Review of the tetrapod burrow fossil record: evolutionary and environmental linkages

RICARDO NESTOR MELCHOR; MARÍA CRISTINA CARDONATTO

Santa Marta

Simposio; IV Simposio Latinoamericano de Icnología (SLIC 2018); 2018

Universidad de Caldas

Fossil tetrapod burrows are relatively common biogenic structures, the oldest ones are Early Permian lysophorid amphibian structures. Most pre-Cretaceous tetrapod burrows have been attributed to therapsids, and a few Late Permian-Early Triassic burrows contain articulated skeletons. A common architecture for Permian to Jurassic tetrapod burrows is in the form of a shallowly inclined ramp with a rounded and not enlarged end, with discrete scratch marks, elliptical cross-section and bilobed bottom (similar to Reniformichnus). Most of Jurassic tetrapod burrows occur in eolian sequences and are commonly small structures (less than 0.20 m wide). The oldest burrow systems assigned to primitive mammals appears in the Early Jurassic Navajo Sandstone of USA. There is a dearth of reports of Cretaceous tetrapod burrows that can be related with the equable climates that existed for most of this period. The more unusual occurrence for the Late Cretaceous is a single dinosaur burrow containing an adult and two juvenile remains of ornithopods, further suggesting denning behavior and parental care. Most of the Cenozoic tetrapod burrows have been attributed to mammals, mainly to Rodentia and Xenarthra and a few examples related to Carnivora. The record of Paleogene tetrapod burrows is meager and can also be linked to benign climate conditions. The Miocene record is more varied and abundant, with a diversification of the architectural patterns and behavioral strategies that, commonly, appeared under volcaniclastic and eolian environments. The early Miocene innovations included long helicoidal and vertical burrows (Daimonelix), complex subhorizontal burrow systems, the first carnivore den, and food caches. The seemingly elaborate pattern consisting of an helicoidal subvertical burrow with a terminal chamber is recorded sporadically since the Late Permian and is probably an adaptation to cope with arid climates and temperature extremes. In the middle Miocene of South America there are cylindrical, subhorizontal, unbranched tunnels with a meniscate backfill interpreted as foraging tunnels of small Dasypodidae (ichnogenus Nagtuichnus). During the late Miocene occurred, also in South America, large mammal burrows of xenarthrans and notoungulates that are related to the appearance of open environments with low vegetation and semiarid and seasonal climate. Pliocene burrows are small (horizontal diameter < 0.25 m) and have been mostly assigned to rodents and notoungulates. The Pleistocene megafauna of South America is also reflected in the burrow trace fossil record in the form of huge cylindrical structures (0.7-2.0 m wide), with elliptical cross-section from Argentina and Brazil. These burrows were attributed to herbivorous mammals that probably acquired a burrowing behavior as consequence of cool and dry climates and/or due the arrival of large carnivorans after the Great American Biotic Interchange. The main driving factor for the occurrence of fossil tetrapod burrows seems to be paleoenvironmental changes related to cool, dry and/or volcanism-influenced settings.