Functional morphology, evolution and systematics of Early Palaeozoic univalved molluscs

Peel, John S.

doi:10.34194/bullggu.v161.6717

Cited by 36 publications

(8 citation statements)

References 21 publications

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“…Muscle scars are grouped into a horseshoe-shape on the dorsal surface; the apex lies outside of this ring in some forms and may overhang the anterior shell margin; in other forms, the apex is sub-central in position. Peel (1991b) recognised three orders, reflecting three different morphological groups, the Tryblidiida Lemche 1957 typically with a pilinaform shell, the Cyrtonellida Horný, 1963 with a planispirally coiled shell, and the Hypseloconida Peel 1991b with a tall cone-shaped shell. The Tergomya is equivalent to the sub-class Tergomya of Horný (1965a, b) and is also very similar to the Tryblidiida of Wingstrand (1985); Tryblidia is broadly used interchangeably with Monoplacophora in neontological usage (e.g., Lindberg 2009;Kocot et al 2020;Ponder et al 2020).…”

Section: Systematic Palaeontologymentioning

confidence: 99%

Tergomyan molluscs from the Early Ordovician of the Llangynog Inlier, South Wales, UK

Cope,

Ebbestad

2023

PalZ

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A low abundance but diverse fauna of tergomyan molluscs is described from the Llangynog Inlier, Carmarthenshire, South Wales. The specimens originate from a single quarry exposing shallow-water siltstone and mudstone beds of the Early Arenig (early Floian) Bolahaul Member of the Ogof Hên Formation. The fauna includes Proplina areniga sp. nov., Proplina? obtusa sp. nov., Celtopileus calvapex gen. et sp. nov. with preserved muscle scars, Costulaconus mirificus gen. et sp. nov., and Hypseloconus? sp. Comparable tryblidiid taxa are mainly found in the Lower Ordovician of Midwestern USA among species of Proplina Kobayashi, 1933 and Gasconadeoconus Stinchcomb, 1986. The tentative assignment of the predominantly late Cambrian Hypseloconus is symptomatic of the many uncertainties regarding the phylogeny of the tryblidiids. The diverse assemblage of organisms preserved at the single locality represents one of the most remarkable faunas of this age worldwide.

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Section: Systematic Palaeontologymentioning

confidence: 99%

Tergomyan molluscs from the Early Ordovician of the Llangynog Inlier, South Wales, UK

Cope,

Ebbestad

2023

PalZ

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“…The rotation hypothesis became entrenched during a time when there were no data that might contradict the assumption of a posterior mantle cavity within a gastropod progenitor, but neither was there strong independent evidence to support this assumption (no unambiguous fossil evidence, no plausible molluscan sister group with a mantle cavity confined to the posterior aspect). Subsequently, living tryblidiid monoplacophorans were discovered (Lemche 1957) and were placed as the most basal lineage of conchiferan mollusks with extant descendants, although Peel (1991) has suggested that monoplacophorans are a polyphyletic assemblage. The mantle cavity of tryblidiids is in the form of elongate lateral clefts running down each side of the foot and uniting at the posterior end.…”

Section: Torsion As a "Grand Controversy"mentioning

confidence: 99%

Modern insights on gastropod development: Reevaluation of the evolution of a novel body plan

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2006

Integrative and Comparative Biology

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More than a century of speculation about the evolutionary origin of the contorted gastropod body plan has been inspired by adult anatomy and by long-standing developmental observations. The result has been a concept of gastropod torsion that I call the "rotation hypothesis." Under the rotation hypothesis, gastropods originated when all components of the visceropallium (shell, mantle, mantle cavity with contained structures, and viscera) rotated by 180° relative to the head and foot. This evolutionary rotation is echoed during early development of patellogastropods and vetigastropods and occurs to some extent during development of more derived clades. However, comparative developmental data on ontogenetic torsion are minimal and I argue that the rotation hypothesis is a tautological argument. More recent studies on representatives from 3 major clades of gastropods suggest that the highly conserved aspect of gastropod development is not synchronous rotation of all components of the visceropallium relative to the head and foot but rather a state of anatomical organization in which the developing mantle cavity is on the right but the shell coil is posterior (endogastric orientation). This conserved state of developmental anatomy has inspired an alternative hypothesis for the evolutionary origin of the gastropod body plan, the "asymmetry hypothesis." Under the asymmetry hypothesis, the gastropod mantle cavity originated from 1 side only of a bilateral set of mantle cavities. The asymmetry hypothesis does not require a saltation event to explain the origin of gastropods, nor does it require that the ancient molluscan precursor of gastropods carried the shell coil over the head (exogastric orientation).

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“…Since the discovery of Neopilina, a living creature, in 1957, it has been involved in research on the evolution of molluscs. The helcionelloids are the earliest molluscs in the Phanerozoic, but there are many concerns about their systematic position (Peel, 1991a;Geyer, 1994;Runnegar, 1996;Skovsted, 2004;Geyer et al, 2019). Most of these univalve molluscs, whose shells were not distorted in the early Cambrian, were originally classified as Monoplacophora (Yu, 1979(Yu, , 1981Jiang, 1980;Mao et al, 1993;Feng et al, 1994).…”

Section: Introductionmentioning

confidence: 99%

“…Peel (1991a, b) classified most of the groups of univalve molluscs whose shells were not distorted as helcionelloid. Peel (1991a) thought that the helcionelloid represented the main pedigree of univalve molluscs distinct from Tergomya, so established a new class Helcionelloida.…”

Section: Introductionmentioning

confidence: 99%

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Helcionelloids from the Cambrian Fortunian of the Northern Yangtze Platform, China

ZHANG,

PANG,

YANG

et al. 2023

Acta Geologica Sinica (Eng)

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A study on the morphological characteristics of helcionelloids and its microstructure from the Nanjiang area (north Sichuan, China) at the northern Yangtze Platform is taken to understand its position in the phylogeny. Helcionelloids fossils from the Kuanchuanpu Formation of the Changtanhe–Maolinzi section include 7 genera and 11 species. There are distinct differences on the degree of shell bending, the apical morphology and apex position, the outline of the apeture, and the height of the shell, which constitute an important identification feature. Though the helcionelloid apices have varying degrees of rotation towards the aperture, they have no any evident distortion. A pair of basally symmetrical muscle scars are discovered on the shells of helcionelloids (such as Bemella simplex), showing distinct difference comparing with those of the stem‐group gastropod (e.g., Pelagiella). The prismatic shell layer of helcionelloids has been observed, which are common components in Cambrian molluscs. They are significantly different from the dense layer microstructure found in contemporary molluscs. The microstructures, muscle scars and morphological characteristics demonstrate that the helcionelloid is a stem‐group of molluscs.

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Functional morphology, evolution and systematics of Early Palaeozoic univalved molluscs

Cited by 36 publications

References 21 publications

Tergomyan molluscs from the Early Ordovician of the Llangynog Inlier, South Wales, UK

Tergomyan molluscs from the Early Ordovician of the Llangynog Inlier, South Wales, UK

Modern insights on gastropod development: Reevaluation of the evolution of a novel body plan

Helcionelloids from the Cambrian Fortunian of the Northern Yangtze Platform, China

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