Nardin et al.—Transitional blastozoan from the middle Cambrian of the Czech Republic
Eichwald, 1840). The grouped arrangement of exothecal feeding appendages (brachioles) at the end of the ambulacral ray or as lateral branches of the ambulacral ray reflects convergences among blastozoans (e.g., Aristocystites Barrande, 1887, Felbabkacystis n. gen., Gogia, Palaeosphaeronites Prokop, 1964 vs. Eumorphocystis Branson and Peck, 1940, Kinzercystis, Macrocystella Callaway, 1877, Trachelocrinus). The trait set of Felbabkacystis n. gen. appears as
fundamentally transitional between the lepidocystoids and the eocrinids, combining both apomorphies (e.g., partially over- grown tessellate region, large epispires, vaulted oral area, roundish lateral periproct) and plesiomorphic features (e.g., imbricate stalk-like appendage) of the blastozoans (Fig. 7). Such atypical morphology reinforces the unity of the blasto- zoans on the possession of brachioles as appendages discon- nected from the body wall cavity, clearly contrasting from arms as outgrowth of the body wall (Ubaghs, 1968, 1975; Sprinkle, 1973; David et al., 2000; but see Zamora and Smith, 2012 for an alternative interpretation). It emphasizes the strong but natural variability affecting the ambulacra-oral surface plate patterns
(embedded, recumbent, erect, or nonmineralized ambulacral flooring plates, see Nardin et al., 2009) and the ambulacral grooves branching and their associated structures plating (Nardin et al., 2010). The stratigraphic scaling of the phylogenetic hypothesis
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distance (parautochthonous to autochthonous material; Ausich, 2001; Gorzelak and Salamon, 2013). Felbabkacystids can be interpreted as epifaunal suspension
feeders, filtering with a small brachiole fan and exploiting the relatively high tier class +5–10cm above the seafloor, in com- parison to other Cambrian echinoderms (Bottjer and Ausich, 1986; Bottjer et al., 2000). The absence of a fully preserved stalk with holdfast in any collected specimen of Felbabkacystis prevents a definitive interpretation of their motility level. However, lepidocystoids, possessing similar stalk, have been interpreted as suspension feeders living attached on hard substrate (pebble, skeletal fragments) (Sprinkle, 1973; Fatka and Kordule, 1990) or sticking (bioglue as ligament fibers of collagen) to firm sediment (Parsley and Prokop, 2004; Dornbos, 2006; Parsley and Zhao, 2010; Kloss et al., 2015). Felbabkacystids may have had a similar mode of attachment (Supplementary Data S2). Felbabkacystids and coeval fauna exhibit an unusual well-
developed ornamentation and/or respiratory structures (epi- spires). Low-level bottom-dweller and shallow sediment sticker taxa (e.g., ctenocystoids, Lichenoides) show thicker thecal plates bearing stronger ornamentation than the higher (5–10 cm) tierers (e.g., Akadocrinus, Felbabkacystis). If felbabkacystids
supports the concept of a rapid early blastozoan diversification (Guensburg and Sprinkle, 1992; Smith et al., 2013). Major homologies defining the derived blastozoans as well as the more basal blastozoan clades seem to appear in the fossil record before the Drumian (Fig. 7). However, the morphological characters defining the clades suggest an asynchronous develop- ment of various versions of plesiomorphic and apomorphic trait sets, as well as strong convergences in the evolution of the plating of the aboral region (as suggested by Sprinkle, 1973) and of the ambulacral architecture (as proposed by Zamora and Smith, 2012).
Paleoecological implications
On some slabs, Felbabkacystis n. gen. co-occurs with a well- preserved individual of the eocrinoid Lichenoides priscus and two individuals of an undescribed ctenocystoid, in addition to small fragments of trilobites (Fig. 4). The relatively large (10mm) specimen of Lichenoides priscus (slab SZ346) shows well-developed epispires and an unusual prominent orna- mentation as large vermicular and branched ridges on the thecal plates and large granules sometimes fused in ridges in the brachiolar plates (Fig. 4.3). Two strongly disarticulated cteno- cystoid specimens are preserved on the same slab as specimen SZ347 (Fig. 4.2). Both are ovoid in shape (8mm in diameter), made of numerous rectangular plates surrounded by 10 triangular plates on the periphery. Ornamentation is relatively dense, being composed of long sinuous ridges over the rec-
tangular plates of the surface and of small granules aligned in fine straight centripetal lines on the frame plates. The good state of preservation (slight disarticulation to full articulation) and the high fragility of the Czech echinoderm material (type 1 echinoderms sensu Brett et al., 1997) would suggest that specimens were probably quickly buried or transported a short
are interpreted as hard-substrate attachers, then the high flexibility of their imbricate skeleton might suggest a strong resistance to moderate lateral bottom current, which is also consistent with the strength of the skeleton of Lichenoides (swollen and strongly ornamented thecal plates). This inter- pretation is in good accordance with the model of latitudinal distribution of Ordovician blastozoans (Paul, 1976), suggesting a higher efficiency of the respiratory structures when slightly lower oxygen settings are reached (e.g., in the colder and deeper environment of the Příbram-Jince Basin). By contrast, taxa with rather small epispires and thin body wall plates tend to occur in the better-oxygenated depositional environments, such as the Impure Carbonate Facies of the Kinzers Formation, which have yielded species with rather small epispires and thin body wall plates (Skinner, 2005; Powel, 2009). Felbabkacystid specimens and their associated fauna (e.g.,
the lepidocystoid Vyscystis, the eocrinid Akadocrinus, the lichenoid Lichenoides, and the enigmatic Cigara; Fig. 1.2) have been collected in transgressive medium- to fine-grained shales in the upper third of the Jince Formation (upper Hypagnostus parvifrons-Paradoxides (P.) paradoxissimus gracilis biozones) at the transition from the trilobite to the agnostid biofacies, sensu Fatka and Szabad (2014). The other highly fossiliferous level of the Jince Formation (not revealing any imbricate blastozoans) occurs in a similar configuration in the Paradoxides (E.) pusillus-lower Onymagnostus hybridus biozones (Fig. 1.2). The richly fossiliferous siliciclastic levels have been interpreted as deposited in a relatively deep (below storm-wave base) and quiet environment of a mixed platform (Fatka and Mergl, 2009). Blastozoan faunas from these two levels are either endemic species or close to typical taxa of Mediterranean peri- Gondwanan margin (e.g., undetermined lichenoid from the Tarhoucht Member of the Jbel Warwmast Formation [global Cambrian Series 3, Stage 5], eastern Anti-Atlas [southern Morocco], Smith et al. (2013); unidentified lichenoid from the Murero Formation [global Cambrian Series 3, Stage 5], Iberian
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