GNATHOSTOMATA Gegenbaur, 1874

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 : 561-564

Superclass GNATHOSTOMATA Gegenbaur, 1874

†Class PLACODERMI M’Coy, 1848 Order ACANTHOTHORACI Stensiö, 1944 Family PALAEACANTHASPIDIDAE Stensiö, 1944

Palaeacanthaspididae gen. et sp. indet. ( Fig. 8 )

MATERIAL. — Specimens GIT 580-22 to 580-25: isolated skeletal elements and fragments of bony plates with “star-shaped” tubercles.

LOCALITY AND STRATIGRAPHIC HORIZON. — Locality (sample) 73, Tonnel’nyj Brook, south of De Long Strait , Chukotka ; Lower Member (1) of Enmakaj Formation, Lochkovian .

DESCRIPTION

Four microremains from the Enmakaj Formation belong to placoderms. ħe best-preserved specimen ( Fig. 8A ; GIT 580-22 ) is an isolated spindle-like skeletal element, twice longer than wide. It has a slightly oval central tubercle, placed asymmetrically, and around it smaller asymmetrical tubercles, forming three rows, except at one of the sides. At the opposite side the tubercles are partly laterally compressed and lamellar. Tubercles have narrow smooth ridges, which are most numerous on the central tubercle (up to 13). ħe ridges end at the sharp tips of tubercles. ħe second remain ( Fig. 8B ; GIT 580-25 ) is rather fragmentary and shows spongy bone with four broken tubercles on it. Ornament of the tubercles is rougher than that of the previous specimen. ħe ridges are in cross section less sharp in comparison with those of the specimen GIT 580-22 . One of the ridges bifurcates at its proximal end. Apices of the tubercles were differently directed. ħe specimen could be a fragment of a larger spindle-like element .

Two remaining specimens of poor preservation show ornament of different type. ħe specimen GIT 580-23 ( Fig. 8C ) is probably a fragment of an exoskeletal plate with a slightly concave margin (?). It is covered with small stellate flat round or elongated tubercles. Number of short ridges varies from 7 to 10. Most of the tubercles are clearly separated from one another.ħe specimen GIT 580-24 ( Fig. 8D ) is a fragment, in which the ornament resembles somewhat that of the specimen 580-23 but is rougher. Stellate tubercles are closely backed. Short ridges end with minute rounded swellings. ħe specimens may belong to different forms.

COMPARISON AND STRATIGRAPHICAL DISTRIBUTION

Identification of the Lower Devonian placoderm microremains is complicated as in many cases the ornament of the carapace plates is not figured in details. However, there are exceptions, concerning acanthothoracids (also called as palaeacanthaspids or radotinids according to their earliest known representatives Palaeacanthaspis Brotzen, 1934 and Radotina Gross, 1950 ). Ørvig’s (1975) paper on the acanthothoracid Romundina Ørvig, 1975 from Arctic Canada, Prince of Wales Island, shows ornament at high magnification. Characteristics of Romundina tubercles are ridges, bearing rows of tiny nodules. Still, not all ridges have necessarily the nodules. Long & Young (1988) have figured similar ornament in the Emsian acanthothoracid Murrindalaspis Long, 1984 (New South Wales, Australia), carrying even finer nodules along ridges. In one of the scales (Long & Young 1988: fig. 9B) the ends of ridges are truncated. ħe Romundina type of ornament is recognized in several acanthothoracid tubercles from the Lochkovian-Pragian of southeastern Australia ( Basden et al. 2000). Some acanthothoracids possess the radotinid type of ornament, consisting of conical stellate tubercles with 4-12 ridges. ħe latter ones bear rows of small round nodules. ħis kind of ornament occurs in a Pragian radotinid from the Armorican Massif, France ( Goujet 1976).

Tubercles similar to those of the above radotinid, i.e. acanthothoracids, can be seen in some Lochkovian buchanosteids from Australia ( Basden et al. 2000: fig. 3). On the other hand, these early buchanosteids resemble in their ornament that of the scales of the Emsian buchanosteid Uralosteus Mark-Kurik & Young, 2003 (Mark-Kurik & Young 2003). Buchanosteids, namely Buchanosteus sp. , are described from the Lochkovian of Guangxi, South China (Wang et al. 1998). However, buchanosteid arthrodires are much more common in the Emsian of many other regions (Mark-Kurik 2004: table 1).

