Bony-fish-like scales in a Silurian maxillate placoderm

Abstract

Major groups of jawed vertebrates exhibit contrasting conditions of dermal plates and scales. But the transition between these conditions remains unclear due to rare information on taxa occupying key phylogenetic positions. The 425-million-year-old fish Entelognathus combines an unusual mosaic of characters typically associated with jawed stem gnathostomes or crown gnathostomes. However, only the anterior part of the exoskeleton was previously known for this very crownward member of the gnathostome stem. Here, we report a near-complete post-thoracic exoskeleton of Entelognathus. Strikingly, its scales are large and some are rhomboid, bearing distinctive peg-and-socket articulations; this combination was previously only known in osteichthyans and considered a synapomorphy of that group. The presence in Entelognathus of an anal fin spine, previously only found in some stem chondrichthyans, further illustrates that many characters previously thought to be restricted to specific lineages within the gnathostome crown likely arose before the common ancestor of living jawed vertebrates.

Fig. 1. Entelognathus primordialis (IVPP V32322).

a Photograph in left lateral view. b Virtual rendering in left lateral view. c Shape space PCA scatter plot showing PC1 versus PC2 of Entelognathus scales (n = 237 flank scales) based on landmark-based geometric morphometrics. anf.s anal fin scales, anf.sp anal fin spine, df.s dorsal fin scales, df.sp dorsal fin spine, llg lateral line groove, md median dorsal plate, Mt.1–12, Morphotype 1 to Morphotype 12, mvs median ventral scale, m.dplv1 first median dermal pelvic plate, m.dplv2 second median dermal pelvic plate, pmd1–4 first to fourth postmedian dorsal scales, p.dplv.l left dermal pelvic plate, p.dplv.r right dermal pelvic plate.

Fig. 2. Scales and fin spines of Entelognathus primordialis (IVPP V32322).

a Morphotype 6 (2D01) in crown view (a₁) and basal view (a₂). b Morphotype 6 (2D04) in crown view (b₁) and basal view (b₂). c Morphotype 6 (3D02) in crown view (c₁) and basal view (c₂). d Morphotype 6 (3D03) in crown view (d₁) and basal view (d₂). e Morphotype 6 (3D04) in crown view (e₁) and basal view (e₂). f First postmedian dorsal scale in crown view (f₁), lateral view (f₂), and basal view (f₃). g Second postmedian dorsal scale in crown view (g₁), lateral view (g₂), and basal view (g₃). h Third postmedian dorsal scale in crown view (h₁), lateral view (h₂), and basal view (h₃). i Fourth postmedian dorsal scale in crown view (i₁), lateral view (i₂), and basal view (i₃). j Median ventral scale in crown view (j₁), lateral view (j₂), and basal view(j₃). k Dorsal fin spine in crown view (k₁), lateral view (k₂), and basal view (k₃). l Anal fin spine in crown view (l₁), lateral view (l₂), and basal view(l₃). adp anterodorsal process, k keel, lh lingulate humps, p peg, r ridges, s socket.

Fig. 3. 3D virtual model of a Morphotype 4 scale (V32323.5).

a 3D virtual model in crown view. b 3D virtual model with canal system in crown view. c Canal system in crown view. d 3D virtual model with canal system in lateral view. e Canal system in lateral view showing the forked vascular canals. f–h CT slices in crown view showing the ascending vascular canals. cf concealed field, rm recrystallized minerals, fvc forked vascular canals, vc vascular canals.

Fig. 4. Body plan of Entelognathus primordialis (IVPP V32322).

a 3D virtual models of scales and spines in left lateral view. b Squamation and body plan in lateral view. c Reconstruction of Entelognathus primordialis in lateral view. d Matrix plot showing the height of flank scales. e Matrix plot showing the length of flank scales. f Matrix plot showing the aspect ratio of flank scales. anf.s anal fin scales, anf.sp anal fin spine, db.a anterior area of dorsal belt, db.p posterior area of dorsal belt, df.s dorsal fin scales, df.sp dorsal fin spine, Ll lateral line scale row, mb.a anterior area of middle belt, mb.c caudal area of middle belt, mb.m middle area of middle belt, mb.p posterior area of middle belt, mb.pc precaudal area of middle belt, mdb median dorsal belt, Mt.1–12, Morphotype 1 to Morphotype 12, mvs median ventral scale, pmd1–4 first to fourth postmedian dorsal scales, p.dplv dermal pelvic plate, tf.s tail fin scales, vb.a anterior area of ventral belt, vb.d dorsal area of ventral belt, vb.m middle area of ventral belt, vb.p posterior area of ventral belt, vb.v ventral area of ventral belt, 1–24 column number of the scales, 1D–3D first, second and third rows of scales dorsal to the lateral line scale row, 1V–7 V first to seventh rows of scales ventral to the lateral line scale row.

Fig. 5. Summary phylogeny of early jawed vertebrates showing the transformation in scale morphology and squamation.

This cladogram is simplified from the strict consensus tree of the 100,000 most parsimonious trees (Supplementary Fig. 6). The character transformations mapped on the tree are based on the results of phylogenetic analyses. Drawing sources: Parayunnanplepis; Sigaspis; Brochoadmones; Andreolepis; Moythomasia^,; Polypterus; Guiyu; Osteolepis.