Fordilloidea

Fordilloidea is an extinct superfamily of early bivalves containing two described families, Fordillidae and Camyidae and the only superfamily in the order Fordillida. The superfamily is known from fossils of early to middle Cambrian age found in North America, Greenland, Europe, the Middle East, Asia, and Australia.The Paleobiology Database Fordillidae entry
accessed 4 January 2012The Paleobiology Database ''Fordilla'' entry
accessed 4 January 2012 Fordillidae currently contains two genera, ''Fordilla'' each with up to three described species while Camyidae only contains a single genus ''Camya'' with one described species, ''Camya asy''. Due to the size and age of the fossil specimens, Fordillidae species are included as part of the Turkish Small shelly fauna.

Description

''Fordilla'' are small bivalves with valves that are equal in size and suboval in shape. In size, ''Fordilla'' specimens reach a total shell length of up to and a height of . The shells are compressed laterally and the back edge is slightly broadened. The rear adductor is less developed and smaller than the front adductor, while the small pedal retractor muscle scar is positioned near the front adductor scar. The valve hinge is usually straight to slightly convexly curved and each valve will have at most one tooth present. The external surface of the shell occasionally shows faint ribbing.

Similar to ''Fordilla'', species of ''Pojetaia'' are small, with valves to less than in length. ''Pojetaia'' species have an overall shape which is suboval, with the subequal valves slightly elongated. The ligament is straight with an umbo which is central to subcentral. In contrast to ''Fordilla'', the rear adductor muscle was larger and more developed than the front adductor, with pallial muscles arranged along the valve margins. Also in contrast to ''Fordilla'', valves of ''Pojetaia'' possess between one and three teeth, with up to two teeth per valve. The exteriors of the shells show faint ribbing and fine comarginal growth lines.

''Camya'' is based on the fossils of two juvenile specimens which are both incomplete due to only the left valve of each being recovered. The valves have a distinct subtriangular shape and possess a long straight hinge. The umbo is positioned notably anterior on the shell and the beak is bracketed by two teeth of indistinctly pyramidal shape. The presence of the teeth was later questioned in a 1998 study by G. Geyer and M. Streng and cited the lack of preserved muscle scars as reason to suspect the placement of ''Camya asy'' in Bivalvia.

The inner shell layers of ''Fordilla'' and ''Pojetaia'' species both consist of layers of carbonate which is akin to the laminar aragonite layer found in extant monoplacophora. The structuring is similar to shell layering found in the extinct genera '' Anabarella'' and '' Watsonella'' which is thought to suggest members of the phylum Mollusca developed nacre independently several times. Of the four accepted bivalve genera to have been described from the Cambrian, Fordilloidea contains three, ''Camya'', ''Fordilla'', and ''Pojetaia'', with the remaining genus ''Tuarangia'' in the possibly related order Tuarangiida.

References

{{Taxonbar, from=Q5468083

Bivalve taxonomy
Prehistoric animal superfamilies
Prehistoric bivalves
Cambrian molluscs
Fossil taxa described in 1975