Isolated and fragmentary fossil findings: clues for fossil fish diversity and keys for further research

GOUIRIC CAVALLI, S.; CARIGLINO, BÁRBARA; IGLESIAS, ARI; REGUERO, MARCELO

Congreso; SCAR Open Science Conference; 2022

Argentinian Antarctic expeditions to the Longing Gap locality (Antarctic Peninsula) where the Ameghino Formation (Kimmeridgian–Berriasian) crops out, were carried out in four summer expeditions (2016, 2018, 2020, and 2022) resulting in the finding of abundant fossil actinopterygians, including complete bodies as well as fragmentary specimens. Here, we report Late Jurassic isolated scales and scale patches of marine actinopterygians. Four morphotypes are recognized: 1) large rounded (=elasmoid) of amioid-type; 2) large, rhombic (=ganoid); 3) small, rhombic scales with four or five enameloid ridges; and 4) rectangular (higher than wide), ganoid scales with well-developed peg and socket structure. These scales are attributed to Amiiformes, Lepisosteiformes , †?Ameghinchthys, and †Aspidorhynchiformes, respectively. Accordingly, this is the first amiiform record for the Antarctic continent, and the southernmost records for both, Amiiformes and Lepisosteiformes worldwide. Delicate amiiforms scales and the few well-preserved †Aspidorhynchiformes scales found isolated in black shales suggest early post-mortem or in-life detachment and short flotation in calm waters. The associated crustacean molts, complete pyritized fishes, and delicate small ammonites further indicate the reductive nature of the environment. The Lepisosteiformes †?Ameghinchthys, and †Aspidorhynchiformes scale patches, lack the enameloid in many of the scales and show cracking, polish, and abrasion evidencing postmortem erosion. Despite being retrieved isolated and fragmentary, the fossil material is highly informative, providing a glimpse of the actinopterygian´s marine diversity during Jurassic times, their depositional environment, and taphonomic history. *Project funding: PICT 2017-0607 to MR and PICT 2019-02419 to SGC.