The Helochelydridae are an extinct family of stem-turtles known from fossils found in North America and Europe spanning the Early to Late Cretaceous.[1][2]
| Helochelydridae Temporal range: | |
|---|---|
 | |
| Helochelydra nopcsai skull | |
 | |
| Shell of Naomichelys | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Pantestudines |
| Clade: | Testudinata |
| Clade: | Perichelydia |
| Family: | †Helochelydridae Nopsca, 1928 |
| Genera | |
See text | |
| Synonyms[1] | |
Solemydidae Lapparent and Murelaga 1997 | |
Description
The skull, shell and osteoderms of helochelydrids are covered in small, cylindrical protuberances, which are a distinctive characteristic of the group.[3] They are thought to be terrestrial, based on the presence of limb osteoderms (granicones) and bone histology.[4] Their skull morphology is dissimilar to that of extant tortoises, suggesting an omnivorous habit similar to that of box turtles.[5]
Taxonomy
Helochelydridae includes all turtles that are more closely related to Helochelydra than Sichuanchelys, Meiolania, or extant turtles.[6] Although referred to as Solemydidae in recent literature on extinct turtles, Helochelydridae has priority over Solemydidae. They are placed as part of the clade Perichelydia.[7] Some recent studies have recovered them as paracryptodires,[8][9] though other studies have found them to be more basal than paracryptodires.[10][11]
Genera
- Aragochersis[12] Escucha Formation, Spain, Early Cretaceous (Albian)
- Helochelydra Wessex Formation, England, Early Cretaceous (Barremian)
- “Helochelydra” anglica (Lydekker, 1889) Purbeck Group, United Kingdom, (Berriasian)
- “Helochelydra” bakewelli Mantell, 1833 Tunbridge Wells Sandstone, England, (Valanginian)
- Helochelys Grünsandstein Formation, Germany, Late Cretaceous (Cenomanian), Cambridge Greensand, England, Albian-Cenomanian, France, Spain, Cenomanian
- Trachydermochelys phlyctaenus Seeley 1869 (revised Joyce, 2022[13]), Cambridge Greensand, England, Albian-Cenomanian
- Naomichelys Cretaceous (Aptian-Campanian) North America[6]
- Plastremys Upper Greensand Formation, Cambridge Greensand, England, Albian-Cenomanian, Spain, Albian France, Cenomanian
- Solemys France, Spain, Late Cretaceous (Campanian-Maastrichtian)
- Trachyaspis turbulensis Bergounioux 1957 Gargallo, Spain, Early Cretaceous (Aptian-Albian) (possibly synonymous with Plastremys rutteri)[13]
Indeterminate remains most similar to “Helochelydra” anglica and “Helochelydra” bakewelli have been reported from the Berriasian aged Angeac-Charente bonebed of France.[14]