Non-technical Summary
Because of the low likelihood of preservation and collection, few larval and juvenile specimens of extinct crinoids are known. This substantially hinders our understanding of crinoid ontogeny, which can in turn limit our understanding of the evolutionary history of crinoids. Reported herein is an exceedingly small (aboral cup ~1.3 mm high) crinoid larva from Ordovician strata of southern Ontario, Canada. This new specimen is comparable in size and developmental stage to an early pentacrinoid larva of living crinoids. It is also the geologically oldest crinoid larva known.
Introduction
Understanding the adult morphology of organisms is essential for describing taxa and delineating the evolutionary history of life. In many groups, very little is known about morphologic change during the ontogeny of individual species, which in many cases can be an obstacle to framing homology statements in adults.
Larval and juvenile crinoids are very rare in the fossil record, which is undoubtedly the consequence of both collection bias and taphonomy. For example, the aboral cup of the pentacrinoid larva described herein is 1.3 mm high and just barely visible to the naked eye (the total preserved specimen height, including arms, aboral cup, and a partial column, is ~4.2 mm). The small piece of limestone with this juvenile crinoid was collected because it contains trilobites.
Taphonomy is also a critical factor. Based on actualistic studies, most echinoderms disarticulate rapidly after death (e.g., Meyer, Reference Meyer1971). The rate of disarticulation is area-dependent, so smaller articular surfaces with less connective tissue decay more rapidly than articulations with larger areas. Thus, preservation of complete or nearly complete fossil echinoderms is typically only possible when burial is rapid and permanent as a so-called ‘ecological snapshot’ (e.g., Taylor and Brett, Reference Taylor and Brett1996; Ausich, Reference Ausich2021). Examples of documented fossil cystidean larvae, pentacrinoid larvae, and early juvenile crinoids include Devonian specimens (Kammer et al., Reference Kammer, Bartels and Ausich2015; Ausich and Göncüoǧlu, Reference Ausich and Göncüoğlu2020; Bohatý and Ausich, Reference Bohatý and Ausich2025), a Mississippian crinoid (Ausich and Wood, Reference Ausich and Wood2012), and Pennsylvanian specimens (Pabian and Strimple, Reference Pabian and Strimple1985; Peters and Lane, Reference Peters and Lane1990). Later stage juveniles (crown height ~7.5 mm) are also known from the Brechin Lagerstätte in Protaxocrinus laevis (Billings, Reference Billings1857) (Wright et al, Reference Wright, Cole and Ausich2019).
Herein, we report a Late Ordovician early pentacrinoid larva (UMMP 84792) that is among the smallest Paleozoic pentacrinoid larvae known. This specimen is comprised of arms, aboral cup, and a partial column. The new larval specimen is a dendrocrinid eucladid crinoid from the Ordovician (Katian) of southern Ontario, Canada and is the oldest (geologically) known crinoid larva.
Location and stratigraphy
This new specimen is from the Lafarge Cannifton Agg Quarry in Cannifton, Ontario (44.206117, −77.370731). This region of southern Ontario is known for Ordovician strata with both rare trilobite species and rich echinoderm faunas (e.g., Billings, Reference Billings1859; Springer, Reference Springer1911; Brett and Taylor, Reference Brett, Taylor, Hess, Ausich, Brett and Simms1999; Sumrall and Gahn, Reference Sumrall and Gahn2006; Hessin, Reference Hessin2009; Ausich et al., Reference Ausich, Wright, Cole and Koniecki2018; Blake and Koniecki, Reference Blake and Koniecki2018; Cole et al., Reference Cole, Ausich, Wright and Koniecki2018, Reference Cole, Wright and Ausich2019, Reference Cole, Wright, Ausich and Koniecki2020; Wright et al., Reference Wright, Cole and Ausich2019). The juvenile crinoid specimen reported here is from the upper Verulam Formation.
Materials and methods
As noted, the juvenile specimen described is very small; it was discovered while preparing the slab with a light air abrasive to expose a specimen of the trilobite Ceraurus globulatus Bradley, Reference Bradley1930 that is close to the new crinoid specimen.
