Pisocrinidae

Triacrinus elongatus, from Bundenbach Fossilien.

Belongs within: Inadunata.

The Pisocrinidae are a group of small crinoids known from the Middle Silurian to Upper Devonian, with the cup composed primarily of three relatively lage plates.

The Pisocrinidae: babyface crinoids
Published 16 December 2018

One question that I haven’t yet found an answer to is why the Palaeozoic marine fauna seems to have included so many filter feeders. Cystoids, blastoids, graptoloids… so many of the distinctive taxa occupying this niche would be gone by the period’s end, without leaving any clear analogues behind them. What was the cause underlying this abundance? Is it simply a misapprehension caused by the filtering effect of history, with the modern fauna containing fewer major lineages but no fewer actual species? Is it the distorting lens that causes us to tend to assign a higher ‘rank’ to those lineages arising earlier in time, whatever their practical levels of disparacy? Or was there actually something different about what could be found in Palaeozoic seawater?

Reconstructions of short-armed and long-armed species of Pisocrinus, from Rozhnov (2007).

The Pisocrinidae are one of those distinctive Palaeozoic marine groups, known from around the world during the Silurian and Devonian. As crinoids, they were perhaps not as immediately unfamiliar to the modern eye as some of the other taxa that could be found at that time, but they were certainly different from any modern crinoid. The majority of the crinoids that have ever lived can be assigned to one of two main clades. One, the cladid lineage, includes all the crinoids alive today. Pisocrinids belong to the other major lineage, the disparids, which were prominent for most of the Palaeozoic era but failed to make it past the end of the Permian. Disparids differed from cladids in that their calyx included a single circlet of plates (the inferradials) beneath the circlet of the radials (the large plates making up the main body of the calyx) whereas cladids (at least to begin with) had two such circlets. Many disparid sublineages showed a tendency towards reduction and/or simplification of the calyx. In pisocrinids, most of the calyx was made up of just three plates: two large radials (representing the A and D rays of the basic crinoid calyx) and a greatly enlarged B inferradial. The B, C and E radials were all reduced in size. The arms of pisocrinids mostly lacked lateral pinnules and were undivided; one genus, Cicerocrinus, had bifurcating arms bearing lateral ramules (Moore et al. 1978). The length of the arms varied considerably between species: in some they were quite short and broad, in others they were remarkably long. Because their derived morphology made it difficult to compare pisocrinids to related families, their origins have been regarded as mysterious. Rozhnov (2007) suggested a derivation from an earlier, more typical crinoid family, the Homocrinidae, via paedomorphosis, possibly as a result of the evolution of a longer larval period in the life cycle (he specifically suggested that this extended larval phase may have allowed the ancestors of pisocrinids to spread across the Iapetus Ocean between the then-existing continents of Laurentia and Baltica). A direct pisocrinid-homocrinid connection was not supported in the phylogenetic analysis of disparids by Ausich (2018) but Rozhnov’s overall model of pisocrinid paedomorphosis remains a possibility.

Assemblage of Triacrinus, from here.

During the Silurian, pisocrinids were among the most abundant, if not the most abundant, groups of crinoids. They were found in a variety of habitats but were particularly abundant around reefs in deeper waters. At first glance, the non-pinnulate arms of pisocrinids appear poorly suited for filter feeding, and one might be inclined to propose a more tentacular method of obtaining food items. However, such a method would seem unlikely for the short-armed species, whose arms would have been almost entirely inflexible. Even the long-armed species sometimes had arms made up of relatively long segments whose flexibility may have been limited. An alternative possibility, I suppose, is that in life pisocrinids may have had long tube feet that took the place of the missing pinnules. Meanwhile, the absence of the pinnules meant that the arms could be lain tightly alongside each other when the crown was closed. Earlier authors presumed that, because of their preference for deeper waters, pisocrinids were rheophobic (that is, they were found in places where the water lacked a noticeable current). However, Ausich (1977) proposed that they were low-energy rheophilic, seeking locations where a moderate but steady current prevailed. The current would provide a steady supply of organic particles that could be captured by the crown, and the ability to close the arms tight would protect the oral region during occasional bouts of rougher conditions.

Systematics of Pisocrinidae

Characters (from Moore et al. 1978): Small crinoids with globose or conical cup. Basals three or five; three small radials (corresponding to B, C and E compound radials of Homocrinidae) and two large radials, inferradial of B ray hypertrophied and displaced obliquely to right, forming with large A and D radials most of cup. Tegmen poorly known, anal opening above cup. Arms atomous and nonpinnulate or with two main ramule-bearing rami to a ray.

Pisocrinidae [Cicerocrinidae, Pisocrinacea, Pisocrinoidea, Triacrinidae]
|--Eocicerocrinus sevastopuloi Donovan 1989SG93
|--Calycanthocrinus Follman 1887ML78
| `--*C. decadactylus Follman 1887ML78
|--Jaekelicrinus Yakovlev 1949ML78
| `--*J. bashkiricus Yakovlev 1949ML78
|--Pisocrinus de Koninck 1858 [incl. Triacrinus Ringueberg 1884 non Münster 1839]ML78
| |--*P. pilula de Koninck 1858ML78
| `--P. yassensis Etheridge 1904 [incl. P. yassensis var. lobata Etheridge 1904]F71
|--Cicerocrinus Sollas 1900 [incl. Lagarocrinus Jaekel 1901]ML78
| |--*C. elegans Sollas 1900ML78
| `--*Lagarocrinus’ osiliensis Jaekel 1901ML78
|--Parapisocrinus Mu 1954 [=Ollulocrinus Bouška 1956]ML78
| |--*P. ollula (Angelin 1878) [=Pisocrinus ollula, *Ollulocrinus ollula]ML78
| `--P. quinquelobusML78
`--Triacrinus Münster 1839 [incl. Trichocrinus Müller 1856]ML78
|--*T. pyriformis Münster 1839ML78
|--T. altus (Müller 1856) [=*Trichocrinus altus]ML78
|--T. elongatusML78
|--T. granulatus Münster 1839SG93
`--T. regnelliML78

*Type species of generic name indicated

References

Ausich, W. I. 1977. The functional morphology and evolution of Pisocrinus (Crinoidea: Silurian). Journal of Paleontology 51 (4): 672–686.

Ausich, W. I. (in press, 2018) Morphological paradox of disparid crinoids (Echinodermata): phylogenetic analysis of a Paleozoic clade. Swiss Journal of Palaeontology.

[F71] Fletcher, H. O. 1971. Catalogue of type specimens of fossils in the Australian Museum, Sydney. Australian Museum Memoir 13: 1–167.

[ML78] Moore, R. C., N. G. Lane, H. L. Strimple, J. Sprinkle & R. O. Fay. 1978. Inadunata. In: Moore, R. C., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt T. Echinodermata 2. Crinoidea vol. 2 pp. T520–T759. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

Rozhnov, S. V. 2007. Changes in the Early Palaeozoic geography as a possible factor of echinoderm higher taxa formation: delayed larval development to cross the Iapetus Ocean. Palaeogeography, Palaeoclimatology, Palaeoecology 245: 306–316.

[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.

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