Acrotretida

(from the Early Cambrian)

Appearing in the earliest Cambrian (Terreneuvian Epoch), these are among the very first hard-shelled organisms. Through their long history (spanning the entire Phanerozoic) they remain quite distinct from other inarticulate brachiopods.

"Chitinous or calcareochitinous, more or less circular, conical, or flattened shells with a circular pedicle opening in the apex of the ventral valve. There is no listrium, but a pseudocardinal area is present." Twenhofel, W. H., and Shrock, R. R., Invertebrate Paleontology, 1935. p.287

Acrotretids are usually round with a convex, cap-shaped brachial valve, and a flat pedicle valve. They resemble the paterinids in general outline and very small size, but are distinguished by calcium carbonate shell, the more distinctly conical form of the pedicle valve and particularly the presence of a minute pedicle foramen, located at or just behind the apex. The posterior slope of the pedicle valve commonly bears a distinct trough; but because the pedicle does not emerge at the base of this trough, next to the valve margin, the indentation cannot be interpreted to denote pressure of the pedicle against the outer side of the valve.

Life posture of Acrotreta is shown at the left. The pedicle emerges from a rounded opening between the apex of the pedicle valve and the nearest shell margin.

The basal acrotretids were most common from the Early Cambrian to Late Ordovician, with a  few stragglers managing to continue on until the Devonian (Frasnian Age), when they died out, along with many other forms of life in the Frasnian-Famennian mass extinction. One specialized group, the Discinida, remain.

Systematics

In non-cladistic schemes, there is only one superfamily.

Acrotretoidea (Early Cambrian-Devonian)

page uploaded 7 June 2002
checked ATW090411
(originally uploaded on Kheper site uploaded 11 May 1999)
page by M. Alan Kazlev (Creative Commons Attribution license) 1999-2002