorhinella has typical lycosuchid characteristics; its snout is relatively low, short and broad with only five upper incisors and few postcanine teeth. ^[3] Unlike some other lycosuchid specimens, neither specimen of Simorhinella exhibits a pair of so-called "double canines", ^[a] and it only has a single erupted pair (although Broom did initially identify two pairs in the young holotype). ^[1] The ventral, tooth-bearing edge of the maxilla (alveolar margin) is notably convex, with a slightly concave curvature in front of the canine where it joins the premaxilla. Like Lycosuchus, the alveolar margin is separated from the sides of the maxilla by a discrete flange of bone, however it is less developed in Simorhinella compared to the sharp maxillary ridge or crest of Lycosuchus. ^[3]

The eye sockets (orbits) are relatively small and widely separated across the roof of the skull by the broad frontal bones, with a deep suborbital bar formed by the jugal underneath them. The frontals have a prominent ridge running down the middle between the eyes, flanked by a depression on each side. Behind the eyes, the temporal region is large (only slightly shorter than the snout at 133 millimetres (5.2 in) versus 182 millimetres (7.2 in)) with large temporal fenestra (openings in the skull where jaw muscles attached), although the zygomatic arches that border the fenestra are not preserved. The intertemporal bar between the fenestra is compressed and raised into a tall, strongly arched sagittal crest made up mostly of the parietal bones that rises up behind the pineal foramen ("third eye"), although the crest is relatively broader than that of Lycosuchus and uniformly wide. The jaw joint, or quadrate trochlea (formed by the quadrate and quadratojugal bones) is unusual in Simorhinella, with a single smooth, rounded surface instead of the typical condition in therocephalians (including Lycosuchus) where it is clearly divided into two lateral and medial condyles separated by a groove. ^[3]

The vomer of the palate is one of the most characteristic bones of Simorhinella due to the raised crest running down the middle for much of its length (along with smaller accessory ridges on either side). This crest is found in both specimens of Simorhinella but not in Lycosuchus, and is what most readily distinguishes the two genera. The front portion of the vomer divides the two choanae (the internal nostrils), which are relatively narrow and compressed by a bulbous internal extension of the maxilla around the canine roots. Behind the vomer are the paired pterygoid bones, which like other lycosuchids (but unlike other early therocephalians) each bear a row of well-developed palatal teeth on their transverse process—four on each in Simorhinella—while the rounded pterygoid boss at their centers lack teeth. The pterygoids surround the open interpterygoid vacuity between them, with prominent ventrally projecting ridges around its rim. This differs from Lycosuchus where the bounding ridges are directed down and outwards. On either side of the pterygoids and vomer are the palatine bones, which (like Lycosuchus) have a raised rugose region along their edges running parallel to the toothrow of the maxilla, bordered on the other side by a narrow ridge. ^[3]

Broom's illustration of the immature holotype

The holotype is much smaller but nonetheless shares the same characteristic traits in the preserved portions (most distinctively the vomerine crest), although its overall form and proportions are much more exaggerated. Like many other juvenile tetrapods, the orbits are much larger and the snout is shorter, measuring only 21 mm long from the preserved tip to the orbits but 28 mm across at the eyes. Indeed, the snout is so proportionately short that the toothrow extends beneath the orbits, despite having only three or four postcanines. Its sutures are also much more open, including a broad gap between the nasal bones, evidence of its very young age. It differs from the adult in that it only has four upper incisors, rather than five. ^[3] The mandible is poorly preserved, although the symphysis where the two sides meet is broad and Broom's original description in 1915 suggested it was potentially deep as well. He identified three incisors, one canine and three postcanines in each mandible. ^[1]

