| Scymnosaurus | |
|---|---|
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| Illustrations of theholotype snout ofScymnosaurus ferox,SAM-PK-632, byRobert Broom in 1903 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Therapsida |
| Clade: | †Therocephalia |
| Family: | †Lycosuchidae |
| Genus: | †Scymnosaurus Broom,1903 (nomen dubium) |
| Type species | |
| †Scymnosaurus ferox | |
| Assignedspecies | |
Scymnosaurus is adubiousgenus oftherocephaliantherapsids from theMiddle Permian of what is nowSouth Africa based upon the fossils of large, butindeterminate, early therocephalians. The genus and itstype speciesS. ferox was named byRobert Broom in 1903, followed byS. watsoni in 1915. A thirdspecies,S. major, was named byLieuwe Dirk Boonstra in 1954, who also referred many more specimens to the genus asScymnosaurussp.
The genusScymnosaurus and its species are all considerednomina dubia today, meaning the fossils have no distinguishing (diagnostic) traits to define each species or to unite them together as a distinct genus. Indeed,Scymnosaurus includes specimens that have since been determined to belong to two separatefamilies of carnivorous early therocephalians. Most specimens, including those ofS. ferox andS. major, represent indeterminate (incertae sedis) specimens from the familyLycosuchidae, while the sole skull ofS. watsoni belongs to an indeterminate member ofScylacosauridae. ThoughScymnosaurus watsoni represents a scylacosaurid, because the type speciesS. ferox is identifiable as a lycosuchid, the genusScymnosaurus itself is regarded as a dubious lycosuchid.
Most specimens attributed toScymnosaurus are fragmentary, often only partial snouts and jaws. The genus was originally defined in part upon a specific sharedtooth count, which is now known to be unreliable for diagnosing early therocephalians and the same dental formula is now known to be typical of lycosuchids in general. The only other consistent feature to unite these fossils is their large size, and though not diagnostic, they notably include some of the largest known specimens of any therocephalians, with the largest specimens representing skulls estimated to exceed 40 cm (16 in) in length.
Scymnosaurus and itstype speciesS. ferox were named in 1903 byRobert Broom for theholotype specimen SAM-PK-632, originally collected on an expedition by J. R. Joubert from an unknown locality in 1881 and now housed in theIziko South African Museum (SAM).[1][2] It is a tightly closed (occluded) partial snout and jaws consisting of only the very front portion around theincisors andcanines up to and including the thirdpostcanine tooth, with the entirety of the snout above the roots of the teeth missing such that the internal anatomy of the palate and roots of the teeth are exposed. Though fragmentary, the specimen had (at the time) a uniquedental formula (count and types of teeth)—five incisors, one canine, and only three small postcanines on each side—that Broom used to defined the genus. SAM-PK-632 was notable for its large size, described by Broom as the second largesttheriodont (then all carnivoroustherapsids) known at the time afterTitanosuchus (now known to be adinocephalian) and belonging to an animal he believed to be the size of ahyena.[1] Indeed, it remains one of the largesttherocephalian skulls on record, as though it is incomplete the preserved portion of its snout is proportionately comparable to that of the relatedSimorhinella, one of the largest therocephalians recognised today with a total skull length of 37 cm (15 in).[3]
Additional specimens were later referred toS. ferox byLieuwe Dirk Boonstra; SAM-PK-9084 in 1953, collected by Boonstra from locality Riet Kuil 387 in 1929, and SAM-PK-3430 and 4341 in 1954, collected bySidney H. Haughton in 1916 from Janwillemsfontein and Stinkfontein, respectively.[2][4] Both SAM-PK-3430 and 4341 are similar specimens to the holotype, preserving occluded snouts and jaws as far as the postcanines (though slightly more completely), while SAM-PK-9084 is a more complete but distorted skull and jaws that includes most of the jaws but only the snout and eyes of the skull, missing the temporal region, along with partial limb bones. Boonstra referred all three toS. ferox on the basis of sharing the same dental formula and estimating that all three were of similar size to the holotype.[4][5] However, Boonstra misinterpreted the dental formula of SAM-PK-4341, and it has been reidentified as a specimen of thescylacosaurid therocephalianGlanosuchus.[6]