According to our interpretation these few placoderm remains from the Chukotka Enmakaj assemblage belong probably all to acanthothoracids. Acanthothoracids are particularly characteristic of the Lochkovian. ħey are reported from Australia, North America and numerous regions of Eurasia, including the present day Arctic. Goujet (1998) mentioned that on the Prince of Wales Island, the Canadian Arctic, at least three different forms of these placoderms, one of them being Romundina , occur in the Lochkovian. In this region acanthothoracids can also be met together with actinolepid arthrodires.

In result of the reassessment of the paper by Mark-Kurik (1974) it can be said that two different acanthothoracids (one of them probably Romundina ) come from the Pshenitsyn Formation of Kotelnyj Island, New Siberian Archipelago, Russia. Four trunk armour plates of latter material, i.e. MD, left ADL, right complex plate (AL + Sp + AVL) (Mark-Kurik 1974: fig. 1), and another complex plate (Mark-Kurik 1974: pl. II, fig.1) belong to a smaller acanthothoracid. ħe figures 1 to 7 of the same paper (Mark-Kurik 1974) show ornament of a larger acanthothoracid. ħe Pshenitsyn Formation is dated by Cherkesova (1988) as Lochkovian. ħe left ADL plate (Mark-Kurik 1974: figs 1-9, pl. II: 7) was erroneously identified as the equivalent plate of an arctolepid(?) arthrodire. ħis misinterpreation was repeated in the figure 5.3 of the paper of Blieck & Janvier (1993).

In the Lower Devonian of the Tajmyr Peninsula, westwards of the New Siberian Islands, acanthothoracids occur on four levels of the Lochkovian Ust’-Tareya Regional Stage (Mark-Kurik 1994). ħree levels are in the Uryum Beds, the fourth one belonging to the upper part of the Tolbat Beds (Mark-Kurik 1994: fig. 48). Acanthothoracid skull roof and trunk armour plates fromTajmyr were compared with those found in the northern part of the Siberian Platform (Norilsk area, Kureika and Koldy River outcrops) and the Timan-Pechora province (Vozej and Lekejyaga drill cores), NE of European Russia (Mark-Kurik 1994: figs 49, 50). According to Goujet (pers. comm. to EMK 1999) some of these acanthothoracids are evidently not Romundina species (e.g., those in Mark-Kurik 1994: fig. 49A from the Tareya River, and fig.49B from the Koldy River). Goujet considered them to be similar to a new acanthothoracid from the Jauf Formation of Saudi Arabia. ħis acanthothoracid is now published under the name Arabosteus variabilis (Olive et al. 2011), and is dated as Pragian-early Emsian. In the Timan-Pechora province Goujet (1997) reported the presence of two forms, resembling the Saudi Arabia acanthothoracid, occurring together with a third one, practically undistinguishable from Romundina . In addition to acanthothoracids the actinolepid arthrodires have been found in the Lochkovian and Pragian of above province. Tsyganko et al. (2000) mentioned also the occurrences of radotinids in the Lochkovian Ovinparma Regional Stage of the same province. It can be concluded that acanthothoracids are more common in the Lochkovian than in the Pragian and Emsian.

Clade TELEOSTOMI Bonaparte, 1837 †Class ACANTHODII Owen, 1846 Order ISCHNACANTHIFORMES Berg, 1940 ? Family ISCHNACANTHIDAE Woodward, 1891 ? Genus Garralepis Burrow, 2002

Figures

FIG. 8. — Acanthothoraci: Palaeacanthaspididae gen. et sp. indet.: A, specimen GIT-580-22; B, specimen GIT 580-25; C, specimen 580-23; D, specimen 580-24. All specimens from locality (sample) 73, Tonnel’nyj Brook, Lower Member (1) of Enmakaj Formation, Lochkovian. Scale bars: A, C, 500 µm; B, 400 µm; D, 200 µm.