NEW MODEL FOR HOW PARACRINOIDS (ECHINODERMATA, BLASTOZOA) DEVELOPED THEIR ASYMMETRICAL, UNISERIAL AMBULACRA

Paracrinoids (mostly Ordovician) are a medium-sized class (~ 25 genera) of stemmed echinoderms with unusual thecal and ambulacral morphology. Unlike most other blastozoans, the theca has numerous plates not organized into horizontal circlets or showing much pentameral symmetry. The only circlets that can be recognized are the three basals around the stem facet and the 4-6 orals around the mouth. Another distinctive feature is the 2-5, uniserial, asymmetrical ambulacra (ambs) extending outward from the mouth bearing mostly uniserial erect brachioles on the left side and a food groove on the right. Most paracrinoid ambs are recumbent on the thecal plates, but a few groups have ambs that become erect armlike structures usually bearing uniserial brachioles.

In 2007, Sumrall and Wray proposed that the earliest echinoderm classes developed pentameral (5-sided) symmetry in their ambs based on the fossilized ontogeny shown by early juveniles: first, 2 lateral branches from the mouth, next, 3 branches (1 anterior, 2 lateral), and last 5 branches (1 anterior, L & R lateral branches split), producing a distinctive “2-1-2” pattern. The 5 ambs show slight differences in how appendages branch off; 3 ambs (A, C, and E) show only alternation, 2 ambs (B and D) have the 1st 2 branches to the left, then only alternation, a pattern known as Loven’s Law). Sumrall and Wray were most interested in how different ambs were subsequently lost in various echinoderm classes, going from 5 to 4, 3, 2, or 1. They argued that 2 ambs were formed when amb A failed to develop and the 2 lateral branches (DE and BC) failed to divide. This origin works well in echinoderm classes with biserial ambs, such as rhombiferans, but not in paracrinoids where all brachioles branch off one side. I propose that the ancestral paracrinoid instead lost A, C, and E (the 3 normal ambs) at an early stage of development, leaving only ambs B and D with the first 2 juvenile appendages on the left side. Because the appendage asymmetry (and food groove location) was already established, all later newly inserted brachioles followed the same pattern, forming 2 long, asymmetrical ambs extending down the theca or 2 shorter S-shaped ambs curving around the summit. A few paracrinoid genera added 1-2 additional amb branches near the summit, and many of these genera with 2-4 ambs tilted the mouth off the thecal axis.