In 1915, Broom named the speciesS. watsoni from a skull in the collections of theNatural History Museum, London catalogued NHMUK PV R 410, originally purchased in 1878 fromThomas Bain, a pioneering road engineer from South Africa with an interest in Karoo geology.[2] This skull had already been published upon the year prior byDavid M. S. Watson, who described the bones of its palate and doubtfully assigned the specimen toLycosuchus.[7] Having examined the skull after furtherpreparation, Broom instead assigned NHMUK PV R 410 toScymnosaurus as a new species. He did so the basis of it sharing a similar dental formula asS. ferox, in spite of the fact the skull is missing the front end of the snout, including the incisors and most of the canines, whileS. ferox is known mostly from just this region. Broom acknowledged there was room for doubt in assigningS. watsoni toScymnosaurus because of this, but nonetheless believed it andS. ferox to be "certainly closely allied".[8] In his 1932 bookThe Mammal-like Reptiles of South Africa and the Origin of Mammals Broom remained doubtful of its placement inScymnosaurus, but retained it in the genus nonetheless.[9]S. watsoni was further examined by Watson in 1921 and again by Boonstra in 1934, who together revised the anatomy of itsocciput,braincase and palate.[10][11]
In 1954, Boonstra named a new genus and species of early therocephalianPristerosaurus microdon (now ajunior synonym ofMairasaurus)[12] and reassignedS. watsoni to this genus as thenew combinationPristerosaurus watsoni, claiming that it could not be included underScymnosaurus (orLycosuchus as Watson originally proposed).[4] Boonstra later backtracked on this assignment and revertedS. watsoni toScymnosaurus once again in a 1969 paper without comment.[2] When revising the taxonomy of scylacosaurids in 2023, Christian Kammerer commented thatS. watsoni indeed closely resemblesMairasaurus (i.e. "Pristerosaurus") proportionately, but as the latter is known only by one similarly sized specimen more information on its anatomy would be needed to investigate any potential affinities as Boonstra had suggested.[12]
In 1954 Boonstra named a third speciesS. major from two incomplete but very large partial snouts he each estimated to belong to skulls over 400 mm (16 in) long when complete.[4] Both skulls were collected by Boonstra in 1929 from two separate expeditions into the Karoo, the first recovering the holotype SAM-PK-9005 at the Kleinkoedoeskop locality and later the second skull SAM-PK-10556 from Knoffelfontein, where it was originally discovered by the farm's owner.[2][4] SAM-PK-9005 is also associated with somepostcranial bones, namely parts of theshoulder andhip girdles and the ends of several limb bones that Boonstra went on to describe in 1964.[13] Boonstra includedS. major inScymnosaurus on the basis of its large size and similar dental formula, but only differentiated it fromS. ferox by larger size and subtle perceived differences of the jaw and the bones of the skull. These included slightly fewer postcanines (2-3 versus 3-4), a supposedly steeper "chin" at themandibular symphysis (where the two jaw bones meet), and thefrontal bone (a bone on the roof of the skull between the eyes) not reaching the edge of theeye socket.[4]
Boonstra referred five more specimens toScymnosaurus in 1954, though he was unable to determine if they belonged to eitherS. ferox orS. major and so only referred them toScymnosaurussp. Most specimens were collected by Boonstra himself and his colleagues on various trips during the first half of the 20th century; SAM-PK-9126 collected from Voelfontein in 1929, SAM-PK-11459 from Buffelsvlei in 1939, SAM-PK-11833 from Lammerkraal in 1947, and SAM-PK-11961 from Dikbome (date unspecified).[2] It is not known when the fifth specimen in the museum's collections SAM-PK-8999 was collected, or by who. Like most other specimens underScymnosaurus they consist of only the snout and occluded lower jaws to varying degrees of completeness and preservation quality, and were referred toScymnosaurus mostly due to their dental formula.[4][6]