Postcranial skeleton

Only a small part of the postcranial skeleton is known for Simorhinella, namely three portions of the vertebral column (one from the neck, rear of the back, and tail), a partial scapula and an ulna from the right shoulder and forelimb. The scapula is a tall and narrow bone, similar in shape to those of other therocephalians. A notable feature is a rearward protuberance above the glenoid (shoulder joint) that was likely the attachment of the triceps muscle. This protuberance is uncommon in therocephalians but is seen in other lycosuchid specimens (including the holotype of the dubious Zinnosaurus ). The inner surface is split by a prominent ridge, possibly marking out muscle attachments. ^[9] The ulna measures 149.6 millimetres (5.89 in) long and is fairly straight and stout with a short and broad olecranon process at the elbow. Such an olecranon is distinctive from other therocephalians, in which it is typically absent or only weakly developed, but is also seen in the large lycosuchid SAM-PK-9005 ("S. major"). Its presence in large lycosuchids like Simorhinella may then be due to its large body size. The ulna is marked by two well developed muscular fossae on both its inner and outer faces where flexor and extensor muscles attached, respectively. ^[3]

Classification

Simorhinella has historically been a difficult taxon to classify, largely due to it only being known by one specimen of a young juvenile for almost a century. In 1915, Broom originally allied it with other small therocephalians traditionally grouped together in the wastebasket taxon Scaloposauria (or Scaloposauridae depending on the classification scheme). "Scaloposaurs" have since been disbanded, with the group being recognised as a polyphyletic collection of various small therocephalians and juveniles of disparate lineages artificially grouped together. ^[10] This brought the potential validity of Simorhinella into question, but the taxon was upheld by Mendrez in 1975, who considered it distinct enough to allocate it to its own monotypic family (albeit one of uncertain classification) as the sole member of Simorhinellidae. Its relationships to other therocephalians, however, remained ambiguous. ^[6]

Simorhinella was systematically identified as a member of the family Lycosuchidae by Abdala and colleagues after they re-assessed the taxon in 2014, as it possesses a suite of traits that are considered diagnostic of the group. These include skull proportions (a short, broad snout, with a wide skull roof between the eyes and deep suborbital bar), tooth count (five or fewer pairs of incisors and postcanines each with no precanines in the maxilla), and palatal anatomy (presence of a single row of teeth on the transverse process of the pteygoid but none on the prominent and round pterygoid boss, a large interpterygoid vacuity with tubercle in front of it, a rugose maxillary ridge on the palatine, and choanae constricted by an internal maxillary boss around the canines). Many of these traits are shared with Lycosuchus, and Simorhinella is predominantly distinguished from it by the bony crest of the vomer. Other traits that distinguish simorhinella from Lycosuchus include the uniformly wide sagittal crest, a proportionately narrower vomer behind the choanae, which are themselves more greatly pinched by the canine boss, and a smooth quadrate trochlea without clearly divided lateral and medial condyles. ^[3]

However, this assignment was only comparative, and Abdala and colleagues did not perform a phylogenetic analysis to test this hypothesis cladistically. Notably, Abadala and colleagues highlighted some features of Simorhinella that are seen in the more derived scylacosaurids and eutherocephalians (i.e. the clade Scylacosauria) but not in Lycosuchus, its presumed closest relative. These include the characteristic crest on the vomer, as well as other features of the palate such as the vertical orientation of the bony walls of the interpterygoid vacuity. These raise the possibility that Simorhinella is in fact more closely related to scylacosaurians than it is to Lycosuchus (a position that would render Lycosuchidae as currently recognised paraphyletic). ^[3] Simorhinella has yet to be included in a phylogenetic analysis of therocephalians to test this possibility. ^[8]

