ICHNOLOGICAL DIVERSITY IN THE EARLY JURASSIC OF THE CONNECTICUT VALLEY, EASTERN NORTH AMERICA.

 

EMMA C. RAINFORTH, Ramapo College of New Jersey, 505 Ramapo Valley Rd., Mahwah NJ 07430 and Lamont-Doherty Earth Observatory of Columbia University, 61 Rt. 9W, Palisades NY 10964; (201) 684-7209, erainfor@ramapo.edu.

 

The Connecticut Valley of Massachusetts and Connecticut yielded the earliest-discovered fossil footprints from the United States, published on extensively by Edward Hitchcock between 1836 and 1865. This founding father of ichnology amassed a stunning collection, still housed at Amherst College, forming the basis of taxonomic works in which he named and renamed over 90 genera and 200 species, in a total of 281 combinations. Navigating the nomenclatural maze, only about 20 genera and 10 species are actually invalid. However, detailed taxonomic studies allow subjective synonymization of many of these taxa. In addition, the ichnotaxa can be broadly grouped into two categories - Hitchcock's Pachydactyli and Leptodactyli (broad and thin-toed prints respectively). The Pachydactyli are footprint taxa with distinct phalangeal pad impressions; associated features of the footprints (e.g. scale impressions) and substrate (e.g. raindrops) indicate that many of these taxa represent "true tracks" or, at worst, very shallow underprints. As such, they allow a detailed picture of the trackmaker's pedal anatomy to be obtained. In contrast, the Leptodactyli are taxa with narrow digits, usually lacking phalangeal pads impressions, and in many cases include footprints penetrating multiple layers. Where a three-dimensional view is obtained, it can be seen that the morphology varies vertically (typically digit divarication decreases downwards). The morphology of the Leptodactyli is thus heavily influenced by the substrate conditions (apparently, surfaces that were not microbially bound) and kinematics of foot emplacement and withdrawal. Pedal anatomical details (beyond "number of toes") are not ascertainable, and thus trackmaker affinity not determinable at high taxonomic resolution.

In order to gain insight into the tetrapod components of these Early Jurassic ecosystems, in which identifiable skeletal remains are rare, the pachydactylous taxa are clearly of greater use. With subjective synonymization of the Pachydactyli, we find the following morphotypes to be commonly represented: Anomoepus (ornithischian), brontozoids (Grallator, Eubrontes, etc.; theropods), Otozoum (prosauropod), and Batrachopus (crocodylomorph); prints attributable to lacertilians and amphibians are rare or absent. Of these four groups, the brontozoids are the most diverse ichnotaxonomically at both genera and species levels; a case can be made for reducing the number of currently-valid genera from eight to one, although species-level analysis remains to be undertaken. Ongoing refinements in lithostratigraphic correlation and high-resolution stratigraphy are providing a framework on which detailed biogeographic and biostratigraphic analyses can be conducted.

 

 

For Tracking Dinosaur Origins: The Triassic/Jurassic Terrestrial Transition, St. George, Utah, March 14-16, 2005.