Echinoencrinitidae

Belongs within: Cheirocrinidae.
Contains: Callocystitidae.

The Echinoencrinitidae are a group of Lower Ordovician to Upper Silurian cystoids with a generally distinctly shaped theca, with short ambulacra and a protruding periproct. Inclusion of the Scoliocystinae, possessing advanced disjunct pectinirhombs with vestibule rims, in the Echinoencrinitidae has been subject to debate (Paul & Rozhnov 2016).

Small-mouthed cystoids

Published 11 September 2023

Imagine yourself swimming through oceans of the early Ordovician. You’re in what will eventually become the Baltic Sea, though it currently lies somewhere in the lower subtropics of the Southern Hemisphere (it’s over 450 million years ago, that’s plenty of time for things to move a long way). Among the countless animals carpeting the sea floor, you find something small but curious. A flexible stem leads to a multi-faceted central body, looking a bit like a distorted woggle. From the top of the woggle emerge a dozen or so mobile appendages, little armour-plated tentacles that grasp at passing food particles. The animal you have just found is an echinoencrinitid.

The Echinoencrinitidae are a family of cystoids, one of the many groups of odd-looking quasi-sessile echinoderms that did not make it past the end of the Palaeozoic. In life, cystoids typically had the basal stalk and upper brachioles (‘armlets’) described above, but these often became disassociated after death, leaving just the central theca. It is the semi-globular appearance of the resulting fossils that inspired the name of ‘cystoid’. Echinoencrinitids are distinguished from other cystoids by the shortness of their ambulacra, the feeding grooves from which the brachioles emerged. These are restricted to the uppermost surface of the theca, not extending down the sides as in other cystoids. Each ambulacrum contained no more than three brachioles (Paul 2015). The periproct, a little circlet of plates on one side of the theca that in life would have surrounded the anus, is relatively small but typically protuberant (Kesling 1967). I haven’t been able to learn just what was the significance of the reduced ambulacra and number of brachioles, but it must surely have been related to their mode of feeding. Perhaps echinoencrinitids were more actively grabbing particles out of the water column than acting as passive filterers. As for the protruding periproct, it resembles structures found in other echinoderms that allow faecal matter to be ejected away from the body and not find its way back to the mouth.

Echinoencrinitids first appeared in the early Ordovician, at which time they appear to be unique to the Baltoscandia region of modern northern Europe. And if you ask some researchers, this is where they begin and end. Questions remain over the status of the Scoliocystinae, a group known from the early Silurian of western Europe and the earliest Devonian of North America. The scoliocystines were excluded from the Echinoencrinitidae by Broadhead & Strimple (1978) and moved to the related family Callocystitidae, on the basis of differences in structure of the pore rhombs (diamond-shaped structures on the theca that would have functioned in respiration). However, Paul (2015) later maintained them as echinoencrinitids, evidently regarding the ambulacral similarities as more significant. If the scoliocystines are indeed echinoencrinitids, then the family’s fossil record is marked by some remarkable gaps, disappearing from one region only to reappear in a different part of the world some tens of millions of years later. If the scoliocystines are not echinoencrinitids, then we have an intriguing case of evolutionary convergence. Either way, this is a family that presents us with some fascinating questions.

Systematics of Echinoencrinitidae

Characters (Paul & Rozhnov 2016): Reduced oral area, with theca composed of 26 or 25 plates arranged in five circlets according to formula: 4 basals (B, plural BB), 5 infralaterals (ILL), 5 laterals (LL), 4 or 5 radials (RR), and 7 orals (OO), with disjunct pectinirhombs.

