Jasinoski Sandra C, Abdala Fernando
Evolutionary Studies Institute, University of the Witwatersrand , Johannesburg , South Africa.
PeerJ. 2017 Jan 10;5:e2875. doi: 10.7717/peerj.2875. eCollection 2017.
Non-mammaliaform cynodonts gave rise to mammals but the reproductive biology of this extinct group is still poorly known. Two exceptional fossils of and , 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 , 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 was identified. Taking this comprehensive survey into account, the two previously described cases of parental care in and 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 are approximately only 37% of the associated adult size; whereas in , 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 and , intraspecific aggregations were much more common in , suggesting it regularly lived in aggregations consisting of both similar and different aged individuals.
非哺乳形犬齿兽类演化出了哺乳动物，但这个已灭绝类群的生殖生物学仍然鲜为人知。50多年前曾简要描述过两个非同寻常的化石，分别是 和 ，化石包含与成年个体紧密关联的幼年个体，被作为非哺乳形犬齿兽类亲代抚育的例证。然而，这两块早三叠世化石在近期关于化石记录中亲代抚育的讨论中基本被排除在外。在此，我们在对这两个基干犬齿兽类群中发现的其他聚集情况进行广泛调查的背景下，重新分析了这些化石。我们的分析揭示了 在其他六个明确的聚集案例，包括未成熟个体或成年个体间同龄个体的聚集，以及亚成年个体和成年个体间不同龄个体的聚集。相比之下， 只确定了另外一个聚集案例。综合考虑这项全面调查，之前描述的 和 中亲代抚育的两个案例得到了证实。这些幼年个体是每个类群已知最小的标本，成年个体与两个相关幼年个体之间的体型差异是所有聚集中最大的。 的幼年个体大约只有相关成年个体体型的37%；而在 中，幼体至少是相关成年个体体型的60%。在每个案例中，两个幼年个体体型相似，表明它们来自同一窝。尽管 和 都存在亲代抚育，但种内聚集在 中更为常见，这表明它经常生活在由不同龄个体组成的聚集中。
