Karksilepis parva gen . et sp . nov . ( Chondrichthyes ) from the Burtnieki Regional Stage , Middle Devonian of Estonia

Dermal scales of a new chondrichthyan were discovered in four levels in sandstones of the Härma Beds (lower part of the Burtnieki Regional Stage, Middle Devonian) in the Karksi outcrop of South Estonia. The classification of Karksilepis parva gen. et sp. nov. Märss is still uncertain.


INTRODUCTION
Chondrichthyan scales are rare in Lower-Middle Palaeozoic rocks.Chondrichthyan-like scales, however, were described by Sansom et al. (1996) from the Ordovician of Colorado, USA, and by Young (1997) from central Australia.The earliest Elasmobranchii gen.nov.has been listed from the Early Llandovery (Early Silurian) of the Siberian Platform, Russia (Karatajūtė-Talimaa & Predtechenskyj 1995).Niualepis and Elegestolepis conica have been identified in the Middle Llandovery of South Yakutia and the Bratsk Region, Siberian Platform (Novitskaya & Karatajūtė-Talimaa 1986; Karatajūtė-(Lower Devonian) strata of Eastern Mongolia (Wrona & Nyamsuren 1998) and in the Lower Emsian (Lower Devonian) of Gornyi Altai of Western Siberia, showing the wide stratigraphical range of the whole group (Rodina 2002).
In the course of lithological studies of Devonian sediments in Estonia, based upon outcrops and drill core material, a rich collection of vertebrate microfossils has been obtained.Notably abundant material has been collected from the Karksi outcrop in the Halliste old valley, near Karksi castle (Viljandi county) (Fig. 1).The sandstones here belong to the lower part of the Härma Beds of the Burtnieki RS and are famous for fish fossils.Vertebrates described in earlier studies include psammosteids (Obruchev & Mark-Kurik 1965;Mark-Kurik 1968, 1995), placoderms (Karatajūtė-Talimaa 1963), acanthodians (Valiukevičius 1998), and sarcopterygians (Vorobyeva 1977).Chondrichthyans have not been mentioned or described so far from these beds.In this paper we describe scales of a new chondrichthyan from the Karksi outcrop, Estonia.The authorship of the new taxon Karksilepis parva is attributed herein to Märss.

MATERIAL AND METHODS
Six samples (200-400 g) were collected from four levels of the Karksi outcrop (Fig. 2) and examined.All samples, those previously collected by A. Kleesment in 1990 and1997 and the ones from two new levels (samples 02-1 and 02-2) contained abundant vertebrate microremains.Scales and tesserae or fragments of plates of heterostracans, placoderms, acanthodians, chondrichthyans, and osteichthyans (sarcopterygians and actinopterygians) were found (see Table 1).Psammosteids predominated, especially plate fragments and scales of Tartuosteus maximus, Pycnosteus tuberculatus, and Ganosteus stellatus.The diversity of acanthodians was high (16 taxa), and Nostolepis gaujensis scales were found for the first time in the Burtnieki RS (they were absent only in sample 02-3).Chondrichthyans were fairly rare but present in all six samples.In addition to the scales of the new taxon described herein, the samples contained teeth of an elasmobranch.As there was no indication that these teeth might belong to the taxon described below (except their co-occurrence in the samples from the same outcrop), they will be described elsewhere.Scales, plate fragments, and teeth of Sarcopterygii were found as well.Actinopterygians were represented by rare scales of Cheirolepis sp.(Kleesment et al. 2003;Märss et al. 2003).
The sandstones in the Karksi outcrop were very weakly cemented and did not need acid preparation.Samples were carefully washed to remove muddy (< 0.01 mm) particles.Then the material was fractioned and sieved; the size ranges were 0.1-0.25 mm, 0.25-0.5 mm, 0.5-1 mm, 1-2 mm, and > 2 mm.The two finest fractions were separated to heavy and light parts using bromoform (2.89 g/cm 3 ).The two heavy fractions obtained, and the two coarsest fractions were investigated using the binocular optical microscope.All microremains with well-preserved sculpture and shape were picked out.The microstructure of chondrichthyan scales was examined in thin sections or by submerging elements into immersion liquids.A selection of scales was photographed using the scanning electron microscope JEOL-JSM-940-A.The described collection is housed in the Institute of Geology at Tallinn University of Technology under the collection numbers GIT 383 + specimen number.

