Interaction between dendritic radix structures of pelmatozoan echinoderms on a hardground in the middle Silurian (Wenlock) of southeastern Indiana, USA

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).…”

“…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.…”

Silurian Diploporitans as Substrata:

“…These features allow Eucalyptocrinites radices to be confidently differentiated from similar attachment structures, such as the dendritic radix structures of the hemicosmitid rhombiferan Caryocrinites, which has a larger, trilobate lumen and radicles that lack lumina. Instead, Eucalyptocrinites radices are solid rods of unsegmented calcite (Brett, 1978(Brett, , 1981(Brett, , 1984Thomka and Brett, 2015a, 2015b, 2019.…”