Madtsoiidae: monophyletic clade or grouping of convenience?

MARK J POWERS; FERNANDO FABIO GARBEROGLIO; CALDWELL, MICHAEL WAYNE

Ottawa

Congreso; Canadian Society of Vertebrate Palaeontology Annual Meeting; 2023

Canadian Society of Vertebrate Palaeontology

Madtsoiidae is currently defined as an extinct family of snakes that share several vertebral characteristics. They appeared in the Late Cretaceous of Gondwana and reached a nearly global distribution by the Eocene, frequently acquiring large body sizes and utilizing narrow intercontinental seaways for dispersal. Early discovered specimens were referred to Boiidae or Pythonidae due to general similarities of vertebral proportions. However, frequent observations of several key vertebral features such as the absence of accessory prezygapophyseal processes, the presence of laterally expanded para-diapophyses and the presence of parazygantral foramina led to the hypothesis that Madtsoiidae represents a valid clade of snakes. Due to cranial material being quite rare, most named taxa within Madtsoiidae aredescribed from isolated or segments of articulated vertebrae, including the type species Madtsoia bai from Eocene deposits of the Chubut province, Argentina. Because of the paucity of well-preserved specimens for madtsoiids, species diagnoses and phylogenetic relationships are resolved almost entirely on vertebral characters, which can show considerable variation along the vertebral column of an individual. The variability of madtsoiid synapomorphies across the vertebral column has not been critically assessed due to the lack of cervical (anterior trunk) and caudal (post-cloacal) vertebrae. An articulated series of madtsoiid cervicals from upper Eocene deposits of the Divisadero Largo Formation in the Cuyo Basin of the Mendonza province, Argentina, are referred to Madtsoia sp. based onidentified madtsoiid synapomoprhies (parazygantral foramina), age (mid-late Eocene) and proximity to the holotype locality. Comparison of this specimen to others from extinct and extant snake clades reveals a wide range of putative madtsoiid synapomophies are variably expressed in non-madtsoiid clades. The variability of these “synapomorphies” across a single vertebral series demonstrates problematic degrees of intraspecific variation when diagnosing isolated vertebrae. These observations reinforce the poor utility of vertebral characters in snake phylogenetics, and stress the importance of relatively complete specimens in constructing species diagnoses.