GEOLOGICAL BACKGROUND
The Middle Devonian Burtnieki RS is distributed in southeastern Estonia (Fig. 1).The total thickness of the stage is 60-95 m.The sequence is represented by light (white, yellowish, pinkish, and greyish-brown) finegrained weakly cemented cross-bedded sandstones with siltstone and clay interlayers.The stage is divided into three successive cyclic sandstone complexes, the Härma, Koorküla, and Abava beds (Kleesment 1995;Kleesment & Mark-Kurik 1997).The lower unit, the Härma Beds, is 13-28 m thick.The beds cropping out in Karksi belong to the lower part of the Härma Beds.The cross-bedded sandstones of the Härma Beds with the stratotype section in Härma settlement at the Õhne River are usually well sorted and fine-grained, where the size fraction 0.1-0.25 mm clearly dominates (75-93%; Kleesment 1995).
The sandstones in the Karksi outcrop and coeval sediments at the Ahja River and in the Tõrva area have the same composition as the Härma Beds.Sandstones of the Härma Beds possibly accumulated in regressive conditions of the Devonian basin in a fluvially dominated subaqueous delta plain environment, which was under influences of low-amplitude eustatic and tectonic movements, repeatedly interrupted by short subaerial periods (Kleesment 1997;Plink-Björklund & Björklund 1999).
The part of the Karksi outcrop that yielded fish microremains is a small sandstone scarp 3.5 m high and 6 m long.About 20 m upstream, in the upper part of the slope there is another, smaller fish-bearing locality.The levels with abundant fish microremains are represented by moderately sorted fine-to medium-grained sandstones which contain both particle types in equal amounts and up to 10% coarse and very coarse detrital material.In the coarse-grained (diameter 0.5-2 mm) detrital material, heterostracan, placoderm, and sarcopterygian plate frag-ments are fairly worn.Acanthodian and chondrichthyan scales are concentrated in beds with the grain size of 0.2-0.5 mm and are not worn, which gives evidence of more rapid accumulation of sediment.Probably the reworking and weathering of fish bones and plates took place in shore face environments, where under lowamplitude eustatic movements, sea level fluctuations caused repeated alternation of marine and non-marine conditions and intense redeposition and reworking of the detrital component.Interlayers with rich, wellpreserved fish microremains possibly deposited in the nearshore realm during short periods of intense influx of detrital material.Such rapid burial in nearshore deltaic conditions resulted in the accumulation of well-preserved fish microremains.In these layers, besides moderate sorting of detrital material among fine-grained particles, subrounded to subangular quartz grains dominate (85-90%); in the coarse-grained fraction, rounded grains make up only 25-30%, referring to intense influx and rapid deposition.Such high concentrations of fish remains have been reported from nearshore marine (Davies et al. 2007), deltaic (Mancuso 2003), and lake deposits (Smith & Swart 2002).

Class Chondrichthyes
Order et Family incertae sedis Derivation of name.Latin parvus, meaning small, an allusion to the size of the scales.
Occurrence.In four levels in Karksi outcrop (see Table 1 and Fig. 2).
Diagnosis.Scales very small to medium-sized, quadrangular (rectangular to rhomboidal), 0.25-0.8mm long and 0.25-0.7 mm wide.Three to eighteen elongate, partly overlapping longitudinal odontodes on the scale surface; odontode width 0.05-0.1 mm and length 0.15-0.4mm.Lower face of basal plate convex.Very fine dentine tubules in odontodes branch from wider dentine canals, and the latter in turn from pulp canal; vascular canals form network beneath odontodes in the basal plate; openings of vascular canals situated around odontodes.Flattened bone cell lacunae form lamellar structure of the basal plate.Sharpey fibres very fine.