Classification and terminology
The superordinal and ordinal classifications follow Cole (Reference Cole2017, Reference Cole2018), Wright (Reference Wright2017a, Reference Wrightb), and Wright et al. (Reference Wright, Ausich, Cole, Rhenberg and Peter2017). Lacking a comprehensive, phylogeny-based classification for the Crinoidea below the ordinal level, family-level classification follows Moore and Teichert (Reference Moore and Teichert1978).
Morphologic terminology follows Ubaghs (Reference Ubaghs1978), Ausich et al. (Reference Ausich, Wright, Cole and Sevastopulo2020), and Ausich and Donovan (Reference Ausich and Donovan2023). All measurements are in mm.
Repository and institutional abbreviation
The specimen described herein is deposited in the collections of the University of Michigan Museum of Paleontology (UMMP).
Systematic paleontology
Class Crinoidea Miller, Reference Miller1821
Subclass Pentacrinoidea Jaekel, Reference Jaekel1918
Infraclasss Inadunata Wachsmuth and Springer, Reference Wachsmuth and Springer1885
Parvclass Cladida Moore and Laudon, Reference Moore and Laudon1943
Magnorder Eucladida Wright, Reference Wright2017a
Superorder Cyathoformes Wright et al., Reference Wright, Ausich, Cole, Rhenberg and Peter2017
Cyathoformes incerta sedis: ‘Dendrocrinida’ Bather, Reference Bather1899
Dendrocrinida indet.
Material
UMMP 84792.
Occurrence
Lafarge Cannifton Agg Quarry in Cannifton, Ontario, Canada; Ordovician (Katian).
Remarks
The Ordovician pentacrinoid larva of the crinoid reported herein consists of a slightly crushed aboral cup with attached arms and column. The slightly compacted aboral cup is ~1.3 mm high. It is comprised of infrabasal, basal, and radial plates (Fig. 1). The proportions of these cup plates are 22%, 50%, and 28%, respectively. Infrabasal plates are ~1.4 times higher than wide, and basal plates are ~1.4 times higher than wide. The radial plates are approximately as high as wide with peneplenary radial facets. Interestingly, the first primibrachial width is only ~50% of the width of the radial facet. Two arms are preserved with the one on the left having five short brachials with the terminal brachial rounded distally, which was judged to indicate that this arm is complete. Brachial plates are slightly wider than high. The incomplete column is heteromorphic and ~2.4 mm high.
Figure 1. Pentacrinoid larva from the Brechin Lagerstätte, Ordovician (Katian) of southern Ontario, Canada (UMMP 84792): (1) camera lucida drawing; black shading is for radial plates; (2) photograph of specimen. Scale bar for both images.
Very small crinoids are of two basic types, microcrinoids and larval crinoids. Microcrinoids have an aboral cup height ≤ 2 mm (Sevastopulo, Reference Sevastopulo, Ausich and Webster2008). Microcrinoids are not small forms in a growth series of otherwise larger crinoids because there are no larger counterparts of these small crinoids. Thus, they are considered adults. The two primary groups of microcrinoids are the Allagecrinidae (Disparida) and Codiacrinidae (Cyathoformes).
In contrast, larval crinoids, e.g., the specimen described herein, are early growth stages of typical-sized crinoids. Doliolaria larvae are released from crinoid ovaries into the water column and eventually settle to become very small larvae attached to the sea floor or another organism by a column and holdfast. The earliest benthic larva is the cystidean larva, which is comprised of only aboral cup plates, orals (now termed primary peristomial cover plates, see Sumrall and Waters, Reference Sumrall and Waters2012; Kammer et al., Reference Kammer, Sumrall, Zamora, Ausich and Deline2013), stalk, and holdfast (e.g., Mortenson, Reference Mortenson1920; Lahaye and Jangoux, Reference Lahaye and Jangoux1987; Holland, Reference Holland, Giese, Pearse and Pearse1991; Eleaume and Ameziane, Reference Eleaume, Ameziane, Ferel and David2003; Nakano et al., Reference Nakano, Hibino, Oji, Hara and Amemiya2003; Haig and Rouse, Reference Haig and Rouse2008; and Amemiya et al., Reference Amemiya, Omori, Tsurugaya, Hibino, Yamaguchi, Kuraishi, Kiyomoto and Minokawa2014). A benthic larva is termed a pentacrinoid larva after arms appear on the radial plates.
