A Beginner's Guide to Blastoids

No, not that. These (photo by DanielCD):



Blastoids are small but reasonably common Palaeozoic (Silurian to Permian) fossils. The name means, roughly, 'bud-like' and refers to the common resemblance of the fossils to flower buds. However, blastoids were not plants but echinoderms, animals of the same phylum as modern crinoids, starfish and sea urchins.


Pentremites, the best-known blastoid. Photo from here.

Most blastoid fossils are less than an inch in diameter, and all have a very clear pentaradial arrangement (the following account is primarily based on Beaver et al., 1967). The theca (the main body) is very solidly built from a very regular arrangement of plates, with five ambulacra (feeding grooves) running down the sides (the ambulacra are what house the tube feet in living echinoderms). At the top of the fossil where the ambulacra meet is a central opening that in life would have led to the mouth, with a number of other openings around it. The largest of these was the anus while the smaller openings are known as the spiracles.



The spiracles are connected to the hydrospires, a series of folds of the internal body wall underlying the ambulacra (reconstruction above from Schmidtling & Marshall, 2010) that are generally believed to have function in respiration. Presumably, water was drawn into the hydrospires on either side of the ambulacra and expelled through the spiracles. Two orders of blastoids are distinguished based on the arrangement of the hydrospires and spiracles. In members of the order Fissiculata, the hydrospires open directly to the outside world through a series of slits in the thecal plates; the spiracles are often small or slit-like and may not be readily distinguishable from the hydrospire slits (as far as I can see, anyway). In members of the order Spiraculata, such as Pentremites, the hydrospire slits were internal and entry to the hydrospires was through minute pores on either side of the ambulacra while the spiracles were much larger and more distinct. The remaining internal anatomy (such as the mode of reproduction) remains largely unknown.


In life the theca was only a small (but significant) part of the whole. A slender stem (up to about 25 cm long) attached the blastoid to the substrate while on either side of each ambulacrum was a row of arm-like structures known as brachioles. The brachioles would have captured food particles in the water, transporting the particles down a groove on the underside to the ambulacrum below. The stem and brachioles were comparatively delicate and rarely preserved but the positions of the brachioles can still be otherwise distinguished by the presence of attachment sockets alongside the ambulacra.

Brachioles are also found in a number of other extinct echinoderm groups (such as cystoids and eocrinoids) but are not found in any living echinoderms. Despite a certain superficial resemblance, brachioles are not comparable to the arms of living crinoids. In recent years, it has been proposed that the echinoderm exoskeleton can be divided on morphological and developmental grounds into two distinct components, the axial skeleton which grows through the alternating addition of plates at the distal points and the extraxial skeleton which can grow through the addition of new plates in between pre-existing ones (David et al., 2000). The axial skeleton makes up the ambulacra and associated structures while the extraxial skeleton makes up the remainder of the body wall. Crinoid arms, which carry the ambulacra along their axis and contain radial extensions of the internal coelom, are made up of both axial and extraxial components. Blastoid brachioles, which sit alongside the ambulacra and do not contain coelomic extensions, are entirely axial*. While many authors have suggested that crinoids may be derived from brachiolar echinoderms (particularly cystoids, some of which have a similar arrangement of thecal plates to early crinoids - e.g. Ausich, 1998), proponents of the extraxial-axial division regard the similarities between the groups as convergent; indeed, the phylogeny proposed by David et al. (2000) would place brachiolar echinoderms such as blastoids entirely outside the echinoderm crown group.

*If I understand things correctly, deriving crinoid arms from blastoid brachioles would be a little like deriving human arms from lemur fingernails.

REFERENCES

Ausich, W. I. 1998. Early phylogeny and subclass division of the Crinoidea (phylum Echinodermata). Journal of Paleontology 72 (3): 499-510.

Beaver, H. H., R. O. Fay, D. B. Macurda Jr, R. C. Moore & J. Wanner. 1967. Blastoids. In Treatise on Invertebrate Paleontology pt. S. Echinodermata 1. General Characters. Homalozoa-Crinozoa (except Crinoidea) (R. C. Moore, ed.) pp. S297-S455. The Geological Society of America, Inc. and The University of Kansas.

David, B., B. Lefebvre, R. Mooi & R. Parsley. 2000. Are homalozoans echinoderms? An answer from the extraxial-axial theory. Paleobiology 26 (4): 529-555.

Schmidtling, R. C., II & C. R. Marshall. 2010. Three dimensional structure and fluid flow through the hydrospires of the blastoid echinoderm, Pentremites rusticus. Journal of Paleontology 84 (1): 109-117.