Description
Morphology.The scales are small to medium-sized, quadrangular, with the shape varying from high rectangle to relatively low rhomboid (Fig. 3A, B, E-K, N, P, Q).The length of the scales is 0.25-0.8mm and width 0.25-0.7 mm.The height of the basal plate is about half the height of the scale (Fig. 3L).The scales are covered with odontodes forming polyodontodia.The number of odontodes varies from three (Fig. 3Q) to eighteen (Fig. 3J); their width is 0.05-0.1 mm and length 0.15-0.4mm.The odontodes are attached to the basal plate of the scale.The rectangular high scales carry odontodes in one to two rows (Fig. 3A, B, E, G); the odontodes on rhomboidal scales are less regularly arranged in rows but still up to four rows can be counted (Fig. 3H, J, N), whereas more anterior odontodes partly cover the posterior ones.There is always lateral separation between adjacent odontodes (e.g.Fig. 3A, E, K).The odontodes can be directed horizontally (Fig. 3M) or rise to the posterior (Fig. 3K; see also Fig. 4).All odontodes are covered with fine ultrasculpture of longitudinal striation which starts at the conjunction of the odontode and basal plate and is directed upwards and posteriorwards, towards the peak of the odontode (Fig. 3A-E, G, I, K, M).The basal plate is larger than the area with odontodes.In some scales the basal plate strongly protrudes anteriorly (Fig. 3A).Relatively large deep pores open in the basal plate (Fig. 3G, K-M, P, Q) and make the surface of the plate uneven.The shape of the basal plate depends on the age of the scale.Vascular network canals open anteriorly of the odontodes in high rectangular scales (Fig. 3B, E, G) or around the odontodes in rhomboidal scales (Fig. 3K-M, P, Q).The lower surface of the basal plate can be either concave, smooth (Fig. 3F), or convex (Fig. 3L, M, O).
A few rhomboidal to roundish scales have their basal plates covered with several (two to seven) odontodes with a flat and smooth surface.The length of these scales is 0.45-0.75mm and width 0.3-0.7 mm (Figs 3S-U; 5D).The width of the odontodes is 0.1-0.2mm and length 0.2-0.4mm.The odontodes are oval (Fig. 3T, U) to oblong (Fig. 3S), more than twice as long as their width.The odontodes lie close to each other (Fig. 3T, U), some space may be between the anterior odontodes and the neighbouring ones (Fig. 3S).Anterior odontodes partly cover the odontodes situated behind them; their posterior apices are usually broken (Fig. 3S).On the sides of odontodes the surface is finely striated.The basal plate is convex, slightly larger than the crown (Fig. 3S-U).The pores occur in the facets of the basal plate and on the plate around the odontodes, making the surface of the plate uneven.
Histology.Vertical longitudinal (Fig. 4A-G, J), horizontal (Figs 4H, I; 5A, B), and vertical crosssections (Fig. 5C) were made from both juvenile (Figs 4B; 5A, B) and mature (Figs 4A, C-J; 5C, D) scales.Very fine branching dentine tubules occur in the odontodes (Fig. 4B, D, G, J).They branch from wider dentine canals (Figs 4B-G, J; 5B), and pulp canals, which in turn are connected with the vascular canal network beneath the odontodes ( Figs 4C, E, F, I, J; 5B, C).At least one dentine canal extends into each odontode.Vascular canals open on the surface of the basal plate around the odontodes.The basal plate contains tightly packed flattened osteocyte lacunae (Fig. 3C-F, H, J), which give the basal plate structure lamellar appearance.Very fine Sharpey fibre tubules occur in the basal plate (Fig. 5A).One sectioned scale with flat and smooth odontodes (Fig. 5D) has microstructure identical to that of other scales.

