Kaljoprion a new enigmatic jawed polychaete genus from the Upper Ordovician of Estonia Olle Hints

A new Late Ordovician polychaete genus Kaljoprion, with K. laevaensis gen. et sp. nov. as the type species, is described from the Nabala Regional Stage of the Laeva-18 drill core, central Estonia. The genus has a labidognath-type apparatus with the first and second maxillae equipped with anterior denticles developed into characteristic transversal ridges. Similar dentaries are observed in the jaws of distantly related Rhytiprion, suggesting a common feeding habit of both genera and representing a case of convergent evolution among Palaeozoic polychaetes.

Ordovician scolecodonts (polychaete jaws) of Estonia have been studied for more than ten years (Hints 1998).Together with earlier data from Poland, particularly by Kielan-Jaworowska (1966), the Estonian material has made the Baltic area the best studied region in terms of Ordovician polychaetes (Hints & Eriksson 2007).However, several stratigraphically and ecologically important taxa have hitherto remained under open nomenclature, which notably hinders further studies.In this paper a new enigmatic genus, Kaljoprion, is described, based on the reconstruction of its jaw apparatus.
The material was obtained from open shelf marls and limestones of the Nabala Regional Stage (middle Katian) of the Laeva-18 drill core (central Estonia, ca 20 km NW from Tartu).For detailed geological background see Raukas & Teedumäe (1997).The descriptive terminology used below follows that of Jansonius & Craig (1971).The collection is housed at the Institute of Geology at Tallinn University of Technology (acronym GIT).

