EVALUATION OF THE ONTOGENY AND SEXUAL DIMORPHISM IN A NEW SPECIES OF MIDDLE TRIASSIC DARWINULOCOPINA (CRUSTACEA, OSTRACODA) FROM ARGENTINA by ANA P. CARIGNANO 1,2 , JAVIER ECHEVARR  IA 1,2 and ANA M. ZAVATTIERI 3 1 Consejo Nacional de Investigaciones Cient  ıficas y T ecnicas (CONICET), Buenos Aires, Argentina: apcarignano@fcnym.unlp.edu.ar; javierechevarria@fcnym.unlp.edu.ar 2 Divisi on Paleozoolog  ıa Invertebrados, Museo de La Plata, Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, Paseo del Bosque s/n, F1900BWA, La Plata, Buenos Aires, Argentina 3 Departamento de Paleontolog  ıa, Instituto Argentino de Nivolog  ıa, Glaciolog  ıa y Ciencias Ambientales (IANIGLA), CCT-CONICET Mendoza, Av. Adri an Ruiz Leal s/n., Parque General San Mart  ın, Casilla de Correo 330, M5002IRA, Mendoza, Argentina; amz@mendoza-conicet.gob.ar Typescript received 27 February 2020; accepted in revised form 5 August 2020 Abstract: Darwinulocopina is an interesting group of ostracods that includes some of the first invaders of fresh- water habitats during the late Palaeozoic. The Permian–Tri- assic extinction greatly reduced their diversity, culminating in a single extant family. The darwinulids are regarded as ‘ancient asexuals’ given that a parthenogenetic mode of reproduction is assumed for all 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 assemblage of ostracods from the Middle Triassic of the Cuyo Basin, Province of Mendoza, Argentina, recognizing a new species of Darwinu- locopina, Prasuchonella? huarpe sp. nov. We discuss the tra- ditional length/height and length/width graphical method for identifying ontogeny/sexual dimorphism in fossil ostra- cod assemblages. A geometric morphometric analysis was performed on both lateral and dorsal views of almost 170 carapaces, to evaluate the ontogeny and sexual dimorphism of the species. The best results were obtained from the anal- ysis in dorsal view, discriminating four ontogenetic stages. This revealed a main ontogenetic trend related to the devel- opment of the brooding chamber. Although subtle in differ- ence, female carapaces are wider not only at the brooding chamber, but also along the whole length, compared with the presumptive males. We provide full systematic descrip- tions and discussions, attempting to unify descriptive criteria for recent and fossil darwinulocopine carapaces, and suggest closeness to species previously described from the upper Permian of Russia. As a result, we recommend a review of Mesozoic records of Darwinulocopina, particularly those from the Triassic. Key words: Darwinulocopina, geometric morphometrics, sexual dimorphism, ontogeny, Middle Triassic, Argentina. T HE Darwinulocopina are among the first ostracods to colonize the continental aquatic environments during the Carboniferous and became highly diverse during the late Carboniferous–Permian (Martens et al. 1998; Molostovs- kaya 2000; Horne 2003). They are defined by smooth elongate carapaces, with very narrow normal and radial pore canals, poorly developed calcified inner lamella, hinge with a groove and cardinal teeth on the right valve or adont, and the central muscle scar (CMS) showing a rosette pattern, together with some appendage features observable only in Recent representatives (Sohn 1988). After the Permian–Triassic mass extinction, they suffered a drastic decrease in diversity, reaching present times rep- resented only by a single family with six genera (Rossetti & Martens 1998; Rossetti et al. 2011; Karanovic 2012). Three superfamilies, Suchonelloidea, Darwinuloidea and Darwinuloidoidea, were dominant and diverse during the Permian, with 100–200 species, but only a few genera of the Darwinuloidea and scarcely any species of Suchonella Spizharskyi, 1937 (Suchonelloidea, Suchonellidae) were described for the Triassic (Martens et al. 1998; Molos- tovskaya 2000; Naumcheva & Golubev 2020). The pres- ence of sexual dimorphism in darwinulocopines is generally accepted for Permian and older taxa (Molos- tovskaya 1980, 1982, 1990; Abushik 1990; Neustrueva 2000), but frequently rejected for Mesozoic–Recent repre- sentatives of the group (Martens et al. 2003; Sch€ on et al. 2009). Due to this interpretation, darwinulids are usually © The Palaeontological Association doi: 10.1002/spp2.1340 1 [Papers in Palaeontology, 2020, pp. 1–27]