Scales and Dermal Skeletal Histology of an Early Bony Fish Psarolepis romeri and Their Bearing on the Evolution of Rhombic Scales and Hard Tissues

Abstract

Recent discoveries of early bony fishes from the Silurian and earliest Devonian of South China (e.g. Psarolepis, Achoania, Meemannia, Styloichthys and Guiyu) have been crucial in understanding the origin and early diversification of the osteichthyans (bony fishes and tetrapods). All these early fishes, except Guiyu, have their dermal skeletal surface punctured by relatively large pore openings. However, among these early fishes little is known about scale morphology and dermal skeletal histology. Here we report new data about the scales and dermal skeletal histology of Psarolepis romeri, a taxon with important implications for studying the phylogeny of early gnathostomes and early osteichthyans. Seven subtypes of rhombic scales with similar histological composition and surface sculpture are referred to Psarolepis romeri. They are generally thick and show a faint antero-dorsal process and a broad peg-and-socket structure. In contrast to previously reported rhombic scales of osteichthyans, these scales bear a neck between crown and base as in acanthodian scales. Histologically, the crown is composed of several generations of odontodes and an irregular canal system connecting cylindrical pore cavities. Younger odontodes are deposited on older ones both superpositionally and areally. The bony tissues forming the keel of the scale are shown to be lamellar bone with plywood-like structure, whereas the other parts of the base are composed of pseudo-lamellar bone with parallel collagen fibers. The unique tissue combination in the keel (i.e., extrinsic Sharpey's fibers orthogonal to the intrinsic orthogonal sets of collagen fibers) has rarely been reported in the keel of other rhombic scales. The new data provide insights into the early evolution of rhombic (ganoid and cosmoid) scales in osteichthyans, and add to our knowledge of hard tissues of early vertebrates.

Scales, Enamel, Cosmine, Ganoine, and Early Osteichthyans

“…The rhomboid scale possesses a free exposed field, which is covered by ganoine, enamel or exposes bone, whereas the anterior and dorsal areas surrounding the free area, are covered by surrounding scales and formed by bone; they slope towards the margin. The 'anterior ledge' (Qu et al, 2013) is the anterior overlapped area of a rhomboid scale. The overlapped area extends from the anterior margin of the scale (Qu et al, 2013, fig.…”

“…The scales of Psarolepis overlap like other osteichthyan scales; their margins are steeper and shorter, but not as narrow as those of Cheirolepis. Figure 3F (Qu et al, 2013) shows overlap and overlapping areas like in osteichthyan scales, therefore the normal terminology is applicable.…”

“…Cosmine is a synapomorphy of sarcopterygians (Brazeau and Friedman, 2014, p. 47-48). In contrast, Qu et al (2013) doubted the distinction between pore-canal system in sarcopterygians, the characteristic feature of cosmine, and dentinal canal systems in actinopterygians.…”

“…Friedman and Brazeau (2010) offer no phylogenetic analysis, and overlooked such an analysis published by Schultze and Märss (2004). Qu et al (2013) introduced new terms for the morphology of rhomboid scales without consideration of the relations of the scales to the neighboring ones, as it occurs in rhomboid scales of other osteichthyans. The scales of Psarolepis overlap like other osteichthyan scales; their margins are steeper and shorter, but not as narrow as those of Cheirolepis.…”

“…I would like to show that ganoine and cosmine are useful terms, and that the canal system in sarcopterygian scales is different from that in actinopterygian scales. In contrast, Friedman and Brazeau (2010) argued that the term ganoine is obsolete because one cannot distinguish enamel and ganoine in early osteichthyans, and Qu et al (2013) argued that the pore-canal system, the identifier of cosmine, is not a useful term. I will first present the different scale types in osteichthyans and definitions of histological terms, followed by the systematic distribution of the two kinds of enamel and the canal system to explain the problem in basal osteichthyans, and finally place basal osteichthyans after their scale characters.…”