New early Cambrian sclerites ofLapworthella schodakensisfrom NE Greenland: advancements in knowledge of lapworthellid taxonomy, sclerite growth and scleritome organization

Abstract

The Cambrian Stage 4 upper Bastion Formation of Albert Heim Bjerge and CH Ostenfeld Nunatak, NE Greenland, yielded 34 excellently preserved sclerites ofLapworthella schodackensisamong other small shelly fossils. Lapworthellids have been interpreted as members of the camenellans, a basal tommotiid group. Little is known about this group although the morphological and ultrastructural features of their sclerites allow a potential reconstruction of a lophophorate body plan. The exquisite material from Greenland provides significant new data for the revision of the species taxonomy, but also for the comprehension of the scleritome structure of lapworthellids and the mode of formation of their sclerites. Two morphotypes ofL. schodackensissclerites are identified: one with a simple apex, occurring in sinistral and dextral forms; and one bilaterally symmetrical sclerite with two apices. All bear a similar ornamentation constructed of repeated growth sets consisting of a reticulate inter-rib groove with tubercles, a densely denticulate rib and a striated sub-rib area. The new data on the ornamentation and observations of the laminar shell microstructure ofL. schodackensisenable us to improve the reconstruction of growth in lapworthellids. Finally, the morphological features of the two types of sclerites provide new information for the reconstruction of the bilaterally symmetrical multi-component lapworthellid scleritome with evidence of the fusion of adjacent sclerites during early ontogeny.

Orthrozanclus elongata n. sp. and the significance of sclerite-covered taxa for early trochozoan evolution

“…Camenellan sclerites show continuous variation within a particular morphological category 47 – as do spines in the dorsolateral zone of the Orthrozanclus scleritome. Certain camenellan sclerites 48 exhibit a tuberculate ornament and apical tip that correspond closely to the sclerites of, for example, Halkieria mira (see Figs 4, 6 in ref. 49 ).…”

First discovery of Small Shelly Fossils and new occurrences of brachiopods and trilobites from the early Cambrian (Stage 4) of the Swedish Caledonides, Lapland

“…Two species of Lapworthella have been reported from Scandinavia to date: L. cornu (Wiman 1903), originally based on two sclerites from a glacial erratic collected at Billuden in Uppland, Sweden (Bengtson 1980), but also occurring in Avalonia (Hinz 1987;Landing et al 2008); and L. bornholmiensis (Poulsen 1942) from the Miaolingian of Bornholm, Skåne and Bohuslän in southern Scandinavia (Devaere & Skovsted 2021 [this volume]). The high morphological variability of Lapworthella sclerites has often been emphasized and, traditionally, species have been defined based on differences in ornament, particularly the presence/absence and density of denticles on the co-marginal ribs (Landing 1984;Bengtson et al 1990), but the ornaments on the apical inter-rib groove and apertural slope of the ribs are also useful because they reflect differences in the growth regimen of the sclerites (Devaere & Skovsted 2017). The dense rows of denticles along the comarginal ribs of the sclerites from Luobákte differentiates the material from species lacking denticles, such as L. bornholmiensis, but also from denticulate species with fewer denticles/mm, such as L. cornu (12-25) (Landing et al 2008;Devaere & Skovsted 2017).…”

“…The high morphological variability of Lapworthella sclerites has often been emphasized and, traditionally, species have been defined based on differences in ornament, particularly the presence/absence and density of denticles on the co-marginal ribs (Landing 1984;Bengtson et al 1990), but the ornaments on the apical inter-rib groove and apertural slope of the ribs are also useful because they reflect differences in the growth regimen of the sclerites (Devaere & Skovsted 2017). The dense rows of denticles along the comarginal ribs of the sclerites from Luobákte differentiates the material from species lacking denticles, such as L. bornholmiensis, but also from denticulate species with fewer denticles/mm, such as L. cornu (12-25) (Landing et al 2008;Devaere & Skovsted 2017). The high density of denticles in the Luobákte sclerites conforms with L. schodackensis (Lochman 1956) from Cambrian Stage 4 strata of eastern Laurentia and Siberia, although the density in this species appears to vary widely (35-144 in material from New York State [Landing 1984]; 28-43 in specimens from Siberia [Rozanov et al 2010]; 25-77 in specimens from Greenland [Devaere & Skovsted 2017]).…”

Small shelly fossils and carbon isotopes from the early Cambrian (Stages 3–4) Mural Formation of western Laurentia

“…Devaere & Skovsted (2017) recently redescribed Lapworthella schodackensis (Lochman, 1956) based on collections from North‐East Greenland and noted the presence of tubercles in inter‐rib grooves with a superimposed reticulate network that makes this species more similar to Canadiella than other species of Lapworthella . In addition, the most common sclerite type in L. schodackensis is a pyramidal sclerite with a rectangular cross‐section (B sclerites), which is comparable to the B sclerites of Canadiella .…”

“…Even though Devaere & Skovsted (2017) demonstrated the presence of distinct sclerite types in Lapworthella schodackensis (Lochman, 1956) from Greenland, the genus Lapworthella remains one of the least poorly understood of all camenellan tommotiids, despite its apparently global distribution. Widely differing species concepts have been applied to lapworthellids in the past and combined with a high degree of variability in sclerite shape and ornament, this has led to much confusion (Devaere & Skovsted 2017). We anticipate that renewed study of lapworthellid assemblages in the future will lead to significant taxonomic refinement of this problematic fossil group, as exemplified by the present discovery of the kennardiid affinity of ‘ Lapworthella ’ filigrana .…”