Aggregations and parental care in the Early Triassic basal cynodonts<i>Galesaurus planiceps</i>and<i>Thrinaxodon liorhinus</i>

Abstract

Non-mammaliaform cynodonts gave rise to mammals but the reproductive biology of this extinct group is still poorly known. Two exceptional fossils of <i>Galesaurus planiceps</i> and <i>Thrinaxodon liorhinus</i>, consisting of juveniles closely associated with an adult, were briefly described more than 50 years ago as examples of parental care in non-mammaliaform cynodonts. However, these two Early Triassic fossils have largely been excluded from recent discussions of parental care in the fossil record. Here we re-analyse these fossils in the context of an extensive survey of other aggregations found in these two basal cynodont taxa. Our analysis revealed six other unequivocal cases of aggregations in <i>Thrinaxodon</i>, with examples of same-age aggregations among immature or adult individuals as well as mixed-age aggregations between subadult and adult individuals. In contrast, only one additional aggregation of <i>Galesaurus</i>was identified. Taking this comprehensive survey into account, the two previously described cases of parental care in <i>Galesaurus</i> and <i>Thrinaxodon</i> are substantiated. The juveniles are the smallest specimens known for each taxon, and the size difference between the adult and the two associated juveniles is the largest found for any of the aggregations. The juveniles of <i>Thrinaxodon</i> are approximately only 37% of the associated adult size; whereas in <i>Galesaurus</i>, the young are at least 60% of the associated adult size. In each case, the two juvenile individuals are similar in size, suggesting they were from the same clutch. Even though parental care was present in both <i>Galesaurus</i> and <i>Thrinaxodon</i>, intraspecific aggregations were much more common in <i>Thrinaxodon</i>, suggesting it regularly lived in aggregations consisting of both similar and different aged individuals.