Helochelydridae
The Helochelydridae are an extinct family of stem-turtles known from fossils found in North America and Europe spanning the Early to Late Cretaceous.
Description
The skull, shell and osteoderms of helochelydrids are covered in small, cylindrical protuberances, which are a distinctive characteristic of the group. They are thought to be terrestrial, based on the presence of limb osteoderms and bone histology. Their skull morphology is dissimilar to that of extant tortoises, suggesting an omnivorous habit similar to that of box turtles.Taxonomy
Helochelydridae includes all turtles that are more closely related to Helochelydra than Sichuanchelys, Meiolania, or extant turtles. 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. Some recent studies have recovered them as paracryptodires, though other studies have found them to be more basal than paracryptodires.Position of helochelydrids according to Tong et. al. 2023:
Genera
Aragochersis Escucha Formation, Spain, Early Cretaceous Helochelydra Wessex Formation, England, Early Cretaceous- "Helochelydra" anglica Purbeck Group, United Kingdom,
- "Helochelydra" bakewelli Mantell, 1833 Tunbridge Wells Sandstone, England, Helochelys Grünsandstein Formation, Germany, Late Cretaceous, Cambridge Greensand, England, Albian-Cenomanian, France, Spain, CenomanianTrachydermochelys phlyctaenus Seeley 1869, Cambridge Greensand, England, Albian-CenomanianNaomichelys Cretaceous North America
- Plastremys Upper Greensand Formation, Cambridge Greensand, England, Albian-Cenomanian, Spain, Albian France, CenomanianSolemys France, Spain, Late Cretaceous Trachyaspis turbulensis Bergounioux 1957 Gargallo, Spain, Early Cretaceous