<==Echinoencrinitidae [Echinoencrininae]
    |--Scoliocystinae [Scoliocystidae]SB02
    |    |--Glansicystis Paul 1967SB02, PR16
    |    |--Osculocystis Paul 1967SB02, PR16
    |    |--Schizocystis Jaekel 1895SB02, K67 [Schizocystinae]
    |    |    `--*S. armata (Forbes 1848) [=Echinoencrinus armatus]K67
    |    |--Scoliocystis Jaekel 1899PR16
    |    |    |--*S. pumila (Eichwald 1860) [=Caryocystites pumilus, S. pumilus]PR16
    |    |    `--S. thersites Jaekel 1899PR16
    |    `--Tyrridiocystis Broadhead & Strimple 1978PR16
    |         `--T. chelyon Broadhead & Strimple 1978P15
    `--+--CallocystitidaePR16
       `--+--Gonocrinites Eichwald 1840PR16
          |    |--*G. angulosus (Pander 1830)PR16 (see below for synonymy)
          |    |    |--G. a. angulosusK67
          |    |    `--‘Echinoencrinites’ a. comptusK67
          |    `--G. lahuseni (Jaekel 1899) [=Echinoencrinites lahuseni]PR16
          `--Echinoencrinites von Meyer 1826PR16, K67 (see below for synonymy)
               |--*E. senckenbergii von Meyer 1826 [incl. Echinosphaerites granatum von Schlotheim 1826]K67
               |    |--E. s. senckenbergiiK67
               |    `--E. s. acutangulusK67
               |--‘Eutretocystis’ acutirostrisK67
               |--E. interlaevigatusK67
               |--E. laevigatusK67
               |--E. reticulatusK67
               |--‘*Eutretocystis’ similis Phleger 1935K67
               |--E. simplicatusK67
               |--E. sphaeroidalisK67
               `--E. striatusK67

Echinoencrinitidae incertae sedis:
  Glansicystites baccata (Forbes 1848)SG93
  Glaphyrocystis Jaekel 1899K67
    |--*G. woehrmanni Jaekel 1899K67
    `--G. compressaK67
  Proctocystis Regnéll 1945K67
    |--*P. monstruosa Regnéll 1945K67
    `--P. rossicaK67
  Erinocystis Jaekel 1899K67
    |--*E. sculpta Jaekel 1899K67
    |--E. angulataK67
    |--E. broeggeriK67
    `--E. volborthiK67
  Fusicystis Terentiev in Zuykov et al. 2008PR16
    `--F. magnificus Terentiev in Zuykov et al. 2008P15

Echinoencrinites von Meyer 1826^{PR16, K67} [=Echinoencrinus Volborth 1842^K67, Euchinoencrinites (l. c.)^K67; incl. Eutretocystis Phleger 1935^PR16, Gonocrinus Eichwald 1859^K67, Sycocystites von Buch 1846^K67, Cyclocystites (l. c.)^K67, Sycocystes (l. c.)^K67, Sycocystis Haeckel 1896^K67]

*Gonocrinites angulosus (Pander 1830)^PR16 [=Echinosphaerites angulosus^PR16, Echinoencrinites angulosus^PR16, Sycocystis angulosa^H04]

*Type species of generic name indicated

References

Broadhead, T. W., & H. L. Strimple. 1978. Systematics and distribution of the Callocystitidae (Echinodermata, Rhombifera). Journal of Paleontology 52 (1): 164–177.

[H04] Haeckel, E. 1899–1904. Kunstformen der Natur. Bibliographisches Institut: Leipzig und Wien.

[K67] Kesling, R. V. 1967. Cystoids. In: Moore, R. C. (ed.) Treatise on Invertebrate Paleontology pt S. Echinodermata 1. General characters. Homalozoa—Crinozoa (except Crinoidea) vol. 1 pp. S85–S267. The Geological Society of America, Inc., and The University of Kansas: Lawrence (Kansas).

[P15] Paul, C. R. C. 2015. A cystoid with two left facets: the significance of Tetracystis in the evolution and classification of the Callocystitidae (Echinodermata, Glyptocystitoida). Geological Journal 50: 607–633.

[PR16] Paul, C. R. C., & S. V. Rozhnov. 2016. Revision of Scoliocystis (Rhombifera: Echinoencrinitidae) and related cystoid genera. Paleontological Journal 50 (3): 255–275.

[SG93] Simms, M. J., A. S. Gale, P. Gilliland, E. P. F. Rose & G. D. Sevastopulo. 1993. Echinodermata. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 491–528. Chapman & Hall: London.