In 1964 Boonstra referred many more specimens toScymnosaurus sp.; SAM-PK-11577, 11695, 11957, 12118, 12193, 12262, K352 and K353, with another specimen SAM-PK-11558A only doubtfully referred to "Scymnosaurus?". Almost all of these specimens are based upon only postcranial bones from the skeleton, mainly limbs and their girdles (i.e. shoulder and hips), with SAM-PK-11695 being the only specimen among them to preserve part of the skull comparable to otherScymnosaurus specimens (though SAM-PK-12193 included a partial jaw bone). However, this skull was identified as that ofGlanosuchus by Kammerer in 2023, without any reference to the postcrania. Similarly, Boonstra had previously referred SAM-PK-11957 toPristerognathus in 1954 and originally included a partial skull in the same specimen, but excluded the skull in 1964.[4][13] The skull was later reidentified asIctidosaurus, another early therocephalian now regarded as dubious.[6][12] Unlike the cranial specimens, none of the postcranial specimens have been re-examined and had their taxonomic affinities reassessed since Boonstra's work.
A fourth species once included underScymnosaurus wasS. warreni, named by Broom in March 1907 from a specimen discovered inKwaZulu-Natal, NM 188. Like most otherScymnosaurus specimens NM 188 is an occluded, and somewhat crushed, partial snout and lower jaws (though the lower jaws have since been lost, leaving only impressions). Broom assigned it toScymnosaurus due to its similar dental formula, but regarded the assignment as provisional.[14] In 1932 he recognised that the specimen actually belonged to theakidnognathid therocephalianMoschorhinus and reassigned the species to the genus as the new combinationM. warreni. This species is regarded by subsequent researchers to besynonymous with the type species ofMoschorhinus,M. kitchingi.[15]
Named in 1903,Scymnosaurus was among the first genera assigned to Therocephalia after Broom had not long named theorder earlier that year. At the time, Therocephalia also included the dinocephalianTitanosuchus (among other genera now known to not be therocephalians), which Broom believed to "undoubtedly" be the closest relative ofScymnosaurus.[1][6] As Therocephalia became better established,Scymnosaurus was regarded as representative of typical early therocephalians—though early in the 20th century both Watson andSamuel W. Williston would each classifyScymnosaurus as agorgonopsian without comment in separate publications—and was classified in thefamily "Pristerognathidae" (now correctly known as Scylacosauridae), to which most early therocephalians were thought to belong to.[6][16]: 176 [17]: 242 Scymnosaurus would even be used as the basis of thesubfamily Scymnosaurinae that was proposed by Boonstra in 1969 for large "pristerognathids" likeScymnosaurus.[2]
The historic diagnosis forScymnosaurus mostly relied on large size and a dental formula of five upper incisors, one canine and at most four postcanines. In 1954 Boonstra incorporated additional traits into its diagnosis, but most of the features he listed are generalised among early therocephalians.[4][6] Indeed, when Boonstra revised its diagnosis in 1969 he would limit it entirely to the small size and number of its postcanines.[2] Similarly, he only distinguishedS. major fromS. ferox by slightly different postcanine counts (2-3 versus 3-4), a stronger, steeper symphysis, its larger size, and the supposed absence of the frontal bone from the border of the eye socket.[4]
