Hylonomus aciedentatus (Dawson 1860) lived ~310 mya and was slightly smaller than its its sister, Coelostegus. Both were derived from a sister to Paleothyris and phylogenetically preceded Protorothyris.
Distinct from Paleothryis, the skull of Hylonomus was lower overall. Any remnants of a temporal ledge were absent. The posterior skull did not descend. The mandible rose anteriorly. The jugal strongly intruded into the squamosal. The maxilla was deeper.
The cervical neural spines were taller than the centra. Rod-like garstralia were present. The tail was long and slender, perhaps longer than the presacrals.
The scapulocoracoid was not well ossified.
The pelvis was relatively smaller. Although the ilium was taller than long, the posterior process was retained. Pedal 4.1 was the widest of the proximal phalanges. Pedal digit V was longer. Metatarsals II and III aligned with p1.1.
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