The Main Devonian Field outcrops on the northwestern East European Platform in Estonia, Latvia, Lithuania, northern Belarus and the Leningrad, Pskov, Novgorod and Vologda regions of Russia. This field represents extensive shallow-water deposits, from which a wide variety of Fish fossils have been recovered. Shark (Chondrichthyan) remains are rare however, with the most abundant Shark taxa recorded being Karksiodus mirus, which is known only from five fragmentary teeth from the Middle Devonian deposits at Aruküla Cave and Karksi, both in Estonia.
In a paper published in the Estonian Journal of Earth Sciences on 20 September 2014, Alexander Ivanov of the Faculty of Geology at St. Petersburg StateUniversity and the Institute of Geology and Petroleum Technology at KazanFederal University and Tiiu Märss of the Institute of Geology at the Tallinn Universityof Technology describe several new specimens of Karksiodus mirus from the Karksi locality and five new localities in the Leningrad Region of Russia.
The Karksi locality in South Estonia produced four of the previously described five specimens, and further investigation yielded three new specimens, as well as a number of Shark scales which may belong to the species (there is no way of determining this).
Chondrichthyan remains from the Burtnieki Regional Stage of the Karksioutcrop, Estonia. (A, B) Tooth of Karksiodus mirus, (A) occlusal and (B) oblique basal views.(C–E) Chondrichthyan scales; (C, D) first specimen in (C) lateral and (D) crown views; (E) second specimen in crown view. Scale bars equal 200 µm.Abbreviations: cc, central cusp; ic, intermediate cusp; lvc, large vascular canal opening; tbc, transverse basal vascular canal. Ivanov & Märss (2014).
The first Russian location is on the right bank of the River Lemovzha (a tributary of the Luga), 2.5 km upstream of the village of Khotnezha, where a cliff exposure of sandstones, siltstones, clays and marls 20 m high and 40 m long has produced a variety of Devonian Fish species. A single tooth was recovered from a fossiliferous sandstone layer, which has also produced Chondrichthyan scales assigned to the morphospecies Karksilepis parva. These scales have also been recorded at the Karski locality, making this the second time the two have been found together.
Karksilepis parva; scale in oblique crown view; Lemovzhalocality; Aruküla Regional Stage. Scale bar is 200 µm. Ivanov & Märss (2014).
Karksiodus mirus; Lemovzha locality; Aruküla Regional Stage; (C) lateral and (D) oblique lingual views. Abbreviations:str., striation on tooth; tbc, transverse basal vascular canal. Scale bars equal 200 µm. Ivanov & Märss (2014).
The second Russian location is in Siverskij village on the right bank of the River Oredezh, where an outcrop 10 m high and 60 m long and comprising sandstones with occasional conglomerates has produced a variety of vertebrate fossils, albeit mostly as isolated elements. Two teeth attributed to Karksiodus mirus were recovered from separate sandstone layers here.
Tooth of Karksiodus mirusfrom the Siverskij locality; Aruküla Regional Stage; (A) lingual and (B) labial views. Scale bars equal 200 µm. Ivanov & Märss (2014).
The third Russian location is at Zaitsevo Quarry, 1.2 km to the east of Stroganovo Railway Station in the Gatchina District, where layered sandstones, siltstones and clays have produced a variety of vertebrate fossils, mostly as isolated, and often fragmentary, elements. A single tooth was recovered from a sandstone layer here.
Karksiodus mirus from the Zaitsevo locality; Aruküla Regional Stage; (G) oblique basal and (H) occlusal views. Abbreviations:lvc, large vascular canal opening; tbc, transverse basal vascular canal.Scale bars equal 200 µm. Ivanov & Märss (2014).
The fourth Russian location is from an area of the Kemka River, 9 km upstream of where it meets the Luga, where a series of fossil producing outcrops are found at a point where the river passes through a canyon-like valley. A 7 m high and 40 m long outcrop of sandstones, siltstones, mudstones and clays yielded a single tooth of Karksiodusmirus from a carbonate cemented sandstone layer, which was obtained by dissolving the rock in dilute acetic acid.
Tooth of Karksiodus mirus from the Kemka locality; Burtnieki Regional Stage; oblique basal view.Abbreviations:lvc, large vascular canal opening; tbc, transverse basal vascular canal. Scale bar equals 200 µm. Ivanov & Märss (2014).
The final Russian location is at Novinka Quarry, about 1.1 km south of Novinka Village in the Gatchina District, where a variety of vertebrate remains have been recovered as isolated elements from sandstones, siltstones and clays. Teeth of Karksiodus mirus were recovered from a siltstone layer here.
Tooth of Karksiodus mirus from the Novinka locality; Burtnieki Regional Stage; occlusal view. Abbreviation:cc, central cusp.Scale bar equals 200 µm. Ivanov & Märss (2014).
The additional specimens provide new information on the structure of the teeth of Karksiodus mirus. These are variable in size, ranging from 0.5 to 2.3 mm, and having 2-4 cusps. Internally the teeth have a complex vascular system with four different types of vascular canals.
Karksiodus mirus, microstructure of tooth from Kemka locality; Burtnieki Regional Stage.(A)Longitudinal section from the base and the lateral cusp (details in B and C shown by frames); (B) detail of basemicrostructure; (C) detail of the microstructure on the cusp/base boundary; (D) detail of the microstructure of the cusp. Scale barsequal 100 µm. Abbreviations: avc, ascending vascular canal; dt, dentine tubules; hvc, (sub)horizontal vascular canal; lvc, large vascular canalopening; pc, pulp canal; tbc, transverse basal vascular canal; vco, vascular canal opening.Scale bar equals 200 µm. Ivanov & Märss (2014).
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