DISCUSSION
Karksilepis gen.nov. is similar to osteostracans by having bone cell cavities in its exoskeleton.Osteostracans have distinct radial structure of peripheral sections of the base of tesserae, which are absent on the basal plate of Karksilepis gen.nov.In Karksilepis gen.nov. the upper layer is comprised of orthodentine, in osteostracans (if present) the equivalent layer consists of mesodentine.Karksilepis gen.nov.differs from osteostracans in having a specific vascular canal network in the basal plate below the odontodes.The ultrasculpture of striation is present in thelodonts, heterostracans, osteostracans, and in Karksilepis parva gen.et sp.nov.The convex basal plate of the scales of Karksilepis gen.nov. is superficially similar to the scale base of acanthodians but their crown sculpture and growth of the scale are different.
The greatest similarities can be found between Karksilepis gen.nov.and some chondrichthyan taxa.Chondrichthyans constitute a very diverse group, with the representatives having very different features.In the elasmobranch chondrichthyan taxa Elegestolepis Karatajūtė-Talimaa, Kannathalepis Märss & Gagnier, and Ellesmereia Vieth monodontodia are simple, the crown has one pulp cavity, one neck, and one basal canal, while Karksilepis gen.nov.Märss has complex polyodontodia.A possible elasmobranch Seretolepis Karatajūtė-Talimaa has peculiar lamelliform odontodes on the crown, the base built from aspidine, and basal and neck openings.Knerialepis Hanke & Karatajūtė-Talimaa has the scales whose odontodes consist of mesodentine and the basal plate of aspidine.Vascular canals are developed between the basal plate and the crown, making Knerialepis similar to Karksilepis, but the sculpture in the former consists of posteriorly fusing wedge-shaped flat odontodes.In Altholepis Karatajūtė-Talimaa the crown is formed of a great number of narrow anchor-shaped odontodes pointed posteriorly; the scale crown is built from orthodentine and the base -from aspidine.Ivanelepis Karatajūtė-Talimaa has scales with a few oblong odontodes and neck and basal openings.Arauzia Mader is defined as a possible hybodontiform taxon with rather high scales with upwards directed odontodes of cyclomorial growth and vascular canal openings in the neck and base (Mader 1986) Hanke, pers. comm. 2008) while the scales of other parts of the body have them (see also Karatajūtė-Talimaa 1998).Karatajūtė-Talimaa (1997b) erected the elasmobranch order Lugalepidida based on the characteristics of pectoral and pelvic fins, complex scales in which the odontodes (polyodontodia) of the crown are formed of orthodentine, and the non-growing base of acellular bone tissue; there is no horizontal canal system on the boundary between the crown and the basal plate.According to Karatajūtė-Talimaa (ibid.), this order differs from Mongolepidida, whose representatives have scales with a growing base and odontodes of lamelline forming longitudinal odontocomplexes of the crown.Karksilepis parva gen.et sp.nov. is externally rather similar to Lugalepis multispinata, which comes from the Emsian(?) and Eifelian of Lithuania, Latvia, Belarus, and Kaliningrad District, Russia (Karatajūtė-Talimaa 1997b).Both taxa have scales whose crowns consist of numerous spiniform odontodes.Karatajūtė-Talimaa (1997b) defined such scales in L. multispinata as polyodontodia of synchronomorial type.Nevertheless, these taxa differ in that the upper surface of odontodes of L. multispinata has a longitudinal groove in the medial part, while K. parva gen.et sp.nov.possesses spines with round cross section.Only K. parva gen.et sp.nov.has ultrasculpture of fine striation.The basal plate of the scales of L. multispinata consists of a spongy acellular bone tissue and thin basal lamina, but our new taxon has flattened bone cell lacunae forming the lamellar structure of basal plates.
The scales of Cladodontida also are formed of orthodentine and the basal plate of scales is either of cellular bone tissue (in Cladolepis) or acellular (in Ohiolepis and Protacrodus) (Gross 1973).Karatajūtė-Talimaa (1997b) noted that some Mongolepidida (Mongolepis, Sodolepis) have an acellular bone developed in the base of scales and some (Teslepis) have a cellular bone with fusiform bone cells; the horizontal system of canals is present in both Mongolepidida and Cladodontida.It would be most appropriate to accommodate Karksilepis gen.nov., having orthodentine in the complex odontodes, horizontal canal network beneath the odontodes in the basal plate, and a cellular bone in the base, within the Cladodontida.Difficulties rise when considering the growth of the crown.In Cladodontida odontodes of approximately the same size were added areally, forming rather regular ring-like zones (Karatajūtė-Talimaa 1992), while in Karksilepis gen.nov.they occur in irregular rows.As a conclusion, at present we prefer to leave the family and order for Karksilepis parva gen.et sp.nov.Märss open.

