Vertebrate Taphonomy: first articulated large non-mammalian therapsid from the Triassic Chañares Formation

MANCUSO, ADRIANA CECILIA; PREVITERA, ELENA; MARSICANO, CLAUDIA

Johanesburg

Conferencia; The Palaeontological Society of Southern Africa 2014 meeting, Johannesburg Southern Africa.; 2014

PSSA

The Chañares record is worldwide known by the well-representation of Ladinian tetrapod fauna with small and medium-sized archosauriforms and synapsids. The Chañares tetrapod assemblages document two accumulation modes: (1) attritional mortality assemblage characterized by disarticulated skeletons of the different Chañares groups in similar proportion with dispersion pattern produced by low-energy flowing water; and (2) mass-mortality assemblage dominated by small and medium-sized elements of the fauna in a singular layer with a large quantity of complete or partially articulated skeletons, and rapid burial. The large dicynodonts are poorly represented with skulls and very few isolated postcranial elements. The first nearly complete articulated large non-mammalian therapsid in the Chañares attritional assemblage is reported here. The bearing level is characterized by structureless light-olive gray clay-siltstone deposited in the floodplain environment. The material is an individual comprising fragmentary skull, tusk, cervical and dorsal vertebrates, two fragmentary scapulas, clavicle, interclavicles, ribs and abundant fragmentary indeterminate bones. The skeleton is articulated, although the appendicular bones are missing. The individual displays a ventral-up attitude with the vertebral column forming a sharp curve. The skull is also in ventral-up attitude with a slight rotation and with loss of some ventral cranial bones and mandibles. Scapulas are fragmented and displaced, clavicle and interclavicle are only slightly displaced and the ribs are articulated in life-position. The bones show cracking without loss of superficial tissue suggesting a stage 1 of weathering and transverse fractures without movement that are associated to post-mineralization. The bone modifications are stronger in the bones located in the upper part of the assemblage. The taphonomic history of the specimen starts with a natural death in the ecosystem as there is no evidence of catastrophic death. The well-preservation of the bones observed suggest that the individual died in the exhumed site or in a very close area in the floodplain environment. The carcass was entombed during a short time with a constant sedimentation evidenced by the structureless deposit. The skeleton attitude suggests that the animal could had suffered predation/scavenging and/or high decay processes in the abdominal area that produce the ventral-up exposition although tooth marks are not observed in the preserved bones. The stronger bone modification and the absence of appendicular bones are favoured by the carcass position that exposed portion of the body to taphonomic processes for more time during pre-burial and were the first to be exhumed.