THE OLDEST TETRAPOD FOOTPRINT ICHNOFAUNA, FROM THE LOWER MISSISSIPPIAN HORTON BLUFF FORMATION, NOVA SCOTIA, CANADA

At Blue Beach, Nova Scotia, Canada, the Blue Beach and Hurd Creek members of the lower Mississippian (Tournaisian) Horton Bluff Formation yield an extensive assemblage of tetrapod footprints that is the oldest of all known tetrapod ichnofaunas. Three ichnotaxa were previously reported from Blue Beach (Hylopus, Anticheiropus and Baropezia). New extensive collections indicate that at least six footprint morphotypes are present: (1) very abundant tracks ("Paleosauropus") that are wider than long (pes width/length ~ 50-70/30-40 mm), with clear manus and pes sole impressions, and tetradactyl manus and pentadactyl pes with relatively short, clawed digits; (2) rare, large (pes width/length ~ 120/100 mm) tracks ("Baropezia") in which a large pes sole impression and five short, rounded digits are evident; the original Horton Bluff record of "Baropezia" is reinterpreted to be of non-tetrapod origin, possibly a large rhipidistian, but "Baropezia" is re-established by a new specimen of a somewhat smaller individual; (3) several, very large (pes width/length ~ 80/70 mm) tracks ("Attenosaurus"), which have long, thin scratch-like digit impressions. (4) fairly common tracks ("Hylopus") that have long, thin and sometimes curved digits (pes width/length ~ 40/40 mm) in which relative digit length is well differentiated. (5) fairly common tracks ("Pseudobradypus" -Ex: 'Anticheiropus') that have a long, narrow pes (pes width/length ~ 30/50 mm) and thin, pointed and forward-directed digits and a narrow trackway width, some with a median drag. (6) Rare, small (pes < 20 mm long) tracks with short, blunt digits ("Batrachichnus"). These are the tracks of reptiliomorphs (1-3), reptiles (4-5), and temnospondyls (6). Almost all the Horton Bluff footprints are subaerial tracks of quadrupeds that have forward directed digits and lack median body or tail drags. Furthermore, none of the Horton Bluff tracks show evidence of polydactyly and are thus not comparable with known Devonian tetrapods. The similarity of this site to later ichnofaunas indicates that a major revolution had taken place in tetrapod evolution at approximately the Devonian-Mississippian boundary, with the replacement of the primitive late-Devonian tetrapod chronofauna by a diverse pentadactyl chronofauna capable of fully terrestrial locomotion by the early Mississippian.