Borealosuchus (Crocodylia) from the early Campanian Mooreville chalk reveals new insights into the late Cretaceous fauna of Alabama and the origin of Crocodylian lineages

• Main Author: McCormack, Larkin
• Other Authors: Brochu, Christopher A. (Christopher Andrew), 1967-
• Format: Others
• Language: English
• Published: University of Iowa 2019
• Subjects: Borealosuchus; Crocodylia; Eusuchia; Phylogenetics; Systematics
• Online Access: https://ir.uiowa.edu/etd/6799; https://ir.uiowa.edu/cgi/viewcontent.cgi?article=8298&context=etd

Borealosuchus is a significant genus as it is considered either a basal crocodylian or close outgroup. A new species identified from the early Campanian Mooreville Chalk of Alabama is one of the oldest known crocodylians and extends the stratigraphic range of Borealosuchus from the Maastrichtian minimally into the early Campanian. It co-occurs with the giant alligatoroid Deinosuchus in deposits formed in a marginal marine setting with fluvial input. Its age, along with the phylogenetic position of Borealosuchus, renders this material critical for understanding the morphological conditions and relationships at the root of Crocodylia. The most complete specimen consists of partial cranial, mandibular, and postcranial remains preserving a unique combination of characters including a short dentary symphysis, splenial participation in the symphysis, lack of discrete concavity on the angular dorsal margin for an external mandibular fenestra, and bipartite ventral osteoderms. The external mandibular fenestra was either absent or very small, raising questions about the ancestral condition for this structure in Crocodylia. The new species is, surprisingly, closer phylogenetically to more derived species of Borealosuchus, including B. wilsoni, B. threeensis, and B. acutidentatus, than to B. sternbergii and B. formidabilis. This creates many range extensions and ghost lineages in the clade and suggests substantial unsampled diversity within Borealosuchus. Additional mandibular material from the Santonian Eutaw Formation may be referable to this new species, further increasing its stratigraphic range. A specimen consisting of a partial post-cranial skeleton from the Mooreville Chalk is significantly larger than the holotype of the new species, and is referable to Borealosuchus sp., but is too incomplete for more precise referral. These newly described specimens provide additional information about the faunal assemblages of Alabama during the Late Cretaceous and have broader implications because they are some of the earliest crocodylian material known. Indeed, the Eutaw material appears to be the oldest known occurrence of Crocodylia.