Paleontology - Spring 2001

Echinoderms


The echinoderms are an amazingly diverse and varied phylum. At one extreme, some authors recognize about 25 classes! Most of these are extinct, small, and restricted to the lower Paleozoic. A few are still with us today, several were remarkably abundant at different times. Echinoderms also have some rather unique features: most have a five-fold symmetry, the water vascular system is unique (and adaptable), and the level of sensory and neural development is rather low for such a successful post-Paleozoic group. As you might expect from this preamble, there are a large number of named morphological features and ideas about evolutionary relations.

We will focus our study on a few major groups: echinoids, holothurians, asterozoans, crinoids, and blastoids. You will encounter discussions regarding cystoids (closely related to blastoids) and carpoids (a.k.a. calcichordates). Please do not get too bogged down in the morphological terminology and evolutionary relations.

Classification of the Phylum

The sheer diversity and disparity of echinoderms result in wide differences of opinion regarding the most appropriate high-level classification. For example, blastoids and crinoids are considered as separate subphylums in the text but other authors place them into one subphylum as separate classes. Another result is the use of "extra" classification levels - see the echinoid classification on pages 269-270! Everyone agrees that the echinoderms are a monophyletic group (unlike the case with arthropods) because of their unique 5-fold symmetry. However some groups may be polyphyletic. These uncertainties illustrate the interpretive nature of taxonomic classification schemes.

The text classification is summarized here. The subphylums Crinozoa and Blastozoa are often referred to collectively as the "pelmetazoans". Note that several early Paleozoic classes are not included. They are relatively rare as fossils and may represent echinoderm "experiments" that left no descendants. Some of these odd, short-lived classes combine morphologic features of other classes, making their taxonomic placement difficult. (Consult the Treatise for more information if you are interested.)

Notes on the major groups

Echinoids (sea urchins)

Echinoids are a remarkable group (in fact, they are my personal favorite). There are less than 1000 specimens from the Paleozoic, and only about two genera survived the end-Permian extinction. Despite this auspicious history, the echinoids diversified into over 3600 species during the Mesozoic. Keep in mind that echinoids have no head - at best there is a mouth-anus axis, extremely limited sense organs, no brain, few organs, etc. - but still did remarkably well in the post-Paleozoic.

Echinoid morphology will probably be new to you. You need to keep a few basic points in mind. First, an echinoid skeleton consist of a series of plates that cover a sack-like body. The skeleton has pores through plates for the water vascular system (tube feet and madreporite) and genital pores. Larger openings between plates are for the mouth and anus (periproct). Second, echinoids are either regular (five-fold symmetry, mouth centrally located on the bottom and anus on the top) or irregular (bilateral symmetry superimposed, mouth shifted forward and anus to rear). Sand dollars are a group of irregular echinoids that are even more modified. Third, the echinoid manipulates its environment primarily by the use of tube feet and external spines. These are remarkably specialized in some groups.

The text presents a description of a regular (Echinus) and two irregular (Echinocardium and Mellita) echinoids. Here are some of the main morphological features to note:

The classification (p. 269-270) is rather complex. It illustrates a classification that uses a cladistic approach to define major groups (although it retains paraphyletic groups). Notice that the post-Paleozoic diversification is essentially the Subclass Eucechinoidea. (Take a look at the classification and Fig. 9.7 with the objective of understanding the relations between major groups.)
The subclasses are briefly discussed. Subclass Perischoechinoidea is the primitive group - note the low diversity, flexible tests. Subclass Cidaroidea have some unusual spine types. Subclass Euchinoidea are the main feature - you will find a lot of discussion regarding the function and evolution of specific morphological features. Instead of getting bogged down in all the details, I would recommend trying to identify the morphological change (and what it may mean functionally) for each section. (For example: tube feet become more variable as they became modified for respiration, funnel building, and locomotion.) The final section on Micraster is significant because Micraster has been studied for over a century. Morphological changes were documented by Rowe - since then their functional significance and evolutionary relations have been clarified. In this way, this has become a "case study" of great utility.

Holothurians (sea cucumbers - quite the nickname!)

These "nasty green warty things" need not detain us long. You should note two things: (1) their life habits, and (2) the nature of their preservable skeleton.

Edrioasteroidea

This is a rare Paleozoic group of unusual echinoderms (for which we have no samples). The main point here is their unusual morphology (see Fig. 9.32). They may have an important place in the overall evolution of the echinoderms (section 9.8) since they may be the ancestral group of Subphylum Echinozoa.

Subphylum Asterozoa (starfish and brittle stars)

You are probably familiar with some representatives of this subphylum - the starfish (Subclass Asteroidea). Starfish are highly effective predators - note how they go about consuming bivalves. This is a good example of how echinoderms evolved into a different ecological niche. The brittle stars (Subclass Ophiuroidea) are also quite interesting - the text suggests their main morphological development.

Crinoids (sea lilies)

Crinoids differ from the groups considered so far in that they are sessile and attached. The morphology is rather similar throughout the group, although the many fine variations provide ample room for classification - you will notice the information on calyx morphology (infraradials, supraradials, monocyclic vs. dicyclic) later in the discussion. The opening section provides you a good introduction to how they lived. The main morphological features are:

The descriptions of the main groups of crinoids is a bit term-rich. I would focus on the information on crinoid ecology (mostly in the last few pages).

Subphylum Blastoidoa: Cystoids and Blastoids

This subclass has two major stemmed groups: the cystoids and the blastoids. These are united by having internal respiration using pore structures that allowed water into the calyx. The pores of cystoids (Diploporita and Rhombifera) are used in their classification (see Fig. 9.45) but (please) do not worry about all the terms. Focus on the ecology (p. 304).

The blastoids are a little more familiar (and common). Here the respiratory pores are aligned along ambulacra (water vascular system). Here are three terms to know:

So how did this work? Water entered through small hydropores, flowed along the hydrospires, and exited out the spiracles. Feeding was via the brachioles - food was passed to the central food groove and ingested at the mouth (top of calyx). The ecology of blastoids is relatively simple.

Subclass Homalozoa (carpoids or calcichordates)

This is truly a bizarre group, even for the echinoderms (or are they vertebrates). This section is quite technical but it boils down to one question: are the carpoids early vertebrates? Vertebrates are more similar to echinoderms than any other living invertebrate group (in terms of larval development and organization). Jeffries and co-workers suggested that the carpoids are an offshoot of echinoderms that are (in fact) vertebrates. Even a quick read of the text will reveal that this is controversial - alternative interpretations of the morphology are identified (mouth or anus?). This is a intriguing issue, but it has not be resolved as of yet.

Echinoderm evolutionary relations

The final part of this chapter serves to wind up this fascinating group. The four sections deal with some broader issues: (1) evolution of echinoderms (Fig. 9.51) - quite a challenge in light of the sheer range of body plans; (2) the central role of tube feet in the phylum; (3) what is the point of five-fold symmetry anyway?; and (4) convergent evolution - recall this is an argument for functional significance.

Congratulations - you made it through the chapter!