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The Largest Silurian Vertebrate and Its Palaeoecological Implications

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The Largest Silurian Vertebrate and Its Palaeoecological Implications

Brian Choo et al. Sci Rep.

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.

Figures

Figure 1
Figure 1. Silurian sequence in Qujing (Yunnan, China) with stratigraphic position of Megamastax amblyodus gen. et sp. nov. and other vertebrate taxa (modified from ref. , using Adobe Illustrator 10).
Figure 2
Figure 2. Fossils of Megamastax amblyodus gen. et sp. nov.
(A–E) Holotype mandible (IVPP V18499.1) in (A) lateral, (B) lingular, and (C) dorsal views; close-up of prearticular bone, showing surface ridges (D), and close-up of the marginal dentition in lingual view (E). (F–H) Partial mandible (V18499.2) in (F) lateral, (G) lingular, and (H) dorsal views. (I) Right maxilla (V18499.3) in lateral view. (J) Reconstruction of (i1) Guiyu oneiros (ref. 13) alongside hypothetical silhouettes of (J2–3) Megamastax with superimposed fossil outlines (drawn by B.C.). The (J2) smaller fish is based on the V18499.1 and V18499.3, the (J3) larger on V.18499.2. ar.psym, knob-like parasymphysial structure; Co 1–4, coronoids 1–4; coT 1–8, coronoid teeth 1–8; De, dentary; fo.add, adductor fossa; fo.gl, glenoid fossa; fo.Mk, Meckelian foramen; Id, infradentary; mpl, mandibular pit line; maT, marginal teeth; oaMx, overlap area for maxilla and quadratojugal; Pat, prearticular; sym, area for parasymphysial plate; tr, indented track bordering splenial.
Figure 3
Figure 3. Lingual views of mandibles from selected pre-Emsian osteichthyans.
Except for Megamastax, all are from the Lochkovian Xitun Formation, Qujing, eastern Yunnan, China. (A) Psarolepis romeri, IVPP V8138 (reversed). (B) Achoania jarviki, IVPP V12492.1 (reversed). (C) Jaw tentatively assigned to Meemannia eos, IVPP V14536.5. (D) Styloichthys changae, IVPP V8143.1. (E) Partial dentary of an indeterminable osteichthyan, IVPP V12493 (reversed). (F) Megamastax amblyodus, IVPP V18499.1 (holotype), dark grey = matrix-filled areas. (G) Megamastax amblyodus, IVPP V18499.2 with restored silhouette in black. (A–G) drawn by B.C. ar.psym, knob-like parasymphysial articular structure; ar.Co1–4, articulation for coronoid 1–4; Co1–5, coronoid 1–5; coT1–8, 1st–8th coronoid tooth; fo.add, adductor fossa; fo.gl, glenoid fossa; Pat, Prearticular; sym, area for parasymphysial tooth plate. Scale bars = 5 mm.
Figure 4
Figure 4. Life reconstruction of Megamastax amblyodus consuming the galeaspid Dunyu longiforus (drawn by B.C.).
Figure 5
Figure 5. Competing models of mid-Palaeozoic oxygenation from 500 Ma to 300 Ma.
Vertical blue line indicates minimum age of the Kuanti Formation and Megamastax. (A) From figure 2 in ref. . Estimates of atmospheric O2 over time based on calculations from the GEOCARBSULF model (solid line = modern O2%). (B) From figure 3B in ref. . Mo sediment samples with seawater (SW) values inferred from highly euxinic (red) and mildly euxinic sediments (pink). δ98Mo is a measure of the relative proportions of heavy and light Mo isotopes, with higher values inferring oxygenated oceans (δ98Mo modern SW = 2.3). Solid lines represent 90% percentiles while values above the dashed line require a substantial oxic Mo sink.

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