In 1987, palaeontologist Juri van den Heever re-examined and revised thetaxonomy of all early therocephalians known from cranial remains as part of hisPhD thesis, and could not identify any unique traits that characterisedScymnosaurus or its species. Notably, he identified both size and tooth count alone as an unreliable method for diagnosing early therocephalians, particularly the precise number of postcanines, and so the dental formula ofScymnosaurus was not unique. Further, he determined that the frontals in supposedS. major specimens did in fact reach the eyes, as in supposedS. ferox, and that the poorly preserved dentary symphyses ofS. major could not be shown to actually be different fromS. ferox. Consequently,S. ferox andS. major could not be distinguished, nor couldScymnosaurus as a whole be distinguished from other early therocephalians, and soScymnosaurus,S. ferox andS. major were all therefore declared to benomina dubia (dubious names).S. watsoni likewise lacks any distinguishing features of its own or that would unite it with other species ofScymnosaurus, and was also declared anomen dubium.[6]
All the specimens underScymnosaurus van den Heever examined could only be identified as far as thefamily level oftaxonomy, but most specimens belong to theLycosuchidae, including three of the four specimens ofS. ferox, both specimens ofS. major, and two referred toS. sp. (SAM-PK-8999 and 11961). These specimens are now classified as Lycosuchidaeincertae sedis, or Lycosuchidae indet. On the other hand, the sole skull ofS. watsoni was identified as an indeterminate specimen of Scylacosauridae (Scylacosauridaeincertae sedis), along with the referredS. ferox specimen SAM-PK-4341 that was identified as the scylacosauridGlanosuchus. The remaining specimens, all referred toScymnosaurus sp., are so incomplete that they lack diagnostic traits of either family. Two (SAM-PK-11459 and 11833) are only identifiable as therocephalians by their loose and sloping mandibular symphyses, but as they only preserve the first few incisors it cannot be determined if they are lycosuchid or scylacosaurid (and so are Therocephaliaincertae sedis). The remaining specimen SAM-PK-9126 preserves the five upper incisors of lycosuchids, but is missing its lower jaw and so cannot even be distinguished as a therocephalian or gorgonopsian (which have a fused, steeper mandibular symphysis), and can only be classified to Theriodontiaincertae sedis.[6] A more recent revision of lycosuchid taxonomy by Fernando Abdala and colleagues in 2014 upheld his conclusions, referring almost allScymnosaurus skulls to Lycosuchidaeincertae sedis and identifyingLycosuchus andSimorhinella as the only valid lycosuchids.[3] The revision of scylacosaurids by Kammerer in 2023 also upheldS. watsoni as dubious and only representing Scylacosauridaeincertae sedis, though noted potential similarities toMairasaurus.[12]
Although the geographic source for the holotype itself is unknown, most other specimens ofScymnosaurus consistently recognised by researchers (i.e.S. ferox,S. watsoni,S. major and twoS. sp.) were discovered in theBeaufort West Municipality of theCentral Karoo inSouth Africa (SAM-PK-9084, SAM-PK-9005 and SAM-PK-10556), with two from the neighbouringPrince Albert (NHMUK PV R 4100, SAM-PK-3430) and another from theLaingsburg Municipalities (SAM-PK-11961).[3][6] The remaining specimens originally assigned toScymnosaurus sp. are likewise from Prince Albert (SAM-PK-11813, 9216) and Beaufort West (SAM-PK-11459).[4] Most of Boonstra's postcranial specimens also come from Beaufort West, with two more from Prince Albert (SAM-PK-11957, 12118) and one from Laingsburg (SAM-PK-K352), but with two others from nearFraserburg also (SAM-PK-11695 and SAM-PK-12262). All of these regions belong to theBeaufort Group of theKaroo Basin, and more specifically from areas that exposerock layers of theAbrahamskraal Formation. These correspond to the upperTapinocephalus Assemblage Zone (AZ) faunal zone, in which both lycosuchid and scylacosaurid therocephalian fossils are abundant.[18]
While the source localities for the holotype as well as SAM-PK-8999 (S. sp.) are unknown, lycosuchid fossils are restricted to theTapinocephalus AZ and into the lower layers of the subsequentEndothiodon AZ from rocks of the overlyingTeekloof Formation, though they are mostly known from theTapinocephalus AZ.[3] Scylacosaurids likeS. watsoni have the same upper bound, but are known from older rocks that date to theEodicynodon AZ.[18] The upperTapinocephalus AZ (a.k.a. theDiictodon-Styracocephalus Subzone) is roughly dated to between 262 and 260 million years ago during the lateCapitanian stage of theMiddle Permian period.[3][19]
