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| Monosmilus chureloides
Capobianco, Beckett, Steurbaut, Gingerich, Carnevale & Friedman, 2020
DOI: 10.1098/rsos.192260
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Abstract
Many modern groups of marine fishes first appear in the fossil record during the early Palaeogene (66–40 Ma), including iconic predatory lineages of spiny-rayed fishes that appear to have originated in response to ecological roles left empty after the Cretaceous/Palaeogene extinction. The hypothesis of extinction-mediated ecological release likewise predicts that other fish groups have adopted novel predatory ecologies. Here, we report remarkable trophic innovation in early Palaeogene clupeiforms (herrings and allies), a group whose modern representatives are generally small-bodied planktivores. Two forms, the early Eocene (Ypresian) †Clupeopsis from Belgium and a new genus from the middle Eocene (Lutetian) of Pakistan, bear conspicuous features indicative of predatory ecology, including large size, long gapes and caniniform dentition. Most remarkable is the presence of a single, massive vomerine fang offset from the midline in both. Numerous features of the neurocranium, suspensorium and branchial skeleton place these taxa on the engraulid (anchovy) stem as the earliest known representatives of the clade. The identification of large-bodied, piscivorous anchovies contributes to an emerging picture of a phylogenetically diverse guild of predatory ray-finned fishes in early Palaeogene marine settings, which include completely extinct lineages alongside members of modern marine groups and taxa that are today restricted to freshwater or deep-sea environments.
Keywords: computed tomography, ichthyology, Palaeogene, ecological release, Clupeomorpha, piscivory
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| Figure 2. Cranial anatomy of †Monosmilus chureloides gen. et sp. nov. (GSP-UM 37, holotype). (a) Rendering and (b) line drawing of specimen in left lateral view. (c) Rendering of basibranchial series with hypohyals (dark green) in left lateral view. (d) Rendering of braincase in anterior view, highlighting the vomerine fang. (e) Rendering of right dentary (incomplete) in medial view, with loose tooth crowns (probably replacement teeth) highlighted in red. Colours indicate different cranial regions: neurocranium (purple), jaws, palatoquadrate and cheek bones (blue), dorsal portion of hyoid arch (cyan), branchial skeleton (chartreuse).
Scale bar: 10 mm. Abbreviations: afn, anterior frontal fontanelle; bb1, first basibranchial; bb2, second basibranchial; bb3, third basibranchial; bbtp, basibranchial toothplate; d, dentary; dh, dorsal hypohyal; f, frontal; ecp, ectopterygoid; enp, endopterygoid; h, hyomandibula; io, infraorbital; le, lateral ethmoid; mpt, metapterygoid; mx, maxilla; ors, orbitosphenoid; pal, palatine; pas, parasphenoid; q, quadrate; sp, sphenotic; sr, scleral ring; vf, vomerine fang; vh, ventral hypohyal; vo, vomer.
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| a comparison between Monosmilus chureloides (conservative estimate) and a modern anchovies.
Illustrstion: Joschua Knüppe
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Systematic palaeontology.
Teleostei Müller, 1845
Clupeomorpha Greenwood, Rosen, Weitzman and Myers, 1966
Clupeiformes Bleeker, 1859 sensu Grande, 1985
Clupeoidei Jordan, 1923 sensu Greenwood, Rosen, Weitzman and Myers, 1966
Engrauloidea Grande, 1985
†Monosmilus chureloides gen. et sp. nov.
Etymology: Generic name from the combination of the Ancient Greek mónos (single) and smil'e (knife), referring to the single massive vomerine fang. Specific name from the combination of Churel, the name in Urdu of a shapeshifting vampire-like demon with large fangs or tusks, with the suffix -oides, indicating similarity.
Locality and horizon: Rakhi Nala (locality RN-4) on the east side of the Sulaiman Range in the Dera Ghazi Khan District of western Punjab Province, Pakistan. The ‘lower chocolate clays' of the Domanda Formation (electronic supplementary material, figure S7) were deposited in a shallow coastal marine environment [35]. Nannoplankton stratigraphy (zone NP 15) constrains the Domanda Formation to the Lutetian stage/age of the early–middle Eocene.
Diagnosis: Clupeiform with vomer bearing two large tooth pits; single vomerine fang representing the largest tooth; parasphenoid straight in lateral view; orbitosphenoid antero-ventrally contacting the parasphenoid; occipital region posteriorly elongated; robust and straight maxilla lacking teeth; dentary bearing greatly enlarged, postero-medially recurved caniniform teeth throughout its length; largest dentary tooth approximately 70% of length of orbital cavity; bulbous lateral ethmoids; anterior part of palatine triangular in lateral view; horizontal lamina of the ectopterygoid under the orbit and directly overlying the maxilla; third basibranchial longest element of the basibranchial series.
Notes: The attribution of two closely related species possibly forming a monophyletic group to two separate genera is subjective. †Monosmilus chureloides differs from †Clupeopsis straeleni in several morphological features, including: proportionally larger teeth on vomer and dentary; bulbous (rather than irregularly flattened) lateral ethmoid; postorbital region of the neurocranium longer (instead of shorter) than preorbital region; contact between orbitosphenoid and parasphenoid; larger lateral horizontal lamina of the ectopterygoid; longer and broader endopterygoid; third (instead of second) basibranchial as the longest element of the basibranchial series. We consider these differences sufficient to justify the erection of a new genus for †M. chureloides.
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| Monosmilus chureloides, a large stem engraulid, pursuing some smaller relatives, the size of the average modern anchovies. Our monster with the kindergarten drawing dentition is interrupted by a basal whale, Dalanistes, swooping in for a kill.
DOI: 10.1098/rsos.192260
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| Figure 4. Medium-to-large-bodied predatory actinopterygians in shallow marine environments through time, showing faunal turnover across the K/Pg extinction and the early Palaeogene. †Aspidorhynchids are reported from continental deposits in the Palaeocene, but their last marine occurrences are Maastrichtian. †Enchodontids are reported from Danian marine deposits, but this is possibly due to reworking from Maastrichtian layers. The last occurrence of †Holosteinae extends into the early Oligocene (Rupelian; not shown here). An arrow on the right end of the illustrated range indicates survivorship to the present day. ...
DOI: 10.1098/rsos.192260
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Alessio Capobianco, Hermione T. Beckett, Etienne Steurbaut, Philip D. Gingerich, Giorgio Carnevale and Matt Friedman. 2020. Large-bodied Sabre-toothed Anchovies reveal Unanticipated Ecological Diversity in early Palaeogene Teleosts. Royal Society Open Science. 7(5) DOI: 10.1098/rsos.192260
