THE CANAL SYSTEM IN SCLERITES OF LOWER CAMBRIANSINOSACHITES(HALKIERIIDAE: SACHITIDA): SIGNIFICANCE FOR THE MOLLUSCAN AFFINITIES OF THE SACHITIDS

Abstract

The halkieriids (Sachitida He, 1980) from the Early to Mid Cambrian possess a hollow sclerite with a complex branching canal system. An analysis of the canal system morphology in the halkieriid Sinosachites (Thambetolepis) delicatus (Jell, 1981) from South Australia reveals similarities to the aesthete canal system in the shell plates of chitons, which has been analysed in a number of extant taxa. The compartments, referred to as macro-aesthetes in chitons, and lateral canals in halkieriids, have overlapping diameters and are constrained in morphology by the space of accommodation by maintaining a constant width, whereas length is more variable. Both canal systems are morphologically distinct from shell pores of other lophotrochozoans and known mollusc classes. Similarities in sclerite growth, microstructure and mineralogy further suggest that halkieriids, along with the other sachitids, are molluscs, most likely stem aculiferans (Polyplacophora and Aplacophora).

A molecular palaeobiological hypothesis for the origin of aplacophoran molluscs and their derivation from chiton-like ancestors

“…Older sclerite-bearing taxa, the sachitids (Early -MidCambrian) [54], have been interpreted as aculiferan relatives [24,55 -57], including Halkieria [56,58], Maikhanella [55], Orthrozanclus [59] and Wiwaxia [60,61], all taxa known as partial to completely articulated specimens with a variable number of shell plates and sclerites arranged in morphological zones similar to aculiferans [57]. The sclerites are constructed of longitudinal fibres of presumed aragonite, a structure similar to that of chiton sclerites, and are hollow with a branching canal system that has been compared with the aesthete canals in the shell plates of modern chitons and the sclerites in the extinct multiplacophorans [57]. These forms would extend the aculiferan stem lineage into the earliest Cambrian, eliminating a long aculiferan ghost range.…”

The oyster enigma variations: a hypothesis of microbial calcification

“…Pores characterize punctate brachiopods (Cowen 1966;Peck et al 1986;Peck 1992), many bivalves (the living cardiid Cardium, cyrenoideans, arcoideans, and Archiheterodonta) (Kennedy et al 1970;Waller 1980;Morton 1982;Araujo et al 1994;Isaji 1995;Savazzi and Sälgeback 2004;Malchus 2010). They also occur in some gastropods (Fissurelloidea, Neomphaloidea, and Phenacolepadidae) (Sasaki et al 2003(Sasaki et al , 2008Kiel 2004;Hesz et al 2008) and, as aesthete canal systems, in the plates of polyplacophorans (Fernandez et al 2007) and the sclerites of halkieriids (Vinther 2009). A shell cavity secondarily divided by septa or tabulae is known in putatively photosymbiotic Paleozoic productide brachiopods (Rudwick andCowen 1967, reviewed in Vermeij 2013) as well as in three groups of bivalves (Permian alatoconchids, Late Triassic wallowaconchids, and Late Cretaceous hippuritid rudists) that likely also harbored photosymbionts (reviewed in Vermeij 2013).…”

Chancelloriids of the Cambrian Burgess Shale

“…Qian and Bengtson (1989) suggested this structure to reflect stepwise mineralization of the basal portion of the sclerite. It is different from the fine transverse striations seen on the inside of palmate sclerites of Sinosachites delicatus (Jell, 1981), the 54 latter seemingly being structural features unrelated to growth (Bengtson et al, 1990; but see Vinther, 2009).…”

“…The affinities of the non-chancelloriid coeloscleritophorans are even more controversial than those of the chancelloriids (Bengtson and Conway Morris, 1984;Conway Morris, 1985;Dzik, 1986;Butterfield, 1990;Bengtson, 1992;Bengtson, 1993;Yochelson, 1993;Conway Morris and Peel, 1995;Williams and Holmer, 2002;Bengtson, 2005;Vinther and Nielsen, 2005;Porter, 2008;Vinther, 2009;Smith, 2014), but a sponge affinity has never been seriously entertained for these organisms. Bengtson and Missarzhevsky (1981) based the proposed homology of sclerites within the Coeloscleritophora on the general structure, composition, and mode of formation.…”

“…However, the evidence for crown-group molluscan affinity of the halkieriids (Bengtson, 1992;Vinther and Nielsen, 2005;Vinther, 2009) and the lack of bilaterian characters in the chancelloriids make No. 1 unlikely, and the mounting evidence for homology of coeloscleritophoran sclerites (Porter, 2004;Bengtson, 2005;Porter, 2008) speaks against No.…”