Larval ecology and morphology in fossil gastropods

Abstract

The shell of marine gastropods conserves and reflects early ontogeny, including embryonic and larval stages, to a high degree when compared with other marine invertebrates. Planktotrophic larval development is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multiwhorled shell formed by a free‐swimming veliger larva. Basal gastropod clades (e.g. Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic ‘derived’ marine gastropods (Neritimorpha, Caenogastropoda and Heterobranchia) with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophic larval development was largely absent in the Cambrian and evolved at the Cambrian–Ordovician transition, mainly due to increasing benthic predation. The evolution of planktotrophic larval development offered advantages and opportunities such as more effective dispersal, enhanced gene flow between populations and prevention of inbreeding. Early gastropod larval shells were openly coiled and weakly sculptured. During the Mid‐ and Late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestions, it seems unlikely that the end‐Permian mass extinction selected against species with planktotrophic larval development. The molluscan classes with highest species diversity (Gastropoda and Bivalvia) are those which may have planktotrophic larval development. Extremely high diversity in such groups as Caenogastropoda or eulamellibranch bivalves is the result of high phylogenetic activity and is associated with the presence of planktotrophic veliger larvae in many members of these groups, although causality has not been shown yet. A new gastropod species and genus, Anachronistella peterwagneri, is described from the Late Triassic Cassian Formation; it is the first known Triassic gastropod with an openly coiled larval shell.

Ammonoid Embryonic Development

“…It is often known also as the protoconch (e.g., Owen 1878;Hyatt 1894;Miller 1938;House 1985), but the term should not be used to avoid confusion (Tanabe et al 1994(Tanabe et al , 2008De Baets et al 2012) with the gastropod protoconch, which is not homologous as it can represent both embryonic and/ or larval shell (Frýda et al 2008;Nützel 2014). Some authors have resolved this confusion by referring to the initial chamber as the "ammonoid protoconch" (House 1996).…”

An Early Triassic (Dienerian) microgastropod assemblage from the Salt Range, Pakistan and its implication for gastropod recovery from the end-Permian mass extinction

“…4B, E-H). These dimensions suggest larval planktotrophy for this species (see Nützel 2014). With a height of up to 1.2 mm, the protoconch of Strobeus pakistanensis is rather large.…”

“…With a height of up to 1.2 mm, the protoconch of Strobeus pakistanensis is rather large. However, there are several reports of larval shells in this size range from the Late Palaeozoic to early Mesozoic including examples representing Soleniscidae in which Strobeus is currently placed (Nützel & Mapes 2001, Mapes & Nützel 2009, Nützel 2014 Types. -Holotype (PIMUZ 32926) and 12 paratypes (PIMUZ 32882, 32891, 32892, 32922, 32925, 32927, 32930, 32935, 32938, 32939, 32941, 32944).…”

Lower Cambrian helcionelloid macromolluscs from South Australia

“…As noted by Nützel (2013), the evidence presented by Mus et al (2008) is difficult to reconcile since the protoconch I is not preserved and so no direct comparison can be drawn about the related egg size. There is also the possibility that the abrupt change in shell growth is not associated with the transition to the teleoconch, but rather an unrelated ontogenetic change during adult shell development or even a taphonomic artifact (Nützel, 2014). Based on the egg size parameters (specifically the 250 µm range) and shell shapes examined by Chaffee and Lindberg (1986) all of the species documented herein could not accommodate for fecundities representative of planktotrophy (>7000 eggs produced).…”