Thecideida is an order of cryptic articulate brachiopods characterized by their small size and habit of cementing their ventral valves to hard substrates such as shells, rocks and carbonate hardgrounds

Carbonate hardgrounds

Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor . A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone sequences and distinguished from later-lithified sediments by evidence of...

. Thecideides first appear in the Triassic

Triassic

The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

(Jaecks and Carlson, 2001) and are common today (Lüter, 2005; Lüter et al., 2007).

Taxonomy

Order Thecideida

• Superfamily Thecideoidea Gray 1840
• Superfamily Thecospiroidea Bittner 1890