A radicle solution: morphology and biomechanics of the Eucalyptocrinites ‘root’ system

Abstract

Eucalyptocrinites is one the most common and familiar mid‐Palaeozoic crinoids and is the exemplar of a form with dendritic radicular holdfasts. American museums have hundreds of specimens of Eucalyptocrinites holdfasts from the Silurian Waldron Shale of Indiana and Kentucky, USA. The radix (‘root’) system of Eucalyptocrinites can be described as comprised of links (branches) that meet at nodes. Measured values include the x‐ and y‐coordinates of the nodes, the distances of the nodes from the stem, the angles between branches, branch length and branch width. Rose diagrams show clearly that the root systems are not isotropic but have preferred orientations. Branch angles are highly variable, but cluster around 60 degrees. Branch lengths and distal branch widths are relatively constant, but branch width increases variably towards the column. The branching pattern can be modelled as a self‐similar (‘fractal’) structure. Several specimens labelled as Eucalyptocrinites show a distinct fivefold symmetry without branching and very likely represent a different taxon. The Eucalyptocrinites radix system, along with the probably stiff dististele, most likely functioned as a rigid plate that resisted rotational forces due to currents acting on the crown. Upstream radicles experienced tension, whereas downstream roots were compressed. This force distribution may explain the observed anisotropies in radix morphology. The ‘roots’ of Eucalyptocrinites and other crinoids have long been compared with the root systems of plants. Although there are superficial similarities, there are fundamental differences.

Crinoid encrustation of holocystitid diploporitan echinoderms: strongly asymmetrical Silurian dendritic attachment structures with palaeobiological implications

“…The presence of well-developed radices on only one side of each radial attachment structure illustrates the extent to which the development of relatively complex pelmatozoan attachment structures is influenced by local substrate conditions. Brett (1978) documented changes in Caryocrinites radice morphology in response to encountering bioclasts in muddy softgrounds (see also Thomka & Brett, 2019), and Plotnick et al (2016) interpreted growth of longer radices in two opposing directions in softgrounds as a response to prevailing current direction; however, the present specimens probably represent the most strongly asymmetrically developed radicular attachment structures described in the body of literature on stalked echinoderms. The fact that they occur as encrusting structures on diploporitan thecae indicates that the hard skeletal substratum was preferable to extending radices into fine, soft sediment surrounding this "benthic island, " probably because of greater stability relative to the nearby bioturbated sediments (see Kauffman, 1978).…”

“…1A). Five radices, comprised of numerous component ossicles not surrounded by a substantial cortex of secondary calcite (Brett, 1981;Plotnick et al, 2016), extend outward from a central, vertically oriented columnal, bearing a minute, centrally located, pentalobate lumen (Fig. 1B).…”

“…The overall morphology of the attachment structures, the pentalobate configuration of columnal lumina and presence of radice lumina, and the fact that radices are comprised of discrete ossicles rather than solid rods of undifferentiated calcite allow the encrusting specimens to be attributed to Eucalyptocrinites (see Halleck, 1973;Brett, 1981Brett, , 1984Thomka & Brett, 2015a, 2019Plotnick et al, 2016). This taxon is common in the mudstone lithofacies of the Massie Formation (Frest et al, 1999(Frest et al, , 2011 as well as the immediately underlying carbonate lithofacies, which is capped by a hardground that is encrusted by morphologically similar Eucalyptocrinites attachment structures (Thomka & Brett, 2015a).…”

“…Two famous examples of widespread, long-ranging, and locally abundant pelmatozoan echinoderms that employed "root-like" attachment structures are the monobathrid camerate crinoid Eucalyptocrinites and the hemicosmitid rhombiferan Caryocrinites-both of which are characterized by terminal dendritic structures comprised of multiple, radially oriented, branching radices (sensu Donovan in press; see this paper for a review of radicular attachment structure terminology). The increased surface area provided by the array of laterally directed radices was effective at permitting stability in soft, water-rich muds (e.g., Brett, 1978;Halleck, 1973;Plotnick et al, 2016;Poschmann, 2020), and the tendency toward initial settlement upon solid particles was effective at permitting colonization of bioherms and bioclastrich firmgrounds and hardgrounds (e.g., Brett, 1978Brett, , 1991Thomka & Brett, 2015a, 2019.…”