FIRST RECORD OF A REGULAR ECHINOID (ECHINODERMATA: ECHINOIDEA) FROM THE GAIMAN FORMATION (EARLY MIOCENE), CHUBUT, ARGENTINA

ALLENDE, A.; GIACHETTI, L. M.; PÉREZ, D. E.; CUITIÑO, J. I.; BONAUDO, S.

Congreso; XII Congreso de la Asociación Paleontológica Argentina virtual; 2021

Asociación Paleontológica Argentina

The Cenozoic fossil record of echinoids from Argentina is dominated by irregular echinoids, while regular echinoids are represented by scarce remains. In this regard, isolated plates, spines and other fragments are known from Roca (early Paleocene) and Monte León (early Miocene) formations. More complete specimens are known from Roca, San Julián (late Oligocene) and El Chacay (early Miocene) formations. In this work, we present the first record of a regular echinoid from the Gaiman Formation (early Miocene). This marine unit is composed of whitish mudstones, fine sandstones and tuffaceous beds. The specimen was found in the lower levels of the exposures at Cerro Prismático locality. Other macrofauna recognized in this section includes occasionally oysters, coquina levels with chionine venerids (Chionopsis? sp.), scarce pinnids, small turritellid gastropods and only few terebratulids brachiopods. Also, an abundant ichnofauna is represented by elements of the Cruziana ichnofacies. The echinoid specimen consists of a 13 mm-radio test fragment, comprising approximately a quarter of the whole test. It includes one complete ambulacrum and one complete interambulacrum, while the others are incomplete. On lateral section, the test is slightly domed, being rounded and straight on aborally and adorally views, respectively. The peristome occupies nearly fifty percent of the total radio. Plates are covered with dense granules. Interambulacral and ambulacral plates are hexagonal, with the former twice the width of the latter. One row of shallow primary tubercles is vertically aligned. Each ambulacral plate contains six small secondary tubercles interspersed in a vertical row. These characters allow us to assign the specimen to the Euechinoidea. Due to the poor preservation of the specimen a more precise taxonomic identification is difficult. This record provides new information about this poorly known group in the Cenozoic of Patagonia.