A platysomid occurrence from the Tournaisian of Nova Scotia

Abstract

The Hangenberg extinction has been hypothesized as a first order event in vertebrate evolution; however, information on the earliest Carboniferous vertebrate fauna, crucial in evaluating biodiversity changes, is scarce. Post-extinction recovery has been suggested as the driver of ray-finned fish (actinopterygian) richness increase and differentiation in the Carboniferous. Under this model, actinopterygian postcranial morphology differentiates in the second stage of their radiation. Here, we report on a platysomid occurrence from the Tournaisian of Nova Scotia, Canada. Despite long-standing taxonomic issues with deep-bodied actinopterygians, this specimen represents the earliest known occurrence of one such fish. Its presence in the earliest Carboniferous indicates that actinopterygians were already postcranially differentiated in the aftermath of the Hangenberg. Moreover, this specimen suggests that earliest Carboniferous actinopterygians used multiple locomotory modes; recent data from later Carboniferous taxa suggest that actinopterygian locomotory modes proliferated throughout the Carboniferous. Taken together, these data suggest that early Carboniferous actinopterygians were morphologically, ecologically, and functionally diverse.

Figure 1

Diversity of body shapes among possible independent origins of deep-bodied actinopterygians listed by Sallan and Coates. Lines on the leading margin of fins indicate fringing fulcra. (a) Eurynotiformes. From left to right: Eurynotus, modified from Sallan and Coates, Fig. 15 and Traquair, Plate III, Fig. 1; Cheirodopsis, modified from Sallan and Coates, Fig. 15 and Moy-Thomas and Dyne, Fig. 36; and Amphicentrum, modified from Sallan and Coates, Fig. 15. (b) ‘Lower’ platysomids. From left to right: Platysomus striatus, modified from Haubold and Schaumberg, Fig. 104; and Platysomus superbus, modified from Moy-Thomas and Dyne, Fig. 39. (c) Bobasatraniiformes. From left to right: Platysomus schultzei, drawn from Zidek, Figs. 22 and 24; and Bobasatrania, modified from Schaeffer and Mangus, Fig. 11. (d) Adroichthys, modified from Gardiner, Fig. 15. (e) Aesopichthys, modified from Poplin and Lund, Fig. 12. (f) Frederichthys, modified from Coates, Fig. 8. (g) Discoserra, drawn from Lund, Figs. 5, 7, 8.

Figure 2

Geographic and stratigraphic context of specimen locality. (a) Geography and bedrock geology of Walton, Nova Scotia. Map of Nova Scotia modified from Anderson et al.; map of Walton area modified from Moore et al.. Numbers on map margins are UTM coordinates. (b) Schematic stratigraphic column of Blue Beach and Walton, Nova Scotia modified from Martel et al..

Figure 3

Whole specimen, including posterior flank and tail, of platysomid specimen NSM 017.GF.017.001. Scale bar = 20 mm. (a) Composite photograph of specimen in left lateral view. (b) Trace of latex peel taken from specimen—right lateral view of counterpart. Large dots indicate pores for the lateral line. afr, anal fin rays; ags, anal guard scales; abf, axial basal fulcrum; al, axial lobe; cal, caudal lobe; cbf, caudal basal fulcrum; cr, caudal lobe rays; dfr, dorsal fin rays; dgs, dorsal guard scales; ?ff, fringing fulcra; ra, dorsal fin radial.

Figure 4

Detail of scale and proximal lepidotrichia of platysomid specimen NSM 017.GF.017.001. Scale bars = 2 mm. (a) Detail of flank and lateral line scales. (b) Line illustration of flank and lateral line scales. (c) Detail of scales and lepidotrichia proximal to dorsal fin. (d) Detail of scales and lepidotrichia proximal to anal fin. (e) Detail of scales and lepidotrichia in caudal fin. al, axial lobe; afr, anal fin rays; ags, anal guard scales; cr, caudal lobe rays; dfr, dorsal fin rays; dgs, dorsal guard scales; fs, flank scales; lls, lateral line scales.

Figure 5

Detail of dorsal fin of platysomid specimen NSM 017.GF.017.001. Scale bar = 10 mm. dfr, dorsal fin rays; dgs, dorsal guard scales; ra, dorsal fin radial.

Figure 6

Detail of anal fin of platysomid specimen NSM 017.GF.017.001. Scale bar = 10 mm. afr, anal fin rays; ags, anal guard scales.

Figure 7

Detail of caudal fin and scales of platysomid specimen NSM 017.GF.017.001. Scale bar = 10 mm. abf, axial basal fulcrum; al, axial lobe; cal, caudal lobe; cbf, caudal basal fulcrum; cr, caudal lobe rays; ?ff, fringing fulcra.

Figure 8

Other platysomids bearing anal and/or dorsal guard scales. Scale bars = 10 mm. (a) Platysomus tenuistriatus, SM E 4949 a in left lateral view. Photograph provided by the GB3D Type Fossils Online project, the copyright holder, at http://www.3d-fossils.ac.uk/fossilType.cfm?typSampleId=20005040 under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License. (b) Platysomus sp., FMNH PF 10792 in right lateral view. Photograph kindly provided by Jack Stack. ags, anal guard scales; dgs, dorsal guard scales.