TABLE 1. Overview of the major anatomical conclusions drawn in this manuscript, and the evidence (direct, comparative, functional) used tosupport them.
| Feature | Direct Evidence | Comparative Anatomy | Functional Morphology/Biomechanics |
|---|---|---|---|
| Body Length of 3-4 m | Scaling from OOL, posterior ventrolateral plate, etc. | Scaling from other arthrodires, locations of pectoral and pelvic girdles and size of ventral shield consistent relative to total length in arthrodires | Small gill chamber compared to body size, pectoral fin position as % of total length limited before animal becomes unstable swimmer |
| Deep trunk | Head is distinctly dorsoventrally expanded compared to other arthrodires, trunk armor much deeper than long | Head and armor of Dunkleosteus is distinctly taller compared to other arthrodires, primarily differ in height rather than overall proportions | – |
| Trunk becomes deeper throughout ontogeny | Trunk armor becomes comparatively deeper in ontogenetic series of Dunkleosteus | Other placoderms show similar pattern across ontogeny | – |
| Spine located at a relatively high position on the trunk | Dorsoventrally high cranio-thoracic joint, ridge for horizontal septum on posterior dorsolateral | Foramen magnum and cranio-thoracic joints located at same level in arthrodires | – |
| Anteriorly stiff spine | Partially fused anterior axial skeleton, ventral shield does not allow lateral flexure, head and trunk armor cannot separately move laterally | – | – |
| Absence of scales | Possible scaleless skin in CMNH 8735 | Scales consistently absent in eubrachythoracid fossils, even when preserved in other fishes | – |
| Incomplete lateral line | Main trunk canal does not extend to end of trunk armor, unlike non-eubrachythoracid arthrodires and other placoderms | Other eubrachythoracid arthrodires show a similar condition | Incomplete lateral lines common, convergent trait in living fishes |
| Sclerotic rings at or near surface of eye | – | Sclerotic rings covered in dermal ornamentation in most arthrodires | – |
| Location of dorsal fin | Extent of armor and carinal process mean dorsal fin cannot be dorsal to center of mass | In other arthrodires dorsal fin is invariably posterior to carinal process/submedian dorsal plate | Dorsal fin located dorsal to center of mass in most nektonic vertebrates |
| Comparatively anterior position of pectoral fin | Pectoral fin base preserved in situ in CMNH 8982 and CMC VP8294 | Pectoral fenestra extends more anteriorly on dermal skeleton than other eubrachythoracids | Pectoral fin origin is unusually anterior compared to other fishes at any estimated length |
| Pectoral fin base shows positive allometry relative to body length | Pectoral fenestra becomes larger relative to head and ventral armor throughout ontogeny | Pachyosteomorphs show larger pectoral fins than coccosteomorphs or sharks, thus large pectoral fin in adult Dunkleosteus is not unusual | Pectoral fin shows positive allometry when compared to sharks |
| Pelvic located at posterior end of ventral armor | Pelvic radials at posterior end of ventral armor in CMC VP8294 | Pelvic girdle is invariably located at posterior end of ventral armor in arthrodires | – |
| Small pelvic fins | Pelvic girdle preserved in association with several specimens | Pelvic girdle is much smaller than in other arthrodires | – |
| Caudal peduncle thickness | – | Arthrodires, even Amazichthys, seem to have thicker peduncles than modern fishes | Pelagic fishes tend to have narrow caudal peduncles |
| Larger ventral lobe of caudal fin compared to sharks | High position of spine requires larger caudal lobe for fin to function | Amazichthys has a comparatively larger ventral lobe than sharks | Larger ventral lobe is needed to keep caudal margin out of wake from deep body |
| Lateral caudal keels? | – | Repeated, independent evolution in oceanic vertebrates, including arthrodires (Amazichthys) | Lateral caudal keels present in most pelagic fishes |
| Thunniform body shape | Deep, rounded trunk relative to length, small pelvic fins, well-developed pectoral fins, stiffened anterior spine with well-developed markers for horizontal septum. | – | Dunkleosteus is common in open water habitats with anoxic bottoms uninhabitable by non-nektonic organisms |
TABLE 2. Definitions for life habits used in this study, along with example taxa. Note “pelagic” is preferred over “oceanic” because some lacustrine fishes inhabit open water habitats and behave similarly to oceanic pelagic fishes. Syn. = synonym.
| Term | Definition | Examples |
| Benthic | Spend large amounts of time on the bottom of bodies of water and are often poor or infrequent swimmers | carpet sharks (Orectolobiformes), gobies (Gobiiformes), eels (Anguilliformes), many catfishes (Siluriformes) |
| Demersal | Capable of swimming but typically live close to the sea floor. Often live in and around complex environments like underwater vegetation, coral reefs, or rock crevices. Syn.: epibenthic | groupers (Serranidae), pike (Esocidae), some requiem sharks (e.g., Triaenodon), dogfish and allies (Squaliformes), hexanchiforms (Hexanchiiformes) |
| Neritic | Actively nektonic organisms, but typically live relatively close to the substrate such as in coastal waters or continental shelf. Often inhabit the water column immediately above the substrate. Syn.: “coastal pelagic”, littoral, “near shore” | many salmon/trout (Salmonidae), some requiem sharks (e.g., Carcharhinus), many carangids (Carangidae) |
| Pelagic | Inhabits open waters far from any substrate. Typically specialized for prolonged or fast swimming. Often oceanic but may inhabit large lakes or rivers. Syn.: oceanic, oceanodromus | tunas/many mackerels (Scombridae), billfishes (Istiophoriformes), lamnid sharks (Lamnidae), emerald shiners (Notropis atherinoides) |
TABLE 3. Arthrodire taxa in which the pelvic girdle is either immediately posterior to or otherwise associated with the posterior end of the ventral shield.
