Predatory asteroids and the decline of the articulate brachiopods

Abstract

Various causes, such as increased predation pressure, the lack of planktotrophic larvae, a ‘resetting’ of diversity, increased competition from benthic molluscs and the decline of the Palaeozoic fauna, have been suggested to explain the failure of the brachiopods to reradiate following the Permo‐Triassic mass extinction. Increased predation pressure has hitherto appeared improbable, because typical predators of brachiopods, such as teleostean fish, brachyuran crabs and predatory gastropods, did not undergo major radiation until the late Mesozoic and early Cenozoic. However, new evidence strongly suggests that one important group of predators of shelly benthic organisms, the asteroids, underwent a major radiation at the beginning of the Mesozoic. Although asteroids appeared in the early Ordovician, they remained a minor element of the marine benthos during the Palaeozoic acme of the brachiopods. However, these early asteroids lacked four important requirements for active predation on a bivalved epifauna: muscular arms (evolved in the early Carboniferous); suckered tube feet, a flexible mouth frame and an eversible stomach (all evolved in the early Triassic). Thus radiation of the Subclass Neoasteroidea coincided with both their improved feeding capability and the decline of the articulates. The asteroids were the only group of predators of brachiopods that underwent a major adaptive radiation in the earliest Mesozoic. The asteroids may therefore have contributed to inhibiting a Mesozoic reradiation of the brachiopods. Epifaunal species lacking a muscular pedicle may have been particularly vulnerable. Unlike bivalve molluscs, modern brachiopods show only a limited range of adaptations to discourage asteroid predation. □Asteroidea, Brachiopoda, evolution, predation, functional morphology.

Starfish Diversity in the Wenlock of England

“…Some authors (e.g. Gale 1987; Donovan and Gale 1990; Gale and Donovan 1992; Dean 1999 a ; Shackleton 2005) have interpreted Palaeozoic asteroids as ecologically restricted, unable to exploit the full variety of feeding methods employed by extant starfish, being mainly scavengers or detritivores. However, this viewpoint has not been universally accepted, with Blake and Guensburg (1989, 1990, 1994) arguing that Palaeozoic asteroids were morphologically capable of exploiting a similarly diverse range of life habits to post‐Palaeozoic taxa.…”

Uncovering the holes and cracks: from anecdote to testable hypotheses in predation studies

“…These points render many seemingly attractive hypotheses untestable. For example, Donovan & Gale () suggested that the post‐Palaeozoic decline in rhynchonelliform brachiopods might be attributed, at least in part, to the rise in extra‐oral feeding asteroids. Although that suggestion provoked a certain amount of lively debate (Blake & Guensburg ; Vermeij ; Pratt ; Gale & Donovan ) it is difficult to conceive of an adequate test.…”

“…It is also worth noting that, although modern mussels are important prey items for a whole host of predatory taxa (Harper ), almost nothing has been reported for fossil mytilids, probably because so many of their predators leave little or no trace of their activities (Harper ). For other taxa, such as brachiopods, bryozoans and crinoids, there has been little impetus to study predation in modern settings (Donovan & Gale ) and, for most of these examples, there are probably more well known reports concerning predation evidence of fossil material than extant records. Indeed much of the observation of extant members of these taxa has been undertaken by palaeontologists rather than zoologists, and much of it published in the palaeontological literature (e.g.…”

Evolutionary and ecological significance of Lepidaster grayi, the earliest multiradiate starfish

“…The basis for much of the controversy over the functional morphology of Palaeozoic starfish is that the phylogenetic relationships of the Asteroidea are poorly understood. It is generally agreed that all Palaeozoic taxa belong to the stem (Blake, 1987; Gale, 1987; Blake & Guensburg, 1990; Donovan & Gale, 1990; Blake, Janies & Mooi, 2000; Blake & Hotchkiss, 2004; Shackleton, 2005), with crown‐group starfish known only from the post‐Permian, but no consensus has been reached as to which are the most basal extant asteroids. Some authors (e.g.…”