New histological information on Holoptychius Agassiz, 1839 (Sarcopterygii, Porolepiformes) provides insights into the palaeoecological implications and evolution of the basal plate of the scales of osteichthyans

The porolepiform Holoptychius Agassiz, 1839 from the Upper Devonian is one of the most widely found vertebrates in the Palaeozoic fossil record. Holoptychius is considered to display a more ubiquitous mode of life and a greater potential of dispersion than any other sarcopterygian taxon, consistent with its cosmopolitan distribution and profuse fossil record, mainly represented by scales. Previous studies on the squamation of Holoptychius have discussed the variable ornamentation of the superficial layer; however, the basal plate has been less explored. Here we furnish new histological data on the scales of Holoptychius, focusing on the mineralisation rate and organisation of the lamellar bone. Ground sections show that the basal plate is made of a stacking of thick collagenous layers with a plywood-like structure. Collagenous fibres are parallel within layers, with two successive layers orthogonally arranged and with every second layer rotated by an angle of 36º. This condition, known as a double-twisted plywood-like organisation, is similar to that of other sarcopterygians like Eusthenopteron and extant coelacanths and dipnoans. The new palaeohistological data provide insights into the morphofunctional, palaeoecological and phylogenetical implications of the microstructural characteristics of the scales, adding to our knowledge of the histological diversity of osteichthyans.