The homology, form, and function of the microraptorine lateral pubic tubercle

Abstract

ABSTRACT Microraptorine dromaeosaurids are characterized by a lateral pubic tubercle or process on the midshaft of the pubis. Initially proposed as a tubercle for muscle attachment, this structure has since received little attention beyond identifying its presence or absence in theropods. Examination of this tubercle reveals inconsistencies in texture and topology with attachment sites for pelvic musculature. However, its unevaluated homology precludes meaningful utilization in comparative or paleobiological studies. Here, the homology, form, and function of the lateral pubic tubercle are explored in the context of the extant phylogenetic bracket. This framework, combined with dissection of extant relatives and direct observation of osteological correlates in extant and extinct archosaurs, permits assessment of various tissues that are potentially associated with the microraptorine lateral pubic tubercles. Consideration of pelvic soft tissues reveals that these tubercles are most parsimoniously interpreted as an osteological correlate for pubogastralial ligaments, which anchor the most posterior row of gastralia to the hip in extant crocodylians. Homologous structures that are correlates for pubogastralial ligaments vary widely in morphology across archosaurs. A series of alterations occurred in paravian theropods while undergoing pubic retroversion and ventilatory changes. The relationship between the lateral pubic tubercles and the pubic apron also imposed functional consequences on surrounding tissues, most notably locomotory musculature arising from the pubic apron. This elucidates why the lateral pubic tubercle is only expressed in Microraptorinae.