Palaeoecology

Simorhinella was one of many carnivorous therocephalians that coexisted in the Diictodon-Styracocephalus Subzone of what would become the upper Abrahamskraal Formation. Among them, its known fossil range overlaps with its close relative Lycosuchus—although the latter only appears near the top of the assemblage zone and so only towards the end of the known range of Simorhinella. ^[3] All other contemporary therocephalians are the mostly mid-to-large scylacosaurids, although of them only Glanosuchus is known to approach similar skull lengths to Simorhinella. Other scylacosaurids are the large Alopecognathus , Maraisaurus , Pristerognathus , and Scylacosaurus (with maximum skull lengths over 30 centimetres (12 in)), mid-sized Alopecodon (26 centimetres (10 in)) and the smaller Pardosuchus (17 centimetres (6.7 in)). ^[11] Early therocephalians were both abundant and species-rich, and so were important members of the Tapinocephalus AZ predator guild, although even large genera like Simorhinella were still subordinate to the largest carnivore in the environment, the giant dinocephalian Anteosaurus . Small therapsid carnivores are represented by the diminutive gorgonopsian Eriphostoma and the basal biarmosuchian Hipposaurus , alongside the varanopid "pelycosaur" Heleosaurus . ^[7]

The largest herbivores in the subzone are predominantly tapinocephalian dinocephalians, represented by twelve genera. These include the tapinocephalids Agnosaurus , Criocephalosaurus , Mormosaurus , Moschognathus , Moschops , Riebeeckosaurus , Struthiocephalus , Struthionops , and Tapinocephalus , the two titanosuchids Jonkeria and Titanosuchus (though these forms may have been omnivorous), as well as Styracocephalus . The only other large herbivores known are the bradysaurians, basal pareiasaurs, namely Bradysaurus , Embrithosaurus , and Nochelesaurus . The roles of medium-sized to small herbivores were occupied mostly by anomodonts, including the diminutive "dromasaurs" Galechirus , Galepus and Galeops , as well as a variety of dicynodonts such as Brachyprosopus , Colobodectes , Emydops , Pristerodon and the smaller pylaecephalids Diictodon , Eosimops , Prosictodon and Robertia . There were also some small reptiles, such as the millerettid Broomia , procolophonomorph Australothyris , and the enigmatic Eunotosaurus . ^[7]

The depositional environment of the Diictodon-Styracocephalus SZ was an alluvial plain deposited by the river channels and surrounding floodplains of high-energy braided river systems draining from the Cape Fold Mountains from the south and southwest and flowing north and northeast towards the Ecca Sea. The Ecca Sea was receding during this time, and the shore progradated some 400 km to the northeast across the duration of the subzone's deposition. The climate was semi-arid with seasonally fluctuating rainfall, leading to flash flooding of the perennial rivers and ephemeral flow in smaller distributary channels, with the occasional overbank crevasse splays from the rivers. The freshwater environment supported two semi-aquatic rhinesuchid temnospondyl amphibians ( Rhinesuchus and Rhinesuchoides ) and a variety of fish (the palaeoniscid Atherstonia , Bethesdaichthys , Blourugia , the acrolepid Namaichthys , and Westlepis ), as well as the bivalve Palaeanodonta . Vegetation in this environment was dominated by the woody seed plant Glossopteris and equisetales (horsetails), such as Schizoneura , Phyllotheca and Paraschizoneura . Other woody trees of uncertain affinities are also represented by the trunks of Australoxylon and Prototaxoxylon . ^[7]

Extinction

Fossils of Simorhinella do not reach at the very top of the Abrahamskraal Formation, disappearing from the fossil record in interval corresponding to the Capitanian mass extinction event that occurred approximately 260 million years ago. This mass extinction wiped out much of the tetrapod diversity in the Tapinocephalus AZ, especially large species, including all of the dinocephalians and pareiasaurs. Large early therocephalians like Simorhinella were similarly effected, which disappeared alongside most of the scylacosaurids (of which only Alopecognathus and Pristerognathus survived). Curiously though, its close relative Lycosuchus not only survived the extinction phase but persisted into the recovery phase of the proceeding Endothiodon Assemblage Zone while Simorhinella perished. That some large therocephalians survived suggests that body size alone was not the sole determining factor in the extinction of other large therocephalians like Simorhinella. ^[12]