| Taxon | Clade | Specimens | References |
| Australophyllolepis youngi | Phyllolepida | NMP 160732, NMP 160746 | Long (1983) |
| Cowralepis mclachlani | Phyllolepida | AMF 103753 | Ritchie (2005); Carr et al. (2009) |
| Sigaspis lepidophora | Actinolepidae | MNHN-SVD 210 | Goujet (1973) |
| Actinolepis spinosa | Actinolepidae | — | Mark-Kurik (1985) |
| Groenlandaspidae indet. | Non-Eubrachythoracid Phlyctaeniian | — | J. Long pers. comm. 2023 |
| Holonema westolli | Non-Eubrachythoracid Phlyctaeniian | WAM 96.6.11 | Trinajstic (1999); Trinajstic et al. (2015) |
| Coccosteus cuspidatus | Coccosteid Coccosteomorph | FMNH PF 1673, NHMUK PV OR43617, NHMUK PV P.59854, NHMUK PV NHMUK PV P.61585, P.61589, NHMUK PV P.61758, NMS 1900.12.12, NMS 1901.106.1, NMS 2017.41.4, ROM VP 52664 | — |
| Millerosteus minor | Coccosteid Coccosteomorph | FMNH PF 1089, LDUCZ-V998, NHMUK PV P. 16795, NHMUK PV P.46772, NHMUK PV P.47434, NHMUK PV P.47385, NHMUK PV P.75451, NMS G.1859.33.994 | Desmond (1974); Trinajstic et al. (2015); van Mesdag (2018) |
| Dickosteus threiplandi | Coccosteid Coccosteomorph | NHMUK PV OR 49663 | — |
| Watsonosteus fletti | Coccosteid Coccosteomorph | NMS G.1995.4.2 | Other undescribed specimens, M. Newman pers. comm. September 2022 |
| Plourdosteus canadensis | Plourdosteid Coccosteomorph | MNHM 02-177c, MNHM 02-901 | — |
| Compagopiscis croucheri | Incisoscutoid Coccosteomorph | NHMUK PV P.50947 | Trinajstic et al. (2015) |
| Incisoscutum ritchei | Incisoscutoid Coccosteomorph | NHMUK PV P.50934, WAM 03.3.28 | Dennis and Miles (1981: fig. 21) |
| Dunkleosteus terrelli | Dunkleosteoid | CMC VP8294 | Present Study |
| Heintzichthys gouldii | Aspinothoracidan | AMNH FF 2826 | Dean (1896) |
| Amazichthys trinajsticae | Aspinothoracidan | AA.MEM.DS.8 | Jobbins et al. (2022) |
TABLE 4. Pre-pectoral length as a percentage of total length in several nektonic fish clades. Data represent clade average of species averages to allow equal representation of each species. Ranges represent ranges of species average for all except Dunkleosteus terrelli, which is represented in terms of individual specimens to highlight the lack of individual variation in this proportion. Data in Appendix 4, for additional information and analyses see Appendix 3: section 4.4.
| Taxon | Clade | # Species | #Specimens | % Prepectoral Length |
| Dunkleosteus | Arthrodira | 1 | 5 | 13.1 (11.2–14.6)* |
| Eastmanosteus | Arthrodira | 1 | 1 | 13.3 |
| Amazichthys | Arthrodira | 1 | 1 | 23.1 |
| Coccosteidae | Arthrodira | 3 | 7 | 20.8 (18.6–23.7) |
| Incisoscutum | Arthrodira | 1 | 2 | 21.1 |
| Ichthyodectiformes | Osteichthyes | 7 | 7 | 13.7 (12.1–15.8) |
| Megalopidae | Osteichthyes | 2 | 6 | 18.5 (17.6–19.4) |
| Cheirocentridae | Osteichthyes | 2 | 8 | 14.5 (14.3–14.8) |
| Other Clupeiformes | Osteichthyes | 17 | 51 | 18.6 (15.8–21.8) |
| Salmonidae | Osteichthyes | 12 | 35 | 18.8 (15.3–22.3) |
| Lampridae | Osteichthyes | 4 | 4 | 28.3 (27.3–28.8) |
| Carangiformes | Osteichthyes | 60 | 128 | 21.2 (14.6–27.6) |
| Istiophoriformes | Osteichthyes | 10 | 147 | 21.1 (18.7–22.7) |
| Scombridae | Osteichthyes | 33 | 106 | 23.0 (15.1–29.4) |
| Sphyraenidae | Osteichthyes | 13 | 40 | 25.4 (23.3–28.2) |
| Carcharhinidae | Chondrichthyes | 41 | 234 | 22.1 (18.1–24.6) |
| Lamnidae | Chondrichthyes | 4 | 31 | 26.2 (24.8–27.8) |
| Squalidae | Chondrichthyes | 15 | 37 | 21.8 (18.0–24.8) |
