This jump in diversity has come to be known as the Great Ordovician Biodiversification Event (GOBE) (Webby et al., 2004) and has even been colorfully dubbed “life’s second Big Bang” (O’Donoghue, 2008).

Crinoids are an excellent group to illustrate the patterns of appearance of fossil taxa that account for the GOBE. Crinoids (also known as sea lilies) constitute a class within the phylum Echinodermata, the same phylum in which sea stars and sea urchins are grouped. Crinoids are benthic organisms that feed off detritus and plankton suspended in the water column. Their body plan typically consists of a stalk attached to a crown (Fig. 2). The crown has a central body (theca) from which arms (typically in multiples of five) radiate. These arms can be branched and feather-like, with appendages known as pinnules. Crinoids have a skeleton made up of numerous articulated calcified plates, which increases their potential for fossilization.

Echinoderms are among the invertebrates that contribute to the Cambrian Explosion, but there are no fossils of crinoids in Cambrian layers. However, disparate groups of crinoids appear in the Lower Ordovician (Fig. 3). The number of families and even orders of crinoids increases upward through the Ordovician layers, with some discrete pulses (Ausich & Deline, 2012). However, the main crinoid higher taxa or body plans are already documented from the basal stratigraphic range of crinoid fossils (Ausich & Deline, 2012; Guensburg et al., 2021) (Fig. 3). One could say that crinoids conform to a pattern of disparity before diversity: “Tremadocian [lowermost Ordovician] crinoid lineages became established in distinct regions of morphospace. Subsequent radiation through the Llandoverian [lowermost Silurian] primarily populated the same basic regions of morphospace” (Ausich et al., 2015, p. 947). What we see at the highest taxonomic level, with disparate Cambrian phyla being populated through the GOBE by more diverse lower rank taxa, is replicated at the level of classes within the GOBE, with abrupt appearance of disparate groups populated by greater diversity of families at higher stratigraphic levels.

This abrupt appearance of disparate forms with lack of ancestral intermediates is the fundamental challenge posed by the fossil record to the gradualistic model of an evolutionary tree of life.

Crinoids are a good example of the significant increase in diversity in Ordovician strata, because they mark the appearance of an entire new class of organisms with no underlying Cambrian fossil record. However, a similar increase of Ordovician diversity could be illustrated by many other groups of marine invertebrates (Webby et al., 2004). There has been some discussion to determine if the GOBE represents a pattern of gradual diversification throughout the Ordovician or if there are distinct pulses of diversification correlated across different invertebrate groups (Servais & Harper, 2018; Stigall et al., 2019). Irrespective of the overall pattern of diversity increase within the GOBE, when we zoom in at ordinal or familial level within specific classes (as seen for crinoids), the pattern is clearly punctuated rather than gradual.

Figure 1: Diversity curve of marine invertebrate families through the Phanerozoic fossil record, documenting the ‘Big Five’ mass extinctions (dashed vertical lines) of marine invertebrates (modified from Sepkoski, 1981; Harper, 2006). Arrow points to diversity increase corresponding to the GOBE. “Cambrian,” “Paleozoic,” and “Modern” refer to Sepkoski’s evolutionary faunas. Geological Systems (left to right): C = Cambrian; O = Ordovician; S = Silurian; D = Devonian; C = Carboniferous; P = Permian; T = Triassic; J = Jurassic; C = Cretaceous; T = Tertiary. Image modified after Liang et al. (2022).

References:

Ausich, W.I., 1998. Early phylogeny and subclass division of the Crinoidea (Phylum Echinodermata). Journal of Paleontology, 72(3), pp.499-510. DOI: https://doi.org/10.1017/S0022336000024276.

Ausich, W.I. and Deline, B., 2012. Macroevolutionary transition in crinoids following the Late Ordovician extinction event (Ordovician to early Silurian). Palaeogeography, Palaeoclimatology, Palaeoecology, 361, pp.38-48. DOI: https://doi.org/10.1016/j.palaeo.2012.07.022.