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| Helianthochelys redondita Sterli, Vlachos, Cuitiño, Cerda & Buono, 2025 |
Abstract
Dermochelyids are a unique lineage of turtles where the primary carapace underwent an extreme reduction, with its maximum expression in the extant Dermochelys coriacea. Above the primary carapace (thecal), a secondary (epithecal) carapace formed by thousands of ossicles appeared in the late Paleocene. In general, the fossil record of dermochelyids is incomplete, fragmentary, and patchy. Besides, no indepth and detailed studies on their anatomy led to a poorly resolved alpha taxonomy. In this contribution, we present one of the most complete dermochelyids in the world. Our new turtle was found in the Gaiman Formation (Lower Miocene) in Chubut, Argentina. This Burdigalian record of Patagonia represents the southernmost species in the fossil record of dermochelyid turtles in the Southern Hemisphere in the Miocene. The histological analysis of long bones shows a highly remodeled cortex, a unique condition in turtles. The tissue compaction recorded in this new turtle would suggest it was a very old individual. The phylogenetic analysis recovered the new species in a clade characterized by longitudinal ridges in the carapace and ossicles arranged in a “sunflower” pattern. Exploring the diversity patterns of dermochelyids, we recognize two peaks of diversity and three extinction events, comparable with those observed in other marine tetrapods (e.g., cetaceans, penguins).
Key words. Dermochelyidae. Marine tetrapods. Gaiman Formation. Patagonia. Diversity.
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| Anatomy of Helianthochelys redondita gen. and sp. nov. MPEF-PV 10918 from the Gaiman Formation of Chubut Province, Argentina. 1, dorsal view of the carapace with a reconstruction of the carapace outline. 2, oblique view of the carapace showing the primary, middle, secondary, and tertiary ridges. 3, detail of the carapace showing the sunflower pattern and defining the disc and ray ossicles; white scale bar equals 10 cm. 4, visceral view of the carapace showing the various bones in their recovery position. Black scale bar equals 50 cm. |
Testudinata Klein, 1760 (sensu Joyce et al., 2020a)
Testudines Batsch, 1788 (sensu Joyce et al., 2020b)
Cryptodira Cope, 1868 (sensu Joyce et al., 2020c)
Durocryptodira Danilov & Parham, 2006 (sensu Joyce et al., 2021)
Americhelydia Joyce et al., 2013 (sensu Joyce et al., 2021)
Chelonioidea Baur, 1893 (sensu Joyce et al., 2021)
Dermochelyidae Lydekker, 1889 (sensu Joyce et al., 2021)
Helianthochelys redondita gen. et sp. nov.
Derivation of the name. The generic name, Helianthochelys, comes from the combination of the Greek words ήλιος (Helios, sun) and ανθός (anthos, flower) combined as “sunflower” and the feminine word χέλυς (chelys, turtle), in allusion to the sunflower pattern of ossicles in the secondary carapace. The specific name, redondita, refers to the type locality Estancia La Redonda Chica, hence the Latin diminutive -ita in the end.
Holotype. MPEF-PV 10918, almost complete carapace (secondary ossicles and nuchal bone) with a putative skull fragment (jugal) and postcranial elements (cervical vertebrae 4, 7, and 8, ten thoracic vertebrae, thoracic ribs, two sacral neural arches, three caudal vertebrae, both scapulae, both pubes, both ischia, both ilia, right femur), forming part of the paleoherpetological collection of MPEF (see Sterli et al., 2021 for additional information).
Type locality. Coastal creek located 5 km east of La Redonda Chica House in La Redonda Chica Farm, Chubut province, Argentina (Fig. 2). Age. Gaiman Formation, Burdigalian, Early Miocene (Cuitiño et al., 2023).
Diagnosis (same for genus and species). Helianthochelys redondita is a dermochelyid turtle, based on the presence of a mosaic carapace (psephophore dermochelyid turtles) and the remaining diagnostic synapomorphies listed above. Helianthochelys redondita shares with other more crownward dermochelyids the presence of the characteristic sunflower pattern formed by the ossicles (helianthophore dermochelyid turtles). Differs from Natemys and is associated with more derived helianthophores in the absence of some elongated ossicles and the presence of anteroposterior ridges. Differs from the type of Psephophorus polygonous and is associated with other closely related taxa from the Atlantic coasts (e.g., “Psephophorus” scaldii and Westerschelde specimen from Europe and “Psephophorus” calvertensis from the Atlantic side of North America) due to the absence of change of curvature until the tip of the ridge, in cross-section. Differs from all these Atlantic helianthophores based on the presence of secondary (and even tertiary) anteroposterior ridges between the five primary ridges. Differs from the Westerschelde specimen and “Psephophorus” calvertensis and is more closely related to “Psephophorus” scaldii based on higher counts of “ray” ossicles around the central “disk” ossicle (up to 13 ossicles) and the presence of comparatively thinner, medium-thick (0.6–1.1 cm) ossicles as well. Differs from “Psephophorus” scaldii based on the disk/ray pattern: Helianthochelys redondita has usually three ray ossicles between two successive disk ossicles, whereas “Psephophorus” scaldii has, usually, a single ray ossicle between two successive disk ossicles.
Juliana Sterli, Evangelos Vlachos, José Ignacio Cuitiño, Ignacio A. Cerda, and Mónica Romina Buono. 2025. SOUTHERNMOST DERMOCHELYID TURTLE FROM THE MIOCENE. AMEGHINIANA. 62(4); 1–19. https://www.ameghiniana.org.ar
https://www.ameghiniana.org.ar/index.php/ameghiniana/libraryFiles/downloadPublic/149
https://x.com/mefpatagonia/status/1974240673780916342
