Preliminary notes on decapods preserved in ammonite shells from the Middle Jurassic of Argentina

Abstract

Although rare, the association of fossil decapods with mollusks has been reported since the Triassic by several authors, including interpretations of inquilinism, sheltering and gregarious behaviour (e.g., Fraaye & Jäger, 1995; Klompmaker & Fraaije, 2012; Landman et al., 2014; Fraaije et al., 2020). The aim of this contribution is to document the presence of two erymoids in ammonite shells from the upper Bathonian of Los Patillos Formation, Paso del Espinacito locality, San Juan province, Argentina (Alvarez, 1997; Riccardi, 2008). This unit, dominated by calcareous sandstones and limestones, has been interpreted as an open shelf marine setting (Álvarez et al., 1994; Álvarez, 1997).The first specimen consists of half a carapace of Eryma sp. (Figure 1A-B) preserved in the body chamber of Bullatimorphites (K.) cf. bullatus (d’Orbigny, 1846). The anterior part of the carapace is oriented towards the aperture. Ongoing research using CT scanning may provide further information about the possible presence of addiƟonal exoskeletal elements within the sediment infill, as the lobster is small relative to the ammonoid body chamber.The second specimen consists of a pair of chelipeds of an erymoid, including dactylus, propodus, carpus and a mold of the merus, preserved in a fragment of a body chamber of an ammonoid (Figure 1C). The tip of the fingers are directed towards the phragmocone. The preservation of both chelipeds, along with the massive sediment infill of the body chamber devoid of other fossil remains, suggests that the lobster remains were not introduced into the ammonite shell by an external agent. Instead, the lobster may have entered the vacant shell on its own (either to molt, retreating backward and leaving the shelter, or dying inside it), leaving both chelae together as they were not subjected to transport.As these are the only two records of decapods in Los Patillos Formation, a taphonomic factor is likely preventing their preservation. The use of ammonite shells as shelters may have played an important role in the fossilization of decapods in this unit.