RESULTS
Based on the features of scale sculpture, the vascular canal network, and the placement of the canal openings beneath the odontodes in the basal plate, a new chondrichthyan of uncertain order and family, Karksilepis parva gen.et sp.nov.Märss, is established with specimens from the Härma Beds, Burtnieki Regional Stage, Karksi outcrop, Estonia.This is the first time that chondrichthyan dermal scales were discovered in Estonia.Previously only chondrichthyan buccopharyngeal denticles have been documented from the region (Mark-Kurik & Karatajūtė-Talimaa 2004).The teeth of an elasmobranch from the same outcrop will be discussed in another study.

Fig. 1 .
Fig. 1.Distribution of the Burtnieki Regional Stage and location of the Karksi outcrop.

________________________________________________________________________________________________________Fig. 4 .
Fig. 4. Microstructure of the scales of Karksilepis parva gen.et sp.nov.Märss.A, scale GIT 383-38 in vertical longitudinal section; B, medial part of the scale GIT 383-31 in vertical longitudinal section; C, scale GIT 383-35 in vertical longitudinal section; D, close-up of C; E, scale GIT 383-33 in vertical longitudinal section; F, posterior part of the scale GIT 383-37 in vertical longitudinal section; G, close-up of E; H, I, scale GIT 383-39 in horizontal section; photos focused on different levels to show bone cell lacunae in the middle part of the basal plate (H) and vascular canals on the upper part of the basal plate (I); J, posterior part of the scale GIT 383-34 in vertical longitudinal section.A, F, Karksi outcrop, sample 02-1; B-D, Karksi outcrop, sample 02-4; E, J, Karksi outcrop, sample 90; H = I, Karksi outcrop, sample 97.Scale bars 0.25 mm.

Fig. 5 .
Fig. 5. Microstructure of the scales of Karksilepis parva gen.et sp.nov.Märss.A, B, horizontal section of the scale GIT 383-40; A, the section shows the anterior part of the scale with Sharpey fibre tubules, B, the section shows the vascular canal network beneath the spines in the basal plate; C, vertical cross section of the scale GIT 383-41; D, vertical longitudinal section of the scale GIT 383-36.AC, Karksi outcrop, sample 02-1; D, Karksi outcrop, sample 02-4.Scale bars 0.25 mm.
Karksilepis gen.nov.Märss Derivation of name.From Karksi locality yielding scales of this taxon, and lepis, meaning scale, gender feminine.Type species.Karksilepis parva gen.et sp.nov.Märss Diagnosis.As for type species.
. The cladoselachid taxa Iberolepis Mader and Lunalepis Mader have the scale crown and microstructure somewhat resembling those of acanthodians (ibid.).The possible chondrichthyans Frigorilepis Märss et al. and Wellingtonella Märss et al. have monodontode scales without neck canals.In chondrichthyan scales, the neck canals can be present or absent in the same fish, for example, the fin scales of Polymerolepis Karatajūtė-Talimaa lack neck canals (G.