Specimens ofScymnosaurus represent large to very large therocephalians, among the largest of any therocephalians known.[6] All the skulls referred toScymnosaurus are incomplete, often only partial snouts and associated jaws with only a few preserving theorbital region around the eyes. However, from comparing their proportions to related, more complete therocephalians, the estimates for the complete skull length have ranged from comparable toSimorhinella (i.e. 370 mm (15 in) long) for specimens referred toS. ferox and up to between 450 mm (18 in) and 474 mm (18.7 in) long forS. major—though these estimates are based upon reconstructions made prior to modern classifications.[4] More direct comparisons between the equivalent portions of the snout show that the type specimen ofS. ferox (180 mm (7.1 in) snout) is similar in size toSimorhinella (182 mm (7.2 in) snout), while the snout of SAM-PK-9005 (S. major) measures 226 mm (8.9 in) long.[3] Apart from size,Scymnosaurus was also historically diagnosed by adental formula of five large upper incisors, a very large single canine, and only two to four small postcanines, allserrated. Otherwise, specimens underScymnosaurus have no uniquely distinguishing traits beyond the typical characteristics of the families they belong to (and hence arenomina dubia).[6]
Most specimens ofScymnosaurus, including those ofS. ferox andS. major, are identifiable as lycosuchids (Lycosuchidaeincertae sedis). Like all lycosuchids their snouts are broad and proportionately short compared to scylacosaurids (albeit some specimens are distorted). All of these specimens have five incisors in eachpremaxilla and less than five small postcanines behind the large canine, both characteristic of lycosuchids compared to scylacosaurids, which have more of each. Notably, no specimens attributed toScymnosaurus have the "double canines" seen in many other lycosuchid specimens.[a] However, lycosuchid "double canines" are now recognised to be a product oftooth replacement rather than being the typical appearance, and indeed the broken or resorbed roots of the alternate canines are observed in several specimens ofScymnosaurus. In at least one large specimen ofS. major the upper margin of the orbit is thickened and rugose, obscuring sutures and hence why Boonstra initially thought the frontal did not reach the edge. This appears to be a typical feature of large and mature specimens of lycosuchids, as it is also seen inLycosuchus[6]
In contrast,S. watsoni is identifiable as an indeterminate scylacosaurid. Scylacosaurid skulls are proportionately longer and narrower than lycosuchids, mostly in the snout, and they typically have more teeth, though the skull ofS. watsoni is distorted and the missing front end of the snout precludes identifying its incisor count. However, it does preserve thepterygoid bones of the palate, which lack palatal pterygoid teeth. This is a trait found in scylacosaurids but not lycosuchids, which do have pterygoid teeth.[6]
S. watsoni is somewhat smaller thanS. ferox andS. major, and in 1915 Broom estimated its complete skull length to be approximately 290 mm (11 in).[8] Broom highlighted the large temporal fenestra and very tall and thinsagittal crest between them as distinctive, but tall sagittal crests appear typical of all large scylacosaurid specimens and both its height and the broad fenestra have probably been distorted duringfossilisation (taphonomy). Though relatively complete, its poor state of preservation hinders identification any further than Scylacosauridae indet., though its proportions are similar to those of the rare scylacosauridMairasaurus. However, unlike definitiveMairasaurus specimens its orbits are visible when viewed from above, rather than facing mostly out to the side as is characteristic ofMairasaurus, though the potential taxonomic significance of this difference is unclear.[12]
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