Article: A new primitive tetrapod, Whatcheeria deltae, from the Lower Carboniferous of Iowa
Whatcheeria deltae gen. et sp. nov. is Visean V3b in age and is represented by the second oldest tetrapod skeletal remains so far described from the Americas exclusive of Greenland. Whatcheeria is a primitive tetrapod about 1 m in length including the tail, and is characterized by the following autapomorphies: ornament on cranial dermal bones is absent or extremely light; the parietal foramen is large relative to skull size; the prefrontal forms a thick, raised ridge at the anterodorsal margin of the orbit; and the cleithrum terminates posterodorsally in a robust, posteriorly directed process above a deep notch. Whatcheeria is extremely plesiomorphic in retaining: a cheek with long postoccipital portion; a preopercular; lateral line canals that in places are completely encased in bone; a dentary which extends far posteriorly along the adductor fossa; a prearticular that extends anteriorly almost to the symphysis; a toothed adsymphysial; a row of small Meckelian foramina between the prearticular and infradentary bones; a tabular and parietal that are not in contact; a nearly continuous row of teeth on the vomer, palatine and ectopterygoid; a femur with an oblique ridge having a maximum elevation at midshaft; and a pelvis with an area of unfinished bone anterior to the acetabulum.The phylogenetic affinities of Whatcheeria are uncertain at present, but its best placement may be as the first outgroup to the Anthracosauria sensu Gauthier et al. 1988. This suggestion is supported by the presence of: small post-temporal fenestrae; a tabular horn that continues from the dorsal surface of the tabular; a skull table in which the temporal bones have a strong ventral inflection dorsal to their sutures with the cheek; dermal ornament that, though generally absent, where present is similar to the 'anthracosaur type'; quadrangular orbits; narrow vomers; marginal fangs on the maxilla; an interclavicle with a long parasternal process; and a scapulocorocoid that ossifies from two centres. In the context of this hypothesis of relationships, some of Whatcheeria's observed features are unexpected: pleurocentra that are sometimes composed of dorsally fused antimeres; a region of presacral ribs with flattened shafts and large uncinate processes; and absence of an ossified branchial skeleton.