| Echinerpeton | |
|---|---|
 | |
| Echinerpeton intermedium fossil | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Synapsida |
| Clade: | Metopophora |
| Family: | †Ophiacodontidae |
| Genus: | †Echinerpeton Reisz, 1972 |
| Type species | |
| †Echinerpeton intermedium Reisz, 1972 | |
Echinerpeton is an extinctgenus ofsynapsid, including the single speciesEchinerpeton intermedium from theLate Carboniferous ofNova Scotia, Canada. The name means 'spiny lizard' (Greek).[1] Along with its contemporaryArchaeothyris,Echinerpeton is the oldest known synapsid, having lived around 308 million years ago. It is known from six small, fragmentary fossils, which were found in an outcrop of theMorien Group near the town ofFlorence.[2] The most complete specimen preserves articulated vertebrae with highneural spines, indicating thatEchinerpeton was a sail-backed synapsid like the better knownDimetrodon,Sphenacodon, andEdaphosaurus. However, the relationship ofEchinerpeton to these other forms is unclear, and itsphylogenetic placement among basal synapsids remains uncertain.[3]
Echinerpeton is known from six specimens, five housed in theMuseum of Comparative Zoology and a sixth in theRedpath Museum: the holotype MCZ 4090, which consists of a partial postcranial skeleton and some jaw fragments; MCZ 4091, which includes vertebrae and aninterclavicle; MCZ 4092, a leftmaxilla or upper jaw bone; MCZ 4093, a partial right maxilla; MCZ 4094, including threeneural arches or vertebral spines; and RM 10057, consisting of a right maxilla, neural arch, rib, and aphalanx or finger bone. Since all other specimens besides the holotype are isolated bone fragments, their assignment to the same species is not certain. The maxillae are distinct in having straight lower margins, distinct from the often curved jaws of ophiacodontids and sphenacodontids but similar to the straight jaws of some other synapsids likeArchaeothyris,Haptodus, andVaranops. The dentary or lower jaw bone has a slight upward curve. The teeth of both the upper and lower jaws are small and cone-shaped, some having slightly serrated edges, and are only differentiated by slight differences in length (some other synapsids have teeth that vary greatly and shape across their jaws). The three forward-most dentary teeth are angled slightly outward as in more derived synapsids such asDimetrodon andSphenacodon. Several features, including straight-margined maxillae and simple conical teeth, are also seen in the earliest reptiles.[2] Twenty-three presacral (neck and back) vertebrae are preserved in the holotype, although several may be missing because the typical number of presacral vertebrae in early synapsids is 27. Thecentra or centers of the vertebrae are slightly compressed, similar to the compression seen in the vertebrae of ophiacodontids. There are large spaces between the centra where bones calledintercentra could fit, although no intercentra are preserved in the sixEchinerpeton specimens. The loose connection between the centra and intercentra is one of the primitive features ofEchinerpeton, since other early synapsids have intercentra that fit tightly with the centra as part of an evolutionary progression toward completely fused vertebral elements. The most prominent feature of the vertebrae ofEchinerpeton are their tall neural spines, which can be up to seven times higher than they are wide. They are similar in proportion to the spines ofSphenacodon, althoughEchinerpeton is considerably smaller in overall size. The neural spines of the holotype are thinnest at their tips, suggesting that MCZ 4090 may have been an immature individual with poorlyossified bones. By contrast, MCZ 4094 has neural spines that are thicker at their tips and are slightly larger in size, both of which are possible indications that the specimen represents an adult individual. The first vertebra preserved in the series, theaxis bone, has a neural spine that is low and broad like those of many other synapsids. The axis is most similar to those of ophiacodontids because it widens toward the top, and unlike those of sphenacodontids which widen about midway up the spine and then narrow at the top. To either side of the neural spines are smallertransverse processes, which have struts of bones extending from them that were described as a "webbing." This "webbing" helps connect the vertebrae to the ribs, and is otherwise only seen in ophiacodontids.[2]
Parts of theappendicular skeleton (limbs, hips, and shoulder bones) are preserved inEchinerpeton specimens, including theinterclavicle,scapula (shoulder blade), the lower portion of thehumerus (upper arm bone), theilium (a hip bone), the upper portion of thefemur (upper leg bone), thetibia andfibula (lower leg bones),astragalus andcalcaneum (ankle bones), andmetatarsals (foot bones). The ilium ofEchinerpeton is similar to those of early reptiles in that it is narrow and backward-pointing, while those of sphenacodontids are widened at their front to support the hip's connection with thesacral vertebrae. The astragalus has the same simple L-shape as those of ophiacodontids.[2]
Reisz (1972) tentatively classifiedEchinerpeton as anophiacodontid in its initial description, but later (1986) considered it an indeterminate "pelycosaur".[2][4] Lee (1999) argued that the placement ofEchinerpeton within Synapsida was not certain because it lacked any of the defining features orsynapomorphies present in the group, all of which come from the skull. He claimed that all the features linkingEchinerpeton with synapsids were also present in other basalamniotes, so it could not be placed definitively on the synapsid branch of Amniota (the other amniote branch isSauropsida, or reptiles).[5] Benson (2012) was the first to includeEchinerpeton in a phylogenetic analysis, and considered it a "wildcard taxon" because it had three equally likely positions on the synapsid tree: one as the most basal synapsid, another as thesister taxon of a clade containingCaseasauria,Edaphosauridae, andSphenacodontia, and a third as an ophiacodontid more derived thanArchaeothyris. Benson also found that the inclusion ofEchinerpeton in his analysis was causing largepolytomies, or unresolved relationships, in thestrict consensus tree.[3] Mann and Patterson (2019) described new material ofEchinerpeton, including substantial cranial remains, and recovered it as a member of Ophiacodontidae.[6]
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