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Decline in Diversity of Early Palaeozoic Loosely Coiled Gastropod Protoconchs

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Decline in Diversity of Early Palaeozoic Loosely Coiled Gastropod Protoconchs Decline in diversity of early Palaeozoic loosely coiled gastropod protoconchs JERZY DZIK Dzik J. 2020: Decline in diversity of early Palaeozoic loosely coiled gastropod protoconchs. Lethaia, Vol. 53, pp. 32–46. Quantitative data on molluscan larval conch fossil assemblages of ages ranging from the Ordovician (Argentina and the Baltic region), through Silurian (Austria), Devonian (Poland) to Carboniferous (Texas) supplement knowledge of early planktonic gas- tropods communities transformations. They show that larval shells of the bilaterally symmetrical bellerophontids and dextrally coiled gastropods with a hook‐like straight apical portion of the first whorl initially dominated. Their relative frequency, as well as that of the sinistrally coiled ‘paragastropods’, diminished during the Ordovician and Silurian to virtually disappear in the Late Devonian and Early Carboniferous. Already during the Ordovician, diversity of larvae with gently loosely coiled first whorl increased, to be replaced then with more and more tightly coiled forms. Both the aper- ture constrictions and mortality peaks, probably connected with hatching and meta- morphosis, indicate that the Ordovician protoconchs with hook‐like first coil represent both the stage of an embryo developing within the egg envelope and a planktonic larva. The similarity of the straight apex to larval conchs of hyoliths and advanced thecosome pteropods is superficial, as these were not homologous stages in early develop- ment. □ Conodonts, Ordovician, Gastropoda, Early Palaeozoic, larvae. Jerzy Dzik ✉ [[email protected]], Institute of Paleobiology, Polish Academy of Sciences PAN, Twarda 51/55, 00-818 Warszawa, Poland and Faculty of Biology, University of Warsaw, Aleja Żwirki i Wigury 101, Warszawa 02-096, Poland; manuscript received on 2/01/2019; manuscript accepted on 15/03/2019. The origin and early evolution of gastropods remain 2014; Lindskog et al. 2015) but they decrease in fre- elusive, despite the relatively high fossilization poten- quency through the Silurian and Devonian (Dzik tial of molluscan shells and hundreds of years of 1994b; Nützel & Frýda 2003). The youngest truly palaeontological research on them. The major obsta- rich ‘small shelly fossils’ assemblages known to date cle with interpreting the gastropod fossil record is a are those of Early Carboniferous age. They are sup- relatively low phylogenetic signal that can be recov- plemented in the Late Palaeozoic and Mesozoic ered from the morphology of their post‐larval stages strata by abundant occurrences of larval gastropod, (teleoconchs). They are highly homoplastic, with clo- bivalve and cephalopods conchs with their original sely similar morphologies repeatedly developing in aragonitic walls preserved in black shales, in which unrelated lineages (Wagner & Erwin 2006). To solve fossil mature specimens are virtually missing (Nützel this conundrum, data on both embryonic conchs and & Mapes 2001; Seuss et al. 2012). Although it cannot teleoconchs of the oldest unquestionable gastropods be excluded that some of these minute mollusc are needed. Unfortunately, present knowledge of conchs belonged to adult individuals of micromor- early Palaeozoic gastropod larvae is highly limited. In phic species, at least the overwhelming majority this paper, I try to extend the already available scarce of them represent planktonic larvae. They appar- evidence by recovering a few new assemblages of ently failed to metamorphose after encountering an fossil larvae and tracing how their composition in oxygen‐deficient environment on the sea bottom changes the geological time. (Nützel & Mapes 2001) The mass occurrence of minute mollusc conch Taken together, the fossil record of such kind phosphoritic nuclei is a phenomenon typical for the gives an insight into two important aspects of the early Palaeozoic strata. They are especially frequent gastropod early history. First, a quantitative research in the early Cambrian deposits, and the concept of on the time‐ordered series of planktonic larvae assem- ‘Cambrian explosion’ was inferred to much degree blages may be used to exploit the problem whether from such fossil evidence, frequently referred to as the planktotrophy or perhaps lecithotrophy domi- ‘small shelly fossils’. Secondarily phosphatized micro- nated among early planktonic molluscan larvae and scopic size shells are common also in sediments which of these life strategies is the plesiomorphic one deposited in temperate climate zone of the Ordovi- (Frýda 1999, 2012; Frýda et al. 2008; Nützel 2014). It cian (Hynda 1983, 1986; Nützel et al. 2006; Nützel is also possible to trace the loosely coiled larvae, DOI 10.1111/let.12334 © 2019 Lethaia Foundation. Published by John Wiley & Sons Ltd LETHAIA 53 (2020) Palaeozoic gastropod protoconchs 33 unique for the Palaeozoic, to assemblages with exclu- biological productivity. The action of micro‐organ- sively modern larval morphologies. Moreover, the isms releasing phosphate ions resulted in the fre- second line of inference offers some insight into quent development of phosphatic coatings around rarely accessible data on the evolution of ecosystems. skeletal grains. The same discrepancy between mass The increased mortality of metamorphosing larvae occurrence of larval conchs and extreme rarity of followed by the process of phosphatization of larval their mature counterparts is known also from shells requires a local coincidence of various factors younger strata, although not being so common as in controlling it (relatively low pH and Eh and high the Cambrian or Ordovician (Dzik 1994b). Here, phosphate ions supply). This means that the both already published and newly obtained data on recorded changes in composition of fossil assem- samples with gastropod larvae of ‘small shelly fossils’ blages reflect transformations of larval communities type of preservation are used. adapted to quite a narrow range of physical factors The material is housed at the Institute of Paleobi- that apparently remained similar for hundreds of ology, Polish Academy of Sciences in Warsaw, million years. Poland (abbreviated ZPAL). The main limitation of such attitude to the fossil record is that only rarely fossil larval shells of such Darriwilian San Juan Formation ancient age can be convincingly matched with teleo- conchs of the same species (Bandel et al. 2002). Simi- A sample about 2 kg weight from the top of the San lar larval conch may belong to Palaeozoic gastropods Juan Formation was collected during excursion to the teleoconch of completely different shape (Cook et al. Don Braulio Creek in the Villicum Range near San 2008). This makes phylogenetic studies based exclu- Juan, Argentina, organized in 2013 by Guillermo sively on the teleoconch morphology (Wagner 2002) Albanesi (Córdoba). The rock is a cephalopod lime- not reliable. Rarely a settling larva survived meta- stone with aggregation of endoceratid nautiloids morphosis to grow for some time before being phos- conchs. The top of this unit is represented by a red phatized (Dzik 1984, 1994a), and then, juvenile ochre grainstone and burrowed greenish dark grey teleoconchs may provide crucial information that bioclastic wackestone (Mestre et al. 2013). Its age was can be used to determine generic or family level iden- determined by Mestre et al. (2013) as the Yangtzeplacog tity. This makes studies on earliest gastropod larval nathus crassus Zone. Platform P1 elements with conchs of special interest. elongated posterior process in my collection identify In this paper, I will use the available data on gas- Eoplacognathus zgierzensis (Appendix S1 Fig. 3R), tropod larvae from ‘small shelly fossils’ assemblages which indicates a slightly younger age than the strata to trace quantitatively changes in contribution of with Y. crassus, but still below the E. pseudoplanus openly coiled protoconchs to them to supplement Zone. The whole conodont assemblage (Appendix and specify already identified general patterns (Nüt- S1 Figs 1–3) is different from that in coeval strata of zel & Frýda 2003). I hope that a backward extrapola- the Baltic region but shares with it a set of cos- tion of the observed trend will also allow to mopolitan species. The rock matrix abounds in sec- determine, which shell morphology is the most prim- ondarily silicified (originally phosphatic) nuclei of itive and, implicitly, ancestral to gastropods. gastropod larvae (Fig. 1) already reported and illus- trated by Mestre et al. (2013) who identified a few Material As already mentioned above, the mass occurrence of secondarily phosphatized minute shells is generally interpreted as a result of increased mortality at attempted metamorphosis on the oxygen‐deficient sea bottom, thus in an environment promoting sec- ondary phosphatization (Dzik 1978, 1994a; Nützel & Mapes 2001; Mapes & Nützel 2009). This was a com- mon phenomenon in shelf areas of the early Palaeo- zoic presumably owing to the generally shallower bioturbation of sediments that time. Shelly detritus was then exposed for long enough to enable extensive Fig. 1. A piece of silicified limestone rock matrix from the acid‐ phosphatization, especially in temperate climate resistant residue of the San Juan Formation sample from its top regions close to upwellings, characterized by a high layer exposed at Don Braulio Creek, Villicum Range, Argentina. 34 J. Dzik LETHAIA 53 (2020) Fig. 2. Tentative taxonomic identifications of secondarily silicified juvenile and larval bellerophontid, hyolith and rostroconch conch nuclei from the top of the Darriwilian
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