VERTEBRATE TRACE FOSSIL ASSEMBLAGES FROM THE PENNSYLVANIAN-AGED RHODE ISLAND FORMATION OF EASTERN MASSACHUSETTS

Numerous vertebrate trace fossils of Pennsylvanian age have been collected from the Rhode Island Formation at quarries in Attleboro and Plainville, MA. The fossils are preserved in shales representing lowland deposits, and co-occurring fossil plants (lycopods, sphenopsids, pteridophytes, early gymnosperms) represent typical Pennsylvanian coal-swamp vegetation. Invertebrate trace fossils have also been found at the sites; myriapods are represented by Diplichnites, and archaeognathan insects are represented by the body impression Tonganoxichnus and the trackway Stiaria.

Prior to this study, vertebrate traces from the Rhode Island Formation were known from isolated tracks and trackways of amphibians. However, thousands of vertebrate tracks were collected recently, particularly from Plainville, revealing an ecologically diverse assemblage. Temnospondyls are represented by footprints that range in size from 3-5 mm (Batrachichnus) to 10 cm (Limnopus). Modes of locomotion for the amphibian trackmakers include walking, which produced trackways with tail/abdominal drags, as well as swimming, which produced sets of up to three elongate scratch marks made by the toes (Charachichnos). Other swimming traces, representing fish or amphibians, occur as sinusoidal trails. Probable amniote tracks from Plainville are represented by two track morphs tentatively referred to Dromopus and Notalacerta. It is often difficult to definitively identify amniote trace fossils because some amphibians converged on amniote pedal morphologies, but the Plainville tracks show characteristics that are typically used to distinguish amniotes from amphibians, including: 1) thin toes, 2) pronounced digital pads, 3) sharp ungual marks, 4) elongate fourth digits, and 5) a long stride. Dromopus is attributed to the captorhinomorphs, whereas Notalacerta is attributed to diapsids. If the identification of the Plainville tracks is correct, then two lineages of amniotes are represented at the locality.