Porolepis, Woodward, 1891

Mark-Kurik, Elga, Blieck, Alain, Burrow, Carole J. & Turner, Susan, 2013, Early Devonian fishes from coastal De Long Strait, central Chukotka, Arctic Russia, Geodiversitas 35 (3), pp. 545-578 : 567

Porolepis ? sp.

( Fig. 10 )

MATERIAL. — Specimen GIT 580-27: fragment of bone or scale.

LOCALITY AND STRATIGRAPHIC HORIZON. — Locality (sample) 73, Tonnel’nyj Brook, south of De Long Strait , Chukotka ; Lower Member (1) of Enmakaj Formation, Lochkovian. Cherkesova (1973: table) indicated an occurrence of Porolepis from the interval of the Member IV to the lower part of the Member VIII of the Enmakaj Formation. So, the fish was discovered not in the basal part of the formation, but in a somewhat higher level .

DESCRIPTION ħe Enmakaj assemblage contains a single small fragment of a porolepiform sarcopterygian (GIT 580-27; Fig.10 ). It has a smooth cosmine covered surface penetrated by pore-canals. In cross section the upper cosmine layer consists of the fused goblet-shaped odontodes, some of them showing segments of narrow pulp canals. Flask-shaped cavities separate the odontodes and end higher up with pores ( Fig. 10A ). Below cosmine is a rather compact layer of spongiosa ( Fig. 10B ). ħe fragment can be provisionally identified as belonging to a species of Porolepis .

COMMENTS ON STRATIGRAPHICAL DISTRIBUTION AND TAXONOMY ħe porolepidids are known from the Lower and Middle Devonian of many regions: Rhineland, Baltic area, Spitsbergen, Urals,?Western USA, Canadian and Russian Arctic (including Novaya Zemlya),? Vietnam (Ørvig 1957, 1969b; Mark- Kurik & Novitskaya 1977; Blieck & Janvier 1993; Vorobyeva 2004). In our case occurrences from the Lower Devonian are of particular interest. Earlier it was considered that porolepidids appeared in the Pragian (Siegenian) (Vorobyeva & Obruchev 1964). But re-assessment of age of several local stratigraphical units evidence that they were rather common already in the Lochkovian. Lochkovian stratigraphical units with Porolepis are: the Kureika Formation of the Siberian Platform ( Cherkesova et al. 1994; Matukhin 1995), the Bely Kamen and Uryum Beds of the Ust-Tareya Regional Stage of Taimyr Peninsula (Cherkesova 1994), and the Pshenitsyn River Formation of Kotelnyj Island, New Siberian Archipelago ( Cherkesova 1975, 1988). Mark-Kurik (1974) mentioned the similarity of the Pshenitsyn River Formation fish assemblage to that of the Kureika Formation. Porolepis in the Enmakaj Formation of Chukotka came according to a later age dating also from the Lochkovian (Cherkesova, pers. comm. to EMK 1976) [see here the section “age and correlation”]. Porolepis is abundant in the Enmakaj Formation ( Cherkesova 1973: 279). Two Siberian species have been formally defined: Porolepis taimyrica Vorobyeva, 1963 and P.kureikensis Vorobyeva, 1963 . Ørvig (1969b) paid attention to contradictions in identification of porolepiform scales from different Arctic regions, including Porolepis species mentioned above. Porolepiforms were probably the top predators in the fish assemblage of the Enmakaj Formation. Other predators were represented by several acanthodians.

Figures

FIG. 10. — Porolepis? sp. from the Lower Devonian of Chukotka, Arctic far-eastern Russia, specimen GIT 580-27: A, lateral view; B, dorsolateral view.Locality (sample) 73,Tonnel’nyj Brook,Lower Member (1) of Enmakaj Formation,Lochkovian.Scale bars:200 µm.