Derivation of name.
Named in honour of Prof. Dimitri Kaljo, my former supervisor and a supporter since the beginning of my scientific career; Greek, prion -saw, a common suffix for naming scolecodont-bearing polychaetes.
Diagnosis.Asymmetrical jaw apparatus of labidognath type.Left MI with subtriangular outer face and prominent inner wing that extends for two-thirds of jaw length.Right MI with distinct ramus and bight, suggesting presence of basal plate.First to third denticles in MI developed into transversal ridges, anteriormost one occupying more than half of jaw width.Second maxillae subrectangular with deeply rounded bight.Anterior part of jaw occupied by long transversal ridge, shank with small knob-like denticles, slightly shorter than ramus.
Stratigraphical and geographical range.Upper Ordovician (Nabala Regional Stage) of Estonia.
Remarks.On the basis of characteristic denticulation the jaws of Kaljoprion are most similar to those of Rhytiprion Kielan-Jaworowska, a genus with a placognath-type apparatus.There are nevertheless several important differences, which also suggest that the jaw apparatus of Kaljoprion belongs to labidognath type: (1) symmetrical right and left MII with the ramus and denticulated shank, (2) presence of a prominent inner wing in the left MI, (3) presence of the subtriangular outer wing in the left MI, and the ramus and bight in the right MI, suggesting the existence of the basal plate, and (4) myocoele openings that are slightly enclosed (Figs 1,2).No other polychaete with labidognath apparatus has similar transversal ridges, but some polychaeturids may have anterior denticles of the MI that are slightly transversally stretched (Hints 2000, pl. 2, figs 1, 2).On that basis Hints (2000, pl. 2   Derivation of name.Named after the type locality, the Laeva-18 borehole.
Diagnosis.As for the genus.
Description.Asymmetrical jaw apparatus of labidognath type consisting of paired MI and MII; basal plate most likely present, other jaws unknown.
Left MI: L = 0.22-0.68mm, L/W = 1.8-2.4(holotype: L = 0.65 mm, L/W = 2.0).Outer face subtriangular, its maximum lateral extension located at about 0.3-0.4 of jaw length from anterior.Postero-lateral margin nearly straight or slightly sinusoidal, meeting the ridge before the posterior termination.Inner wing subtriangular, tapering posteriorly, occupying up to one-third of jaw width and reaching to about two-thirds of jaw length from posterior.Posterior margin narrow, convex, placed to the right of the ridge.Dentary composed of two transversal ridges in the anterior part, followed by one large, sometimes transversally slightly widened denticle and 7-11 small nearly equal-sized denticles.Transversal ridges slightly slanting postero-medially; the first ridge corresponds to about 0.75 of jaw width, the second one is half as long as the first.In ventral view, cover extends to about 0.1 of jaw length.In juvenile specimens the first transversal ridge is composed of three separate transversally widened denticles, the length of the second ridge forms about one-third of the first ridge.
Right MI: L = 0.31-0.74mm, L/W = 1.8-2.6.Outer margin directed postero-laterally at anterior 0.3-0.4 of jaw length, then bending steeply and running posteromedially towards the tip of ramus.Ramus extends to 0.5-0.6 of jaw length.Shank is slender, tapering posteriorly.Dentary similar to that in left MI, with two long transversal ridges in anterior part.The first ridge corresponds to 0.7-0.85 of jaw width, the second ridge to 0.5-0.6 of jaw width.The following denticle is larger than Left MII: L = 0.13-0.22mm, L/W = 0.65-0.8.Subrectangular jaw, wider than long, with wide and straight anterior margin.Ramus is more slender and slightly longer than shank, bight is widely rounded.Anterior part of jaw has long transversal ridge similar to those in MI, corresponding to 0.75 or more of jaw width.Shank has 6-11 small equal-sized denticles; sometimes the denticles are indistinct.
Variability.Jaws of K. laevaensis vary rather widely in size.In argillaceous sediments the jaws are commonly compressed and distorted; thus the length/width ratio and the general outline of the maxillae, including the shape of inner and outer wings, are also variable.The denticulation pattern and size of the transversal ridges are more stable, with the exception of ontogenetic changes discussed below.
Distribution.Kaljoprion laevaensis is a common taxon in the upper Mõntu and Saunja formations of the Nabala Regional Stage, accounting for up to 13% of specimens in the assemblage, but has not been recovered from the lowermost part of the Nabala Stage and from the overlying Vormsi Stage, which generally share similar polychaete faunas (e.g.Hints & Eriksson 2007).The limited vertical range indicates biostratigraphical potential of Kaljoprion.However, material from other localities is necessary to ascertain whether the spatial distribution of this species is facies-dependent.
Remarks.Although the apparatus types are not formal taxonomic units, they seem to convey broad phylogenetical relationships (Kielan-Jaworowska 1966).Since the jaw apparatuses of Kaljoprion and Rhytiprion are supposed to belong to different apparatus types, the transversal ridges most likely represent a polyphyletic character that has developed independently in two rather distantly related taxa.
Polychaetes use their jaws primarily for feeding and digging.The feeding habits of extant polychaetes are diverse and not directly related to the presence of jaws.Hence it is generally problematic to infer feeding habits of fossil polychaetes based on the jaws (Eriksson et al. 2004 and references therein).The transversal ridges are, however, exceptional, since they have developed independently in two different genera, suggesting clear functional benefit for a particular lifestyle, most probably a specific feeding type and diet.Bergman et al. (2003) suggested that the transversal ridges in the jaws of Rhytiprion may have been useful for crushing and grinding food rather than grasping or penetrating prey.Another plausible explanation is that the ridges were used for grazing, although hard substrates may have been difficult to find in the muddy facies that both Rhytiprion (see Bergman et al. 2003) and Kaljoprion seem to have preferred.In any case, the great morphological similarity between their jaws most likely indicates that Kaljoprion and Rhytiprion shared a common feeding habit and occupied a similar ecological niche.This is further supported by the fact that both genera co-occur in the studied samples and in similar abundance.Consequently, the transversal ridges can be taken as an example of convergent evolution among polychaetes.In juvenile posterior maxillae of K. laevaensis the first transversal ridge is commonly composed of two or three separate laterally widened denticles.This is especially evident in the smallest jaw recovered (see Fig. 2F, but also Fig. 2E, M, N).Such ontogenetic dimorphism may indicate a change in feeding habit during the life span (not accounting the larval stage).On the other hand, it may also suggest that the ridges have phylogenetically evolved through fusion of several denticles rather than through widening of individual ones.It remains to be studied if the same pattern may also apply to Rhytiprion.

assigned
Kaljoprion tentatively to Polychaeturidae.Hints & Eriksson (2007, table 2) also referred to Kaljoprion as Polychaeturidae?gen.nov. A. The type and only species of Kaljoprion nevertheless differs from polychaeturids in having a shorter posterior margin in the left MI, differently shaped MII, and much more prominent transversal ridges in MI and MII.Therefore this family level assignment remains doubtful.It cannot be excluded that Kaljoprion belongs to a new family, but as only the first and second maxillae are known, it is more appropriate to keep it under open nomenclature for the time being.

Table 1 .
Numbers of individual jaws of Kaljoprion laevaensis gen.et sp.nov.recovered from the Laeva-18 borehole