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 repres ented by the second oldest tetrapod skeletal remains so far described from the Americas exclusive of Greenland. Whatcheeria is a primitive t etrapod about Im in length including the tail, and is characterized by the following autapomorphies: ornament on cranial dermal bones is abs ent or extremely light; the parietal foramen is large relative to skul l 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 postoccipi tal portion; a preopercular; lateral line canals that in places are co mpletely encased in bone; a dentary which extends far posteriorly alon g the adductor fossa; a prearticular that extends anteriorly almost to the symphysis; a toothed adsymphysial; a row of small Meckelian foram ina between the prearticular and infradentary bones; a tabular and par ietal that are not in contact; a nearly continuous row of teeth on the vomer, palatine and ectopterygoid; a femur with an oblique ridge havi ng a maximum elevation at midshaft; and a pelvis with an area of unfur nished bone anterior to the acetabulum. The phylogenetic affinities of Whatcheeria are uncertain at present, but its best placement may be a s 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 th e tabular; a skull table in which the temporal bones have a strong ven tral inflection dorsal to their sutures with the cheek; dermal ornamen t that, though generally absent, where present is similar to the 'anth racosaur type'; quadrangular orbits; narrow vomers; marginal fangs on the maxilla; an interclavicle with a long parasternal process; and a s capulocorocoid that ossifies from two centres. In the context of this hypothesis of relationships, some of Whatcheeria's observed features a re unexpected: pleurocentra that are sometimes composed of dorsally fu sed antimeres; a region of presacral ribs with flattened shafts and la rge uncinate processes; and absence of an ossified branchial skeleton.