The Darwinulocopina comprise an interest group of ostracods which where among the first invaders of freshwater waters during the late Palaeozoic. The Permian–Triassic extinction greatly reduced their diversity, reaching present times represented by one family. The darwinulids are regarded as “ancient asexuals” since a parthenogenetic mode of reproduction is assumed for all the post-Triassic members of the group. However, the high diversity achieved during the late Palaeozoic is often associated with sexual reproduction. Here we studied a monospecific association of ostracods from the Middle Triassic of the Cuyana Basin, Province of Mendoza, Argentina, and recognized a new species of Darwinulocopina, Prasuchonella? huarpe nov. sp. We discuss the traditional length/height and length/width graphical method to recognized ontogeny and/or sexual dimorphism in fossil ostracod assemblages, and performed a geometric morphometric analysis performed on both lateral and dorsal views of near one hundred seventy carapaces. The best results were obtained from the analysis in dorsal view, and four ontogenetic stages were discriminated (A-3, A-2, A-1 juveniles, adult). This allowed recognizing a main ontogenetic trend related to the development of the brooding chamber. Although subtle in differences, female carapaces are wider not only at the brooding chamber, but also along the whole length, compared to the carapaces of the presumptive males. Finally, full systematic descriptions and discussions, attempting to unify descriptive criteria for recent and fossil darwinulocopin carapaces, are provided. As a result, the need for a review of those Mesozoic records of Darwinulocopina, particularly those from the Triassic, is noted.

Several hundreds of carapaces and a few valves were picked under a Nikon SMZ645 stereomicroscope from a single sample. Selected specimens were mounted on stubs using carbon conductive adhesive tape, gold coated and scanned with a JEOL JSM-6360LV Scanning Electron Microscope (SEM) at the Servicio de Microscopía Electrónica del Museo de La Plata, La Plata city, Argentina. After examination under SEM, a total of 167 specimens were retained for study regarding their preservation, mainly those with undeformed dorsal and/or lateral outlines. Length, height, and width for carapaces were measured on SEM images. A geometric morphometric analysis, following the procedures outlined by Zelditch et al. (2004), was performed on the SEM photographs on both dorsal and lateral left views of the carapaces. For the dorsal view, two landmarks were established: 1) the anterior most and 2) the posterior most points of valves contact. The outline for each valve was characterized by a 100 points curve including both landmarks. The anterior most point of the carapace was defined as initial point for the outline on lateral view of the carapace, which was characterized by 200 points. Data was standardized for position, size and rotational differences by means of a generalized least squares Procrustes superposition (Zelditch et al. 2004, p. 113–119). Logarithm with base 10 of Centroid Size—log(CS)—was used as size estimator for all the analyses that required it (Zelditch et al. 2004, p. 78). Digitization of the landmark configurations was performed on TPSdig2.12 (Rohlf 2008), and a Procrustes superposition was done for each data set with CoordGen6h (Sheets 2001). Procrustes coordinates were used as data for a Principal Component Analysis (PCA) using the software PAST 3.23 (Hammer et al. 2001). 

HAMMER, Ø., HARPER, D. A. T. and RYAN, P. D. 2001. PAST: Paleontological Statistics software package for education and data analysis. Palaeontologia Electronica, 4, 1–9.

ROHLF, F. J. 2008. tpsDig2. http://life.bio.sunysb.edu/morph/

SHEETS, H. D.2001. IMP: integrated morphometrics package. Department of Physics, Canisius College, Buffalo, N.Y. - http://www3.canisius.edu/~sheets/morphsoft.html - http://www3.canisius.edu/~sheets/moremorph.html

ZELDITCH, M. L., SWIDERSKI, D. L., SHEETS, H. D. and FINK, W. L. 2004. Geometric Morphometrics for Biologists: a Primer. Elsevier Academic Press, Amsterdam, 443 pp.