Millerosteus, Tristychius, Ferromirum and Dwykaselachus

Millerosteus minor(Stensiö 1959, Middle Devonian, 8.5cm long) was twice the size of Bianchengichthys and half the length of Coccosteus with a relatively larger orbit. Pelvic fins and a tail were preserved. This tiny arthrodire placoderm was close to Psarolepis, and a clade of traditional sharks, including Maghriboselache and Ferromirum.

Ferromirum oukherbouchi (Frey et al. 2020; Late Devonian; PIMUZ A/I 4806; est. 33cm overall) is closest toDwykaselachus in the LRT. Preserved with impressions of soft tissue this is a rare 3D specimen. The large size of the palatoquadrate (= lacrimal) erased the temporal fenestrae. Note the hyomandibular is a distinct 'bone'. Teeth were individually sharp, but coiled to lay each point alongside each side. As the jaws opened the mandibles rotated outwardly to expose more teeth.

Dwykaselachus oosthuizeni (Oelofsen 1986, Coates et al. 2018; Middle Permian) is a larger, later version of Ferromirum.

Tristychius arcuatus (Agassiz 1837; Dick 1978; Coates et al. 2019; Early Carboniferous; 60cm est.) was considered a Hybodus relative, but here nests with Maghriboselache and Devonian ANU V244. The nares point anteriorly. Teeth are nearly absent with only a few in the anterior dentary and premaxilla homolog. Note the large labial cartilages (red) that restrict jaw depression and create lateral walls for the open jaws to create suction.

Agassiz L 1837. Recherches Sur Les Poissons Fossiles. Tome III (livr. 8-9). Imprimérie de Petitpierre, Neuchatel viii-72.

Coates MI, Tietjen K, Olsen AM and Finarelli JA 2019. High-performance suction feeding in an arly elasmobranch. Sci. Adv. 2019; 5 : eaax2742

Coates MI et al. (4 co-authors) 2018. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature 541:208–211.

Dick JRF 1978. On the carboniferous shark Tristychius arcuatus Agassiz from Scotland.
Earth and Environmental Sciences Transactions of the Royal Society, Edinburgh. 70: 63–108.

Frey L et al. (4 co-authors) 2020. A symmoriiform from the Late Devonian of Morocco demonstrates a derived jaw function in ancient chondrichthyans. Nature Communications Biology 3:681 https://doi.org/10.1038/s42003-020-01394-2 www.nature.com/commsbio

Oelofsen BW 1986. A fossil shark neurocranium from the Permo-Carboniferous (lowermost Ecca Formation) of South Africa. Indo-Pacific fish biology: Proceedings of the Second International Conference on Indo-Pacific Fishes: 107–124.

Stensiö EA 1959. On the pectoral fin and shoulder girdle of the arthrodires. K. svenska VetenskAkad. Handl. (9):2.

Zangerl R 1984. On the microscopic anatomy and possible function of the spine-"brush" complex of Stethacanthus (Elasmobranchii: Symmoriida). Journal of Vertebrate Paleontology. 4 (3): 372–378.

wiki/Ferromirum – not yet posted
wiki/Dwykaselachus
wiki/Symmoriiformes
wiki/Millerosteus
wiki/Tristychius