Fossil pentacrinoid larvae and perhaps cystidean larvae have been reported from the Hunsrück Slate of Germany (Kammer et al., Reference Kammer, Bartels and Ausich2015). The smallest of these specimens is ~1.0 mm in height. Preservation of this specimen is insufficient to determine whether the arms were present, but based on its size, it was interpreted to be an early pentacrinoid larva (Kammer et al., Reference Kammer, Bartels and Ausich2015, fig. 2). Slightly larger pentacrinoid larvae in the Hunsrück Slate have short arms that branch once (Kammer et al., Reference Kammer, Bartels and Ausich2015, fig. 2). Devonian cystidean and pentacrinoid larvae have also been reported recently from other regions of the Rhenish Massif in Germany (i.e., the Bergisches Land and the Eifel synclines) (Bohatý and Ausich, Reference Bohatý and Ausich2025). The smallest specimens lack arms and have aboral cups < 2 mm in height. The specimens reported by Bohatý and Ausich (Reference Bohatý and Ausich2025) were the first reported fossil cystidean larvae.
The pentacrinoid larva from Cannifton is slightly larger than those reported from the Hunsrück Slate, and the arms are well defined, although they are atomous rather than branched. Also note that the Cannifton pentacrinoid larva is approximately the same size as the very early living crinoid Metacrinus rotundatus Carpenter, Reference Carpenter1885 reported by Amemiya et al. (Reference Amemiya, Omori, Tsurugaya, Hibino, Yamaguchi, Kuraishi, Kiyomoto and Minokawa2014, fig. 7E). This M. rotundatus specimen is ~167 d past fertilization and has three primibrachials with a single pinnule plate articulated to the second primibrachial. Because three circlets of plates are present in the aboral cup and arms are free above the cup, the Cannifton larva is interpreted as an eucladid crinoid. Adult eucladid crinoids reported from the Brechin Lagerstätte include Cupulocrinus humilis (Billings, Reference Billings1857); Cupulocrinus jewetti (Billings, Reference Billings1859); Dendrocrinus simcoensis Wright, Cole, and Ausich, Reference Wright, Cole and Ausich2019; Grenprisia billingsi (Springer, Reference Springer1911); G. springeri (Moore, Reference Moore1962); Konieckicrinus brechinensis Wright, Cole, and Ausich, Reference Wright, Cole and Ausich2019; K. josephi Wright, Cole, and Ausich, Reference Wright, Cole and Ausich2019; Illemocrinus amphiatus Eckert, Reference Eckert1987; Plicodendrocrinus proboscidiatus (Billings, Reference Billings1857); Praecupulocrinus conjugans (Billings, Reference Billings1857); Protaxocrinus laevis; and Simcoecrinus mahalaki Wright, Cole, and Ausich, Reference Wright, Cole and Ausich2019. However, because morphology can change markedly through ontogeny, it is not possible to positively identify this pentacrinoid. Table 1 lists the adult character states of the Lafarge Quarry eucladid taxa compared to the Brechin pentacrinoid larva. None of the known adults correspond exactly with this postlarval juvenile, making even a tentative genus and species identification problematic. There are no unique characteristics in this Brechin pentacrinoid that would unequivocally allow a comparison to larger crinoids, e.g., the quadrilobate shape of the axial canal in the columns of cupressocrinids (Bohatý and Ausich, Reference Bohatý and Ausich2025). The circular column and the number of primibrachials in this new pentacrinoid larva presumably contrast with other Brechin eucladids (Table 1). Perhaps the two most similar taxa are K. josephi and P. conjugans although critical morphologic characters would have had to develop during growth if either of these species is the correct identification.
Table 1. Character states of the Katian pentacrinoid stage larva compared to sympatric adult crinoids
Based on the size and arm branching, the other quite small crinoids from the Brechin Lagerstätte (Wright et al., Reference Wright, Cole and Ausich2019, fig. 4.4) are much further developed than the pentacrinoid reported here. These other small crinoids should be considered juveniles rather than pentacrinoid larvae.
Acknowledgments
We thank J. Leonard-Pingel, who helped with photography, and the editors and reviewers who improved this manuscript, including J. Bohatý, T.W. Kammer, and S. Zamora.
Competing interests
The authors declare none.
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