Notes

1. "Double canines", i.e. two distinct pairs of simultaneously functional canine teeth, were once thought to be a defining characteristic of lycosuchids. However, it has since been realised that this condition represents the overlapping presence of alternating functional and replacement canines. An alternating pattern of replacement is common amongst predatory therapsids (such as gorgonopsians), though replacement canines co-occur with the functional predecessor much more often in lycosuchids than in other therapsids. ^[8]

References

1. Broom, R. (1915). "On some new Carnivorous Therapsids in the Collection of the British Museum". Proceedings of the Zoological Society of London . 85 (2): 163–173. doi:10.1111/j.1469-7998.1915.tb07409.x.
2. Abdala, F.; Rubidge, B. S.; van den Heever, J. (2008). "The Oldest Therocephalians (Therapsida, Eutheriodontia) and the Early Diversification of Therapsida". Palaeontology . 51 (4): 1011. doi: 10.1111/j.1475-4983.2008.00784.x .
3. Abdala, F.; Kammerer, C. F.; Day, M. O.; Jirah, S.; Rubidge, B. S. (2014). "Adult morphology of the therocephalian Simorhinella baini from the middle Permian of South Africa and the taxonomy, paleobiogeography, and temporal distribution of the Lycosuchidae". Journal of Paleontology . 88 (6): 1139. doi:10.1666/13-186. S2CID   129323281.
4. Boonstra, L. D. (1954). "XXVII.—A Scaloposaurid from the Tapinocephalus-zone". Annals and Magazine of Natural History (Journal of Natural History). 12. 7 (74): 153–157. doi:10.1080/00222935408651711.
5. Boonstra, L. D. (1969). "The Fauna of the Tapinocephalus Zone (Beaufort Beds of the Karoo)". Annals of the South African Museum . 56 (1): 1−73.
6. Mendrez, C. H. (1975). "Principales variations du palais chez les therocephales Sud-Africains (Pristerosauria et Scaloposauria) au cours du Permien Superieur et du Trias Inferieur". Problemes actuels de paleontologie-evolution des Vertebres (in French). Vol. 218. Colloque International CNRS. pp. 379–408.
7. Day, M. O.; Rubidge, B. S. (2020). "Biostratigraphy of the Tapinocephalus Assemblage Zone (Beaufort Group, Karoo Supergroup), South Africa". South African Journal of Geology . 123 (2): 149–164. Bibcode:2020SAJG..123..149D. doi:10.25131/sajg.123.0012. S2CID   225815517.
8. Pusch, L. C.; Ponstein, J.; Kammerer, C. F.; Fröbisch, J. (2020). "Novel Endocranial Data on the Early Therocephalian Lycosuchus vanderrieti Underpin High Character Variability in Early Theriodont Evolution". Frontiers in Ecology and Evolution . 7: 1–27. doi: 10.3389/fevo.2019.00464 .
9. Bishop, P. J.; Pierce, S. E. (2023). "The fossil record of appendicular muscle evolution in Synapsida on the line to mammals: Part I—Forelimb". The Anatomical Record . 307 (5): 1764–1825. doi:10.1002/ar.25312. PMID   37726984. S2CID   262068960.
10. Huttenlocker, A. K.; Botha, J.; Browning, C.; Kulik, Z.; Tshibalanganda, M.; du Plessis, A. (2022). "A Gulliver Scaloposaurus (Therapsida, Therocephalia) from the Katberg Formation of South Africa and its implication for Lilliput assemblages during the Early Triassic recovery". Journal of African Earth Sciences . 196. 104720. doi:10.1016/j.jafrearsci.2022.104720.
11. Kammerer, C. E. (2023). "Revision of the Scylacosauridae (Therapsida: Therocephalia)". Palaeontologia africana. 56: 51–87. ISSN   2410-4418.
12. Day, M.O.; Rubidge, B.S. (2021). "The Late Capitanian Mass Extinction of Terrestrial Vertebrates in the Karoo Basin of South Africa". Frontiers in Earth Science. 9: 15. Bibcode:2021FrEaS...9...15D. doi: 10.3389/feart.2021.631198 .