First occurrence in life position of two Triassic brachiopod species (Thecideida) from NE Italy: Paleoecological considerations

Primer registro en posición de vida de dos especies de braquiópodos triásicos (Thecideida) del NE de Italia: Consideraciones paleoecológicas

Miguel A. Torres-Martínez¹, Francisco Sánchez-Beristain ^2,*, Joachim Reitner ³

¹ Departamento de Paleontología, Instituto de Geología, Circuito de la Investigación Científica, Avenida Universidad 3000, Universidad Nacional Autónoma de México, Coyoacán, 04510, Mexico City, Mexico.

² Unidad Multidisciplinaria de Docencia e Investigación, Facultad de Ciencias, Campus Juriquilla. Blvd. Juriquilla 3001, Universidad Nacional Autónoma de México, Juriquilla, 76230, Querétaro, Mexico.

³ Geowissenschaftliches Zentrum der Universität Göttingen, Abt. Geobiologie, Goldschmidtstraße 3, 37077, Göttingen, Germany.

* Corresponding author: (F. Sánchez-Beristain) This email address is being protected from spambots. You need JavaScript enabled to view it.

How to cite this article:

Torres-Martínez, M. A, Sánchez-Beristain, F., Reitner, J. 2023, First occurrence in life position of two Triassic brachiopod species (Thecideida) from NE Italy: Paleoecological considerations: Boletín de la Sociedad Geológica Mexicana, 75 (1), A250822. http://dx.doi.org/10.18268/BSGM2023v75n1a250822

Manuscript received: June 28, 2022; Corrected manuscript received: August 20,2022; Manuscript accepted: August 26, 2022

ABSTRACT

The San Cassiano Formation from the Dolomites, NE Italy, is a Ladinian–Carnian (Middle – Upper Triassic) lithostratigraphic unit belonging to the Western Tethys domain. In this formation numerous groups of marine invertebrates from reef communities have been reported. In particular, brachiopods are represented by different orders. Thecideids are the most abundant and diverse, with different species of the genus Thecospira. Despite the apparent abundance, only one report exists of a thecideid of the genus preserved in life position: Thecospira tyrolensis. In this study, we describe and discuss the first finding of Thecospira semseyi and Thecospira tenuistriata in life position based on the study of thin sections of Cipit boulders. Seemingly, the Th. semseyi shells were not firmly attached to the substrate by a small cementation surface; therefore, they could have lived embedded into the biogenic matrix. By contrast, Th. tenuistriata displays an umbonal region modified in a cementation area, allowing the attachment to the bio-builder. Most samples don’t show any signal of transport since all specimens are articulated and are still attached to the substrate. Besides, the commissures are partially opened, free of any encrustation of sponge or bryozoan growing. The preservation of brachiopods suggests that the samples of both species were fossilized in life position. Moreover, Th. semseyi and Th. tenuistriata continued to live although the host’s fabric was imbibing the brachiopods’ ventral valve. This shows that thecideids were important components of Tethys reef communities.

Keywords: Thecideid brachiopods, San Cassiano Formation, paleoecology, Triassic, Italy.