Glyptocystitoid Rhombiferans
There are very few examples of high-resolution character-based phylogenetic datasets tied to high-resolution stratigraphic occurrence for invertebrates. I have begun to construct such a dataset in order to investigate questions of sampling quality and the patchiness of the known fossil record. For this project, I have chosen glyptocystitoid rhombiferans (one of the "cystoid" groups). This clade is ideal for such a study for several reasons. 1) They have a standardized skeleton composed of 26-27 primary thecal plates thereby eliminating much of the confusion found in other echinoderm groups that do not have standardized plating; 2) they have a relatively long geological range (Late Cambrian to Late Devonian); 3 there are a relatively small number of species, and 4) they show a wide range of morphological variability that can be used to discover characters for used in phylogenetic analysis. The second phase will require linking all of the known North American callocystitid occurrences to conodont stratigraphy.
The first step in this project (reconstructing the evolutionary relationships of callocystitid glyptocystitoids) is nearing completion. This work suggests that the three ”subfamilies” of callocystitids have no bearing on real evolutionary groupings. These subfamilies were diagnosed primarily of primitive features such as shape of the theca, spacing of the brachioles, or based on convergent features such as branching of the ambulacra. Features that unite true callocystitid clades include the loss of the A ambulacrum, the development of paired hydropores, the loss of two of the respiratory structures.
Preliminary phylogenetic relationships among Callocystitid genera.
Callocystitid Anatomy
Before a phylogenetic dataset can be constructed the anatomy of the clade in question must be thoroughly understood. The following is a generalized account of callocystitid morphology that I hope is useful for those interested in learning more about this group.
Callocystitids are the most derived of Glyptocystitoids. The body or theca is ovoid to lens-shaped and affixed to the substrate by a short stem that is cemented to the substrate with a single-piece holdfast. Two to five ambulacra lie recumbently upon the surface of the theca and food-gathering brachioles attach to these ambulacra. The mouth is located at the summit of the theca where the ambulacra unite. Nearby are the spout-like hydropore and the pyramid-covered gonopore. Unlike other echinoderms, the anus is located on the right side of the theca. For respiration, callocystitids have five or fewer pectinirhombs that pump ambient seawater through the theca in closed canals where gas exchange occurs.
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Reconstruction of the callocystitid Lipsanocystis. AM = ambulacrum, BR =brachioles, DS = distal stem, HF = holdfast, HP = hydropore/gonopore, PR = pectinirhomb, PS = proximal stem, TH = theca. |
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A specimen of the Devonian callocystitid Strobilocystites calvini. Note thestem, holdfast and theca. |
The stem of the callocystitid Lipsanocystis magnus. This stem illustrates the unusual construction of the stem with inner and outer columnals. Note that this gives the proximal stem great flexibility. |
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The ambulacral system of callocystitids is formed from two types of elements: floor plates that rest recumbently upon the thecal surface, and brachioles that attach to the floor plates. This photograph shows the floor plates forming a well-developed food groove for transporting nutrient to the mouth. The food gathering brachioles mount on the small facets along the edge of the food groove. |
The hydropore and gonopore of Strobilocystities calvini. The hydropore and gonopore complex in callocystitids is positioned between three oral plates. In this species the hydropore is split with two separate openings and a sieve-like covering. The gonopore is the small hole between the hydropores and is often preserved covered with a pyramid of small plates. |
Other Glyptocystitoids
There are other clades of Glyptocystitoids as well. These will be incorporated into an expanded version of this project at a later date. Here are a few examples.
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Cheirocystis fultonensis is a typical cheirocrinid glyptocystitoid. Most primitive cheirocrinids hava barrel-shaped theca and well-developed ridges connecting plate centers. |
Echinoencrinites is a typical echinoencrinitid glyptocystitoid. It has a globular theca, but the summit of the theca is greatly reduced and has only few feeding appendages. |
Colin Sumrall
Department of Earth and Planetary Sciences
1412 Circle Drive
Knoxville, TN 37996-1410
Phone: (865) 974-0400
Email: csumrall@utk.edu