Allometry in Late Devonian Podocopa ostracods (Crustacea) and its implications for Ostracoda ontogeny

Podocopa ostracods abounds in various marine and non-marine habitats since the early Palaeozoic and leave diversified fossil records in strata worldwide. They are particularly useful for stratigraphic correlation and paleoenvironmental and paleogeographical reconstructions. However, being the most numerous ostracods during the Palaeozoic, their origins, evolution, ontogeny and phylogeny remain poorly defined. This paper represents the first study of morphological variation in Devonian Platycopida (an order of Podocopa) ostracods using a geometric morphometric approach. Our results identify substantial shape variation for Cribroconcha honggulelengensis from the Upper Devonian Hongguleleng Formation of western Junggar, NW China, demonstrating a remarkable change to those juveniles between stages A-3 and A-2. Using linear regressions between Principal component scores from right lateral view and carapace length, we demonstrate allometric growth patterns for the species C. honggulelengensis.The body plan of C. honggulelengensis developed along with the ontological variable, i.e., from an ovate outline with rare pores to an oblong outline with dense and large pores. Environmental factors play a major role in shape determination during ontogeny of C. honggulelengensis.

Methods
SEM images of each specimen were used to digitalize landmarks, with digitization performed using TpsDig2 v. 2.25 (Rohlf 2016a). A combination of landmarks and semilandmarks or sliding landmarks were used for analysing homologous points' locations together with curves measured on a sample of organisms. Principal component analysis (PCA) was subsequently applied to explore the overall diversity of within-sample shape variation using software  PAST v. 2.13 (Hammer et al. 2001).