BIOCHRONOLOGY OF TRIASSIC VERTEBRATE COPROLITES AND THE LACK OF A SIGNIFICANT TURNOVER IN COPROLITE ICHNOFAUNAS AT THE TRIASSIC/JURASSIC BOUNDARY

Vertebrate coprolites are of biochronological utility. Some workers have been reluctant to apply a binomial nomenclatural scheme to coprolites based on the misconception that feces are not distinguishable, even though wildlife biologists routinely track the distribution of extant taxa based on the distinct morphologies of their feces. A developing binomial nomenclature has been key to utilizing coprolites for biochronology (and other purposes). Most coprolites correspond to higher level taxonomic groups of body fossils and probably represent “orders” or above. Coprolite ichnotaxa have defined stratigraphic ranges that parallel the stratigraphic ranges of the producing animals, so the coprolites have utility in biostratigraphy and biochronology. There are 13 named coprolite ichnotaxa from the Triassic.

Some of the most biochronologically important ichnotaxa for the Triassic are: (1) Hyronocopros is common in the Permian and apparently has its last occurrence in the Middle Triassic as do Rhynchocoprus and Santamariacoprus; (2) Alococopros is abundant and characteristic of the Triassic, although there are reported occurrences from the Permian to the Cretaceous; (3) Dicynodontocopros ranges from the Middle to the Late Triassic (upper Carnian) and it occurs in some faunas as the only evidence of the presence of dicynodonts (e.g., Chinle Group of West Texas and western New Mexico); (4) Heteropolacopros first occurs in the Pennsylvanian, and it is abundant in the Carnian but rare in the Norian and absent in the Rhaetian; and (5) the first occurrence of Strabelocoprus is in the Rhaetian, and the coprolite faunas of this age lack Heteropolacopros.

The base of the Late Triassic is marked by the last occurrence of Hyronocopros and the first appearance of Saurocopros. The end of the Carnian coincides with the last occurrence of Dicynodontocopros and Malericopros and a marked decrease in the abundance of Heteropolacopros and Alococopros. The start of the Rhaetian coincides with the last occurrence of Heteropolacopros and the first occurrence of Strabelocoprus. There is no significant change in coprolite ichnofaunas across the Triassic/Jurassic boundary although Plektecoprus makes its first appearance.