| Dimacrodon | |
|---|---|
 | |
| Illustration showing known material and reconstructed outline of the skull | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | incertae sedis |
| Family: | †Dimacrodontidae Olson & Beerbower,1953 |
| Genus: | †Dimacrodon Olson & Beerbower, 1953 |
| Species: | †D. hottoni |
| Binomial name | |
| †Dimacrodon hottoni Olson & Beerbower, 1953 | |
Dimacrodon is an extinctgenus of non-mammaliansynapsid from the latestEarly PermianSan Angelo Formation ofTexas. It is distinguished by toothless, possibly beaked jaw tips, large lower canines and a thin bony crest on top of its head. Previously thought to be ananomodonttherapsid related todicynodonts, it was later found to lack any diagnostic features of anomodonts or eventherapsids and instead appears to be a 'pelycosaur'-grade synapsid of uncertain classification.
Three specimens ofD. hottoni are known, although they only comprise incomplete portions of thelower jaw andskull.[1] Theholotype specimen consists of a single partialmandible with teeth. The jaw is long with an unusually broad and deepmandibular symphysis, while the jawrami are slender (although the additional material fromD. sp. below suggests that the back of the jaws was also deeper).[2] Unusually, the front of the jaw is toothless and has a rough bone texture, possibly supporting a beak like in dicynodonts, with no pre-canine teeth. Behind the toothless region is a single pair of large caniniforms, while the teeth behind them are much smaller and uniform in size, unlike typical 'pelycosaur' dentition (although see theedaphosauridGordodon), and arethecodont like the teeth of therapsids.[1]
An additional specimen referred toDimacrodon sp. includes more extensive, but still incomplete material from the rear of skull and margins of the upper jaw. The back of the skull is deep and broad, with a largetemporal fenestra that is roughly trapezoid in shape and slopes down and back from the roof of the skull. There is a small, thin parietal crest of bone in the middle of the skull between the eyes and temporal fenestra. What is known of thepremaxilla suggests that it had a broad, flattened plate that opposed the beak-like lower jaw and likewise was also toothless. A jaw fragment containing two large "canines" are also known from this specimen, although it is unknown if it is from the lower or upper jaw. This skull is believed to be the largest-known specimen ofDimacrodon, and was estimated by Olson (1962) to be approximately 50 centimetres (20 in) in length when restored.[2]
Olson and Beerbower tentatively assignedDimacrodon to Therapsida in 1953 (along with other unusual synapsids from the San Angelo Formation) due to its therapsid-like teeth and general resemblance to various therapsids. The jaw structure was similar to thedinocephalianTitanophoneus, the toothless beak-like jaw tips similar to dicynodonts, and they also noted general similarities togorgonopsians as well. However, it could not be attributed to any one of these groups, and so they did not classify it beyond being a probable therapsid. They suggested that it represented a synapsid that had independently reached the therapsid "level of organisation" from the better-known Russian and South African therapsids, reflecting the pre-cladistics method of classification employed at the time.[1] Olson later provided a more definitive classification in 1962 when he assigned it to Anomodontia, specifically to theinfraorderVenyukovioidea, and erected its ownmonogenericfamily, theDimacrodontidae.[2]
Olson also described an additional specimen in 1962 which he attributed to a possible second species asD. sp., based on possible presence of two caniniform teeth in the tooth row, as well as variations in the spacing and shape of the postcanine teeth and the overall size difference.D. hittoni was characterised as having only a single lower "canine" tooth in each jaw; however, the number of upper "canines" is unknown. Two lower "canines" could distinguishD. sp. fromD. hittoni, although it is possible thatD. hittoni had two upper "canines" and the jaw fragment is simply from the upper jaw ofD. hittoni. It is unclear then ifD. sp. truly represents a distinct species or if it is simply a large specimen ofD. hittoni.[2]
Dimacrodon was mistakenly assigned to Anomodontia primarily because of its broad, toothless mandibular symphysis, which resembled the beaks of dicynodonts. Olson also interpreted the deep jaw tips and cheek teeth as resembling those of the Russian anomodontVenyukovia, leading him to group them together in the infraorder Venyukovioidea (although unlikeDimacrodon, venyukoviids possess front teeth).[2] It was also briefly regarded as a dinocephalian inTapinocephalia byRobert L. Carroll in 1988, but this was never elaborated upon.[3] In 1995, Sidor and Hopson revisited the San Angelo therapsids and reported that all of them were likely to be based on 'pelycosaur' material and that none of them were therapsids, includingDimacrodon.[4][5] This re-identification has been accepted and followed since then; however, features of its skull such as the differentiated teeth and large temporal fenestra have been described as therapsid-like. No further studies onDimacrodon have been carried out since then, and it remains unclear what its truephylogenetic relationships are amongst early-derived therapsids.[6][7]
In the upper San Angelo Formation,Dimacrodon co-existed with and was found alongside the abundant giantherbivorouscaseidCotylorhynchus, along with thecaptorhinidRothianiscus and similarly enigmatic synapsidTappenosaurus. The upper San Angelo Formation has been interpreted as a near-shore terrestrial environment, cut by numerous channels and experiencing periods of flooding, during which the bones ofDimacrodon were likely deposited.[2] Olson and Beerbower regardedDimacrodon as acarnivore, although the beak-like jaws were noted as being more similar to the herbivorous dicynodonts.[1]
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