DIRECT EVIDENCE OF THE ROSTRAL ANATOMY OF THE AÏSTOPOD PHLEGETHONTIA, WITH A NEW CRANIAL RECONSTRUCTION

Journal of Paleontology, Mar 2007 by Anderson, Jason S

INTRODUCTION

IN A series of recent papers, colleagues and I (Milner, 1994; Carroll, 1998; Anderson, 2002, 2003a, 2003b; Andersen et al., 2003) have revised Aïstopoda, a collection of elongate, limbless tetrapods from the Paleozoic of Euramerica. Aïstopods are part of a larger, monophyletic assemblage of tetrapods known as Iepospondyls, which are proving to be critical to understanding the early evolution of amniotes and are possibly related to the origins of some, or all, modern amphibians (Carroll, 1995, 200Oa, 200Ob; Laurin and Reisz, 1997, 1999; Laurin, 1998; Anderson, 2001; Ruta et al., 2003; Vallin and Laurin, 2004).

Sound phylogenetic analysis depends upon careful descriptive anatomy, but when working with fossils one also has to contend with variation in the quality of preservation. Thus, in my revision of the genus Phlegethontia Cope, 1871 in these pages (Anderson, 2002) I was forced to extrapolate the shape of the nasal bone based upon the shape of surrounding cranial elements, because no specimen at my disposal permitted direct observation.

Subsequent to publication of this revision, I learned of a large collection of Mazon Creek nodules from Pit 11 in the invertebrate paleontology collection at the Royal Ontario Museum in Toronto. Pit 11 preserves specimens from both the mostly marine Essex and more terrestrial Braidwood faunas (Baird et al., 1985). Many of the nodules in this collection are unopened, and most that are opened and fossiliferous contain remains of invertebrates or plant material. A few, however, contain vertebrate remains, including a recently described enigmatic temnospondyl (Godfrey, 2003). One of the nodules contained the skull and anterior postcranial skeleton of a phlegethontiid aïstopod which, upon inspection, preserves the entire rostrum, including the rarely complete nasal bone. This note documents for the first time the complete shape of the nasal of Phlegethontia, which necessitates a revised cranial reconstruction. Systematic treatment follows Anderson (2002).

Abbreviations.-AMNH, American Museum of Natural History; MCZ, Museum of Comparative Zoology, Harvard University; NMW, Naturhistorisches Museum Wien; ROM, Royal Ontario Museum.

ROM 48445

Description.-The specimen is preserved in a siderite concretion 65 mm in length, with one end (corresponding with the caudal portion of the animal) cut away by a rock saw at some point in the past (Fig. 1). The nodule is represented in part (thicker half) and counterpart (thinner half), preserving the right and left lateral views of the animal, respectively. Mazon Creek fossils are preserved as impressions in the concretion matrix, with the bone replaced by kaolinite. In this instance, the specimen was left unprepared because the specimen is delicate and the kaolinite had already flaked away from the rostral areas; thus Figure 2 is a photograph and line drawing of a negative impression.

The part is more complete than the counterpart, preserving vertebrae which are lost on the counterpart because of weathering (approximately 74 vertebrae compared to 53). Aïstopods have 63 or 64 precaudal vertebrae, the beginning of the tail being identified by a change in the morphology of the ventral centrum from a weak median keel to paired longitudinal flanges surrounding a haemal groove (Gregory, 1948; Zidek and Baird, 1978; Anderson, 2003a), so this specimen preserves the entire precaudal skeleton and proximal 10 or so caudal vertebrae, although specific vertebral identification is not possible with the kaolinite in place.

The skull (Fig. 2) is 7.0 mm in length from occiput to rostrum (measured under a stereomicroscope using dial calipers), and the lower jaw is 5.45 mm in length. Identifiable elements include the premaxilla, nasals, frontal, maxilla, dentary, and a poorly preserved prefrontal. The posterior portion of the skull, which is infilled with kaolinite, preserves no traces of the posterior skull roof bones (parietals, postparietals, etc.) present in Pseudophlegethontia Anderson, 2003b, also present at this locality. This description focuses on the rostral elements.

The premaxilla appears complete. It bears space for seven to nine teeth, consistent with previous descriptions, and contacts the maxilla in an oblique suture that runs from rostroventral to caudodorsal at the level of the prefrontal. The narrow nasal ramus is half as long as the dental ramus, and runs close to the midline. The nasal ramus rises from the dental ramus at a sharp (approx. 75°) angle, which makes the rostrum appear beaklike in lateral view (Fig. 2). This is also seen in AMNH 2564 (Anderson, 2002, fig. 2.1 ). As the nasal ramus passes caudally, this angle decreases to approximately 45°.

The single frontal is slightly rotated to the left, so its morphology is clearly visible. The rostral margin is deeply W-shaped, but not previously described is the fact that the medial process is somewhat shorter than the lateral two processes. Reinspection reveals this morphology to be present in other specimens where this area is visible (e.g., AMNH 2564).