A continental ichnological study across Romer’s Gap (Mississippian Period) in Southern New Brunswick: implications for the colonization of Lacustrine and Alluvial continental environments

Abstract

Vertebrate fossils from continental environments are rare in the Misssippian Period. This includes the interval spanning the Tournaisian and Visean stages historically known as ‘Romer’s Gap’, where the few known sites represent paleoenvironments close to open water rather than inland continental conditions. This thesis uses ichnofossils from the Mississippian-aged strata of southern New Brunswick, Canada as a proxy for biodiversity. Examination of the Horton, Sussex, and Mabou groups in the Martimes Basin has yielded diverse footprint ichnoassemblages representing the earliest-known Carboniferous tetrapods. Additionally, the latest Visean to Serpukhovian Mabou Group has yielded the oldest-known tetrapod burrows. The sedimentological record suggests the tetrapods responsible for trackways lived within forested wetlands on the margins of freshwater lakes and alluvial plains, thus representing the earliest colonization of continental paleoenvironments by tetrapods. Tetrapod burrowing represents the earliest known behavioral response of the group to climate change. Trace fossils demonstrate that diverse tetrapods were already adapted for continental ecosystems earlier in the Mississippian than previously recorded, representing the oldest known evidence for temnospondyls, anthracosaurs, and recumbirostran ‘microsaur’ lineages. Relic Devonian-grade polydactyl tetrapods similar to whatcheerids remained within the alluvial Kennebecasis Formation. The dryland, strongly seasonal alluvial settings of the Stilesville and Enrage formations preserve sparse tetrapod ichnofossils interpreted to have been made by anthracosaurs and recumbirostran microsaurs that were likely better-adapted to continental conditions. This study tests and mostly agrees with recently-established ichnobiochronological models for the Mississippian Period, with evidence of Valentian ichnobiochronological elements extending into the Mississippian Bluebeachian ichnobiochron, suggesting an overlap between them. The present research extends the known chronological distribution of several ichnogenera (Hylopus, Batrachichnus, Limnopus) earlier into the Tournaisian. This study also informally recognizes new tetrapod ichnofossil morphotypes that will be published as new ichnotaxa, including new trackways (‘Brachydactylopus’ and a new polydactyl trackway) and burrows (Reniformichnus ‘stringeri’ and Katarrhedrites ‘peekabooensis’)