ONTOGENY AND SKELETOGENESIS OF CORUMBELLA – INSIGHTS FROM BIOMETRY AND SYNCHROTRON RADIATION-BASED 2D X-RAY DIFFRACTION

Corumbella, a well-known Ediacaran fossil first found in Brazil, is believed to be among the earliest biomineralized animals. However, its original morphology remains a topic of debate. Here we focus on the ontogeny and mineralogy of this enigmatic fossil. We conducted a comprehensive analysis of 27 relatively long specimens from the Tamengo Fm, Corumbá city, Brazil (ca. 541 Ma). Various measurements were taken, including the ‘proximal’ (apical) and ‘distal’ (apertural) width, tube length, segment height, and sinuosity. Ontogenetic changes were assessed using the allometric equation (y = ax^b). To determine the mineral composition, 2D x-ray diffraction was performed at DIAD beamline, Diamond Light Source, Oxfordshire, UK, with an energy of 30 KeV and a beam size of 25 µm. Our findings reveal that width variation (rate of change between ‘proximal’ and ‘distal’ measurements) shows a negative hypoallometric relationship with tube width. Interestingly, the height of the segments remained unchanged despite variations in tube width. Sinuosity values were predominantly close to one, suggesting a mostly straight disposition of the tubes, with rare exceptions. 2D XRD maps highlight the prevalence of calcite composition in the skeletons compared to the clay-rich host rock. No Bragg peaks attributed to aragonite were observed. These results suggest that the rate of growth of Corumbella’s soft-body cross-section decreased throughout its developmental stages. If Corumbella possessed an elongated soft body, this may also imply an increase in the surface area:volume ratio during ontogeny, potentially linked to physiological constraints. Furthermore, the ring heights remained independent of tube width, which may reflect unchanged dimensions of the skeleton-secreting tissues during growth, if the individual ring segments indeed reflect the size of these organs. Although the only phase observed in the fossils was calcite, this may result from diagenetic alteration from other phases. The relationships among late Ediacaran tubular animals remain a subject of contention. However, studies into the ontogeny and skeletogenesis of these groups can offer valuable insights into the emergence of the first wave of biomineralized metazoans in the fossil record, even when phylogenetic considerations are difficult.