The largest Silurian vertebrate and its palaeoecological implications

Abstract

An apparent absence of Silurian fishes more than half-a-metre in length has been viewed as evidence that gnathostomes were restricted in size and diversity prior to the Devonian. Here we describe the largest pre-Devonian vertebrate (Megamastax amblyodus gen. et sp. nov.), a predatory marine osteichthyan from the Silurian Kuanti Formation (late Ludlow, ~423 million years ago) of Yunnan, China, with an estimated length of about 1 meter. The unusual dentition of the new form suggests a durophagous diet which, combined with its large size, indicates a considerable degree of trophic specialisation among early osteichthyans. The lack of large Silurian vertebrates has recently been used as constraint in palaeoatmospheric modelling, with purported lower oxygen levels imposing a physiological size limit. Regardless of the exact causal relationship between oxygen availability and evolutionary success, this finding refutes the assumption that pre-Emsian vertebrates were restricted to small body sizes.

Precise occlusion and trophic niche differentiation indicate specialized feeding in Early Devonian jawed vertebrates

“…The employment of different biting or chewing strategies by coeval ischnacanthiforms at the MOTH locality provides support for the hypothesis that these two species could have coexisted by partitioning trophic niches and specializing in prey sources. This also supports the hypothesis proposed by Choo et al (2014) that early gnathostomes had likely undergone significant trophic niche differentiation by the Devonian.…”

“…These teeth could have been well suited to puncturing soft prey. The medial teeth, however, are blunt, robust, and low crowned, typical of animals interpreted to be durophagous (Pregill 1984;Motani 2005;Choo et al 2014;Crofts and Summers 2014). Two very different forms of teeth on the same tooth-bearing element in the jaw could be indicative of several possible feeding strategies.…”

“…Alternatively, it may have specialized in hard-shelled prey, grasping with its lateral teeth and crushing with its medial teeth. This type of strategy has been proposed for Megamastax amblyodus (Choo et al 2014) and could have also been employed by T. gannitus.…”

Evidence of a specialized feeding niche in a Late Triassic ray-finned fish: evolution of multidenticulate teeth and benthic scraping in †Hemicalypterus

“…Although it is hypothesized that Mesozoic or Paleozoic ray-finned fishes may have been opportunistically feeding on plants or detritus, there is currently no evidence-either via gut contents, ichnological traces, or dentition-to indicate that Paleozoic or Mesozoic actinopterygians had moved into a herbivorous niche that had otherwise been occupied by aquatic invertebrates (Steneck 1983). Instead, it has been inferred that Paleozoic and Mesozoic fishes were carnivorous or omnivorous feeders with generalized teeth either styliform, caniniform, or durophagous in appearance (Schaeffer and Rosen 1961;Tintori 1983;Nursall 1996;Kriwet 1999;Choo et al 2014;Smithwick 2015). With the evolution of highly specialized tooth morphology for scraping the substrate, †Hemicalypterus likely exploited a new ecological niche that would allow it to feed directly on primary producers in an aquatic ecosystem.…”

“…The fossil record for teleosts expressing herbivory extends to the Eocene (∼50 Ma; Bellwood 1996), including fossil representatives of many extant marine and brackish inhabitants (e.g., rabbitfishes, scats; Tyler and Sorbini 1990;Blot 1969). Without previous fossil anatomical evidence to indicate a herbivorous diet, it has been assumed that Paleozoic and Mesozoic fishes were carnivorous or omnivorous as they possess generalized caniniform or styliform teeth, associated with carnivory (Schaeffer and Rosen 1961) or specialized durophagous dentition for crushing invertebrates (e.g., Nursall 1996;Kriwet 1999;Tintori 1983;Choo et al 2014;Smithwick 2015). Evidence of specialized, multidenticulate, scraping dentition is documented here for the first time in the Mesozoic fish †H.…”