A newly recognized fossil coelacanth highlights the early morphological diversification of the clade

Abstract

Previously considered an actinopterygian or an osteichthyan incertae sedis, the Devonian (Givetian-Frasnian) Holopterygius nudus is reinterpreted as a coelacanth. This genus is among the oldest coelacanths known from articulated remains, but its eel-like morphology marks a considerable departure from the conventional coelacanth body plan. A cladistic analysis places Holopterygius as the sister taxon of the Carboniferous (Serpukhovian) genus Allenypterus. Despite the specialized morphology of these genera, they occupy a surprisingly basal position in coelacanth phylogeny; only Diplocercides and Miguashaia are further removed from the crown. A morphometric analysis reveals that coelacanths were anatomically disparate early in their history. Conflicts between this result and those of previous studies challenge the adequacy of systematic character sets for describing historical patterns of morphological variety. Coelacanths have long had an iconic place in the study of vertebrate evolution for their apparent anatomical conservatism over geological time, but Holopterygius provides clear evidence for rapid morphological evolution early in the history of this clade.

Figure 1

Holopterygius nudus Jessen (P 7789), latest Givetian–earliest Frasnian, Bergisch-Gladbach, Germany. (a) Photograph of P 7789a; (b) composite drawing based on P7789a-c; and (c) reconstruction. Bones outlined in grey in (c) were reconstructed based on Allenypterus. Scale bar represents 10 mm. Abbreviations: ano, anocleithrum; bp, basal plate of the second dorsal fin; chy, ceratohyal; cla, calvicle; cle, cleithrum; cop, principal coronoid; ex, extracleithrum; has, haemal arch and spine; gs, indeterminate components of the gill skeleton; nas, neural arch and spine; op, opercle; ot, otolith; pal, palate; pap, parapophyses; pmx, premaxilla; psph, parasphenoid; rd, radials of the dorsal lobe of the caudal fin; rv, radials of the ventral lobe of the caudal fin; sr, skull roof; sym, symplectic; vs, ventral keel scales.

Figure 2

Phylogenetic and morphological relationships between Holopterygius and other coelacanths. (a) Stratocladogram based on a cladistic solution with characters re-weighted by their rescaled consistency indices (N=2; L=90.83143; CI=0.6339; RI=0.8714; RCI=0.5966). Equally weighted solution places Holopterygius and Allenypterus as sister taxa, but cannot resolve the relationships between this clade and other Carboniferous coelacanths from Bear Gulch (N=58; L=245; CI=0.4587; RI=0.7029; RCI=0.3271). Arrow leads to more crownward coelacanths. Grey cladogram in background is that of Forey (1998). (b) Landmarks used in morphometric analysis. 1, tip of snout; 2, posterior margin of postparietals; 3, anterior insertion of first dorsal fin; 4, posterior insertion of second dorsal fin; 5, posterior insertion of second dorsal fin; 6, anterior insertion of epichordal lobe; 7, posterior tip of accessory lobe; 8, anterior insertion of hypochordal lobe; 9, posterior insertion of anal fin; 10, base of pelvic fin; 11, quadrate/articular joint. (c) Plots of scores on RW1 versus RW2, with coelacanths divided into Devonian/Carboniferous (lower plot) and post-Carboniferous forms (upper plot). RW1 describes caudal fin geometry, while RW2 reflects body depth. Coelacanth profiles (clockwise from bottom left): Allenypterus, Libys, Ticinepomis, Miguashaia, Holopterygius. Multiple metrics indicate that early coelacanths are more disparate than stratigraphically younger forms (sum of variances: 0.0392 versus 0.00900; product of variances: 1.11×10⁻⁹ versus 1.75×10⁻¹¹; sum of ranges: 1.03 versus 0.505; geometric mean of ranges: 0.227 versus 0.138).