Life on the seafloor: adaptations and strategies in Stylophora (Echinodermata)

Abstract

Stylophorans are a Palaeozoic group of non-pentamerous echinoderms, morphologically well-adapted to a benthic mode of life on soft sediment seafloors. By developing a thin and wide theca, they successively increased the surface area in contact with the substrate resulting in an even distribution of the body mass resting on the ground, and efficiently preventing the body from sinking into non-indurated sediments (snowshoe strategy). In stylophorans, the body surface is dramatically increased in result of the expansion of two integumentary areas on the lower thecal surface. While infracentral areas are reduced in primitive forms to the detriment of massive marginals; in boot-shape cornutes, infracentral areas are larger, polyplated and framed by very delicate marginals. In heart-shaped cornutes, the reduction of both infracentral areas is compensated by the development of spiny elements on the theca outline. In forms where the degree of bilateral symmetry is high, the left infracentral area is larger than the right area, resulting in an elongated thecal shape. In structural geology, changes of shape of a rock submitted to a strain can be categorized by translating the obtained deformation into a strain ellipsoid plotted on a Flinn diagram, while in biology changes of shapes of organisms are traditionally measured through morphometric analysis. By applying Flinn's principle of strain ellipsoid to biological objects, the present study aims to characterize different life adaptations across stylophorans, observing changes of shape of both infracentral areas interpreted as two ellipsoids. Once plotted on a Flinn diagram, three significantly separated clusters are observed when focusing on the left area. This concerns forms with reduced infracentral areas, highly and weakly asymmetrical forms. According to these results, three new morphological adaptations are described (water strider, flat fish and stream-lined body). These newly described adaptations enabled snowshoe strategist stylophorans to remain stable on top of the seafloor. □ Echinodermata, Flinn diagram, Palaeozoic, snowshoe strategy, strain ellipsoid, Stylophora.

New Middle and Late Ordovician cornute stylophorans (Echinodermata) from Morocco and other peri-Gondwanan areas

“…All cornutes display the typical morphology of stylophorans, with a single flexible feeding appendage (aulacophore) inserted into a flattened test (theca) (e.g. Ubaghs 1963Ubaghs , 1968Ubaghs , 1970Nichols 1972;Chauvel & Nion 1977;Chauvel 1986;Parsley 1988;Sprinkle 1992;Sumrall et al 1997;Ruta 1999b;Sumrall & Sprinkle 1999;David et al 2000;Martí Mus 2002;Lefebvre 2003a;Noailles 2016;Lefebvre et al 2019a). In terms of orientation, the aulacophore, which hosts the mouth, marks the anterior pole of the organism, whereas the theca, which bears the anus, is posterior (Ubaghs 1963).…”

“…This ornamentation (protuberances, knobs, spikes, posterior blades) probably helped anchor or stabilize boot-shaped cornutes on the surface of soft substrates (e.g. Parsley 1988;Lefebvre 2003a;Noailles 2016).…”

“…Ubaghs 1970;Jefferies & Prokop 1972;Chauvel 1986;Martí Mus 2002) In most cornutes, the lower thecal surface is divided into two unequal fields (left and right 'infracentral areas') by an asymmetrical skeletal strut (the 'zygal bar' or 'septum'), extending diagonally from left of the aulacophore insertion (anteriorly) to right of the anal opening (posteriorly) (Figs 1a,c,2a,) (e.g. Ubaghs 1963Ubaghs , 1968Ubaghs , 1970Chauvel & Nion 1977;Parsley 1997;Sumrall et al 1997Sumrall et al , 2009Lefebvre et al 1998;Lee et al 2005;Noailles 2016;. In Ceratocystis and other early sutural pores sometimes occur in the right anterior corner of the upper thecal surface (Phyllocystis crassimarginata ;Thoral 1935;Ubaghs 1968Ubaghs , 1970Ware & Lefebvre 2007).…”

“…The identification of skeletal homologies in stylophorans suggests that similar-looking cordiform, bilaterally symmetrical thecal morphologies were acquired convergently in various cornute lineages (Amygdalothecidae, Chauvelicystidae, Hanusiidae, and Phyllocystidae), probably in response to relatively comparable modes of life (see ; Lefebvre 2001Lefebvre , 2003aLefebvre et al 2006;Noailles 2016). Within stylophorans, the central position of Z is not unique to amygdalothecids: this character also occurs, for example, in hanusiids, mitrates, 'Phyllocystis' jingxiensis, and the Tizagzaouine cornute (see above).…”

Laurentian origin of solutan echinoderms: new evidence from the Guzhangian (Cambrian Series 3) Weeks Formation of Utah, USA

“…All four echinoderm clades originating during the middle Cambrian are characterized by relatively similar, convergent, flatfish, fundamentally asymmetrical ‘homalozoan’ morphologies, strongly departing from the radial body plans of early Cambrian taxa (Sprinkle, 1992; Lefebvre & Fatka, 2003; Lefebvre, Nardin & Fatka, 2015). Most homalozoan taxa are generally interpreted as snowshoe strategists, well-adapted for lying atop soft, soupy substrates (Jefferies & Prokop, 1972; Parsley, 1988; Sprinkle & Guensburg, 1997; Lefebvre, 2003; Lefebvre & Fatka, 2003; Noailles, 2016). In contrast, most early Cambrian echinoderms are considered either as shallow sediment stickers (e.g.…”