Table 1.
Summary of results and description of character states for ancestral state reconstruction.
| Char. State | Centrum shape | Fossil articular surface morphology and fossil hard and soft tissues | Anatomical structures and tissues in living animal | Joint type | Occurrence in this study |
|---|---|---|---|---|---|
| 0 | Amphicoelous, notochordal | I. Notochord foramen (Figs. 3a, S3, S4, S7) | I. Persisting notochord, expanded in intervertebral space (Fig. 1a) | Plesiomorphic amniote | Stem amniotes, basal synapsids, basal reptiles, Sphenodon |
| II. Central concave area covered by thin chondrocyte layer or bone (Figs. 3a, S2, S4d) | II. Area of expanded notochord, possibly incipient nucleus pulposus, separated by cartilage from bone (Fig. 1e) | ||||
| III. Irregularly shaped and loosely packed bodies the size of large chondrocytes or translucent, coarsely crystalline matter in the intervertebral space (Figs. 3a, S7) | III. Possible notochordal or incipient nucleus pulposus cells (Fig. 1d) | ||||
| IV. Peripheral convex area with chondrocyte files (serial cartilage) and intervening bone spicules (Figs. S1b–d, S3b–d, S4b–d, S7c) | IV. Fibrocartilage of annulus fibrosus inserting into serial cartilage and bony spicules (Fig. 1b) | ||||
| IV. Sharpey’s fibres peripheral to articular surface in periosteal bone (Fig. S4b) | IV. Joint capsule (Fig. 1c) | ||||
| 1 | Amphicoelous, non-notochordal | I. Notochord foramen absent (Figs. S8a, S10a, S11a) | I. Notochord reduced or absent | Intervertebral disc |
hupehsuchians, ichthyosaurs, placodonts |
| II. Central concave area of articular surface with thin layer of irregular chondrocytes (Figs. 3d, S10d, S12c) | II. Globular nucleus pulposus separated by cartilage from bone (Fig. 1e) | ||||
| III. Irregularly shaped and loosely packed bodies or grains larger than chondrocytes in the intervertebral space (Figs. 3c, S9b, S11, S12) | III. Possible nucleus pulposus cells or notochord cells (Fig. 1d) | ||||
| IV. Peripheral convex area of articular surface with chondrocyte files (serial cartilage) and intervening bone spicules (Figs. 3e, S8b, S9b, c, S10b, S12a, b) | IV. Fibrocartilage of annulus fibrosus inserting into serial cartilage and bony spicules in articular surface periphery (Fig. 1b) | ||||
| V. Sharpey’s fibres peripheral to articular surface in periosteal bone | V. Joint capsule | ||||
| 2 | Platycoelous | I. Notochord foramen absent (Figs. S19a, S20a, S22a, S23a) | I. Notochord absent (Fig. S5a) | Intervertebral disc | Mammals, eosauropterygians, most archosauromorphs, including most dinosaurs |
| II. Flat central area of articular surface with irregular chondrocyte cover, usually thin (Figs. S19, S20c, S22c, S24a) | II. Lens-shaped nucleus pulposus separated by cartilage from underlying bone (Figs. 2a, b, S5b) | ||||
| III. Irregularly shaped and loosely packed bodies the size of large chondrocytes in the intervertebral space (Fig. S22b) | III. Possible nucleus pulposus cells | ||||
| IV. Peripheral area of articular surface with peripherally inclined chondrocyte files (serial cartilage) and bone spicules (Figs. 3b, 5i–l, S6b,c, S20b, S22d, S24b,c, S25b–e) | IV. Fibrocartilage of annulus fibrosus inserting into serial cartilage and bony spicules in articular surface periphery (Figs. 2c, 5e–h, S5c) | ||||
| V. Sharpey’s fibres peripheral to articular surface in periosteal bone | V. Joint capsule | ||||
| 3 | Procoelous /opistho-coelous | I. Notochord foramen absent (Figs. S15a, S17a) | I. Notochord absent (Figs. 5a, S16a) | Synovial ball-and-socket | Squamates, eusuchians |
| II. Convex/concave joint surface with unorganized chondrocytes or serial cartilage with files normal to joint surface overlying bone, no bony spicules (Figs. 2e,f, 5b–d), S17b,c, S17b,c) | II. Convex/concave joint surface covered by hyaline cartilage bordering on fluid-filled joint space encased by joint capsule (Figs. 2d, 5a–d, S16b,c) | ||||
| III. Sharpey’s fibres peripheral to articular surface in periosteal bone | III. Joint capsule (Figs. 5a, S16b) | ||||
| 4 | Procoelous | I. Notochord foramen absent (Fig. S28a) | I. Notochord absent | Fibrous, bird-type | Dromaeosaurs, birds |
| II. Convex/concave surface with irregular, scattered chondrocytes in bone with fibre insertions (Figs. S26b–d, S27b,c, S28b,c) | II. Convex/concave joint surface covered by irregular, thin cartilage, connective tissue fibers inserting into bone of joint surface | ||||
| III. Sharpey’s fibres peripheral to articular surface in periosteal bone | III. Joint capsule |
Relationship between vertebral centrum shape, observed fossilised tissues (bony tissues, altered cartilaginous tissues, and altered soft tissues), inferred anatomical structures, tissue and cell types, inferred joint type, and systematic occurrence in this study. This table is complimentary to Fig. 1. Other pertinent figures are referenced as well. The lack of a figure reference does not mean that the feature is not present, it simply means that it is not illustrated. Char. state character state.