The Echinoid Directory

Phylogeny and classification


As usual the major obstacle to placing fossil taxa into families is an absence of information about their lantern structure and ambulacral plate compounding. Pedinidae are no exception. Furthermore, it is possible that amongst the early members currently assigned to this family are stem-group members of the Euechinoidea in general.

Three distinct morphologies can be distinguished:-

1. Zardinechinus stands apart from other pedinoids because of its markedly different style of tuberculation. Unlike other pedinoids it has its primary interambulacral tubercles surrounded by a ring of scrobicular tubercles. Furthermore, its ambulacral plating is very primitive, with plate compounding present only adorally and then with primary tubercles not much enlarged. It is either sister group to all other pedinoids, or a more primitive taxon near the base of the Acroechinoidea.

2. Pedina, Hemipedina, Diademopsis, Caenopedina, Palaeopedina, Leptocidaris, Phymopedina and Pseudopedina form a second group of closely related taxa. However, they do not share any derived synapomorphy and are thus unlikely to represent a clade. They have primitive ambulacral plating, in which all elements reach the perradius, and simple uniserial pore zones.

3. Echinopedina, Leiopedina and Loriolipedina form the third group, with more complex ambulacral plating in which one or two elements are reduced to adradial demiplates. Thieulinipedina is an intermediate, linking this group to Pedina. All except Echinopedina have pore zones composed of strongly oblique triads.

In the Treatise on Invertebrate Palaeontology Fell (1966) largely followed the scheme established by Mortensen (1940). However, a number of these genera are now known to be wrongly placed. Micropedina and Dumblea have crenulate tubercles and are best placed along with Pedinopsis and Cottaldia in the stirodonts. Mesodiadema is almost certainly synonymous with Loriolella, and is a stem group irrregular echinoid. Pseudorthopsis is sister group to Orthopsis and presumably like that taxon had keeled teeth. Finally Stenechinus has crenulate tubercles and echinoid plate compounding and is here removed to the stem group Camarodonta.