The youngest representatives of the genus Ribeiria Sharpe , 1853 from the late Katian of the Prague Basin ( Bohemia )

Ribeiria apusoides and Ribeiria johni sp. nov. are described from the late Katian of the Prague Basin (Bohemia) as the youngest representatives of the genus Ribeiria. The Ordovician ribeirioids from Bohemia (Perunica) show close affinities to the ribeirioids from Armorica and Iberia. The functional morphology of ribeirioids, mainly the pedal muscle system, is discussed, based on very well-preserved specimens of R. apusoides. The ribeirioids attained their diversity in the Lower Ordovician, since the Middle Ordovician their diversity declines, and during the late Katian only three genera are known worldwide. They are unknown from the Hirnantian but the last ribeirioids are recorded from the lower Silurian in South China.


INTRODUCTION
The study of rostroconchs was started by Martin (1809) and Sowerby (1815), but the systematic position of the group was for a long time unclear.Rostroconchs were allied mainly to bivalves (Branson et al. 1969) or even to arthropods (Schubert & Waagen 1904;Kobayashi 1933).Pojeta et al. (1972) established them as a separate group of molluscs and Pojeta & Runnegar (1976) presented three orders of rostroconchs, Ribeirioida, Ischyrinioida and Conocardioida.Subsequently Pojeta (1987) included Ischyrinioida within the Ribeirioida.The main reasons for the assignment of rostroconchs to the molluscs are the presence of a protoconch, calcareous shell with the growth lines and the prominent muscle scars, which also show growth increments (Pojeta & Runnegar 1976).In a large monograph (Pojeta et al. 1977) on the Cambrian and Ordovician rostroconchs from Australia their potential usefulness as biostratigraphic tools was presented.The research of rostroconchs continues with modern systematic studies focused mainly on the order Conocardioida (e.g.Hoare 1989Hoare , 1990Hoare , 2000;;Amler 1996;Hoare et al. 2002;Amler & Rogalla 2004;Rogalla & Amler 2006a, 2006b, 2006c).On the other hand, the Ordovican ribeirioids have been an overlooked group and outside comprehensive palaeobiological and systematic studies.
The ribeirioids from Bohemia are not an exception: Schubert & Waagen (1904) described the genera Ribeiria Sharpe, 1853and Ribeirella Schubert & Waagen, 1904(now Technophorus Miller, 1889) from the Ordovician of the Prague Basin and assigned them to arthropods.These two genera were also figured in Perner (1903).Pojeta & Runnegar (1976) briefly described and figured all known species of rostroconchs, including also species of ribeirioids from the Ordovician of the Prague Basin (Czech Republic).The rich material of rostroconchs from the Czech Republic is one of the best-preserved materials.It includes ribeirioids from the late Katian (Králův Dvůr Formation), among them the worldwide youngest representatives (R. apusoides Schubert & Waagen, 1903 and Ribeiria johni sp.nov.) of the genus Ribeiria.

SYSTEMATIC PALAEONTOLOGY
Abbreviation.NM -specimens deposited in the National Museum, Prague.

MOLLUSCA
Class ROSTROCONCHIA Pojeta, Runnegar, Morris & Newell, 1972Order RIBEIRIOIDA Kobayashi, 1933 Family RIBEIRIIDAE Kobayashi, 1933 Genus Ribeiria Sharpe, 1853  Pojeta & Runnegar (1976) show the main aspects of functional morphology in rostroconchs.The determination of the morphological orientations in extinct groups is always difficult and therefore the orientations of the shells in bivalves were used as a certain example for describing the morphological orientations in rostroconchs.

FUNCTIONAL MORHOLOGY IN RIBEIRIOIDA
The system of muscles in bivalves also served as an example for the interpretation of the musculature in rostroconchs, which seems to work fairly well in the Ordovician Conocardiida like Eopteria.On the other hand, the system of muscle scars in Ribeirioida is more different from the system of muscle scars in bivalves.
Ribeiria apusoides from the Ordovician of Bohemia possesses a very well-preserved posterior median muscle scar (Fig. 1D3, F3) lying across the midline of the shell and serves to retract the foot.In R. apusoides the posterior median retractor is very large, extending almost from the umbo to the posterior notch, where it is more deeply inserted.The posterior median retractor is formed by two lobes (Fig. 1D3, F3) and differs markedly from the bivalve pedal musculature.
Side muscle scars are rarely preserved and their function is unclear.According to Pojeta & Runnegar (1976), they can help with moving the foot or support gills.Side muscle scars connect median muscle scars and they are in a similar position as accessory muscle scars in some Ordovician bivalves considered as infaunal burrowers (Babinka, Coxiconchia or Praenucula) (Fig. 2).
In bivalves these muscles serve as a support of retractors or can provide a firm attachment of the pericardial region to the shell during vigorous movement of the foot.They can also hold the muscular floor of the visceral sac (Heath 1937;Polechová 2013).The function of side muscles in Ribeirioida, also considered as infaunal burrowers, could be similar.
The anterior median muscle scar lies on the posterior part of the pegma (Pojeta & Runnegar 1976, pl. 5, fig. 4).Very rarely (seen in two specimens) this muscle scar is preserved also in R. apusoides.Similar to the posterior median muscle scar, it serves probably to retract the foot and therefore the function of the pegma could be in supporting this muscle.The bivalves Redonia or Nuculites as well show a myophoric plate, which supports the anterior adductor muscle.The pedal muscles are deeply inserted in the shell and have more chance to be preserved than pallial muscles, which are very rarely preserved also in bivalves.
The decrease in the diversity not only in Ribeirioida, but also in the whole group of rostroconchs was explained as a competition of this group with the Ordovician bivalves (Pojeta 1979), which are mostly considered similarly to rostroconchs like infaunal burrowers.According to Pojeta (1979), the main competitive advantage for bivalves could be their adaptation to burrowing because they could better establish and keep their position in the sediment.Thus they could burrow into a wider variety of substrates.This idea was disproved by Cope (2004), who showed that (1) initial early Ordovician bivalve radiations took place in Gondwana where there were few rostroconchs, (2) the early Middle Ordovician rostroconch decline was principally extra-Gondwanan (where there were few bivalves) and (3) the late Ordovician increase in rostroconch diversity, particularly in the Laurentian carbonate shelves, coincided with the arrival of bivalves on these shelves, where the latter rapidly diversified.
Furthermore, in conocardiids no apparent decrease in diversity is observable during the Ordovician, because the number of genera is almost the same (Early Ordovician -four genera, Middle Ordovician -four genera, Late Ordovician -five genera).The situation is different in ribeirioids; they reached the peak of their diversity in the Early Ordovician.Unlike the rostroconchs, the bivalves diversified significantly during the Middle Ordovician and Late Ordovician (e.g.Babin 1993;Polechová 2013) and the diversity during the Ordovician increased and reached almost 200 genera (Cope & Kříž 2013) in comparison with rostroconchs and their 26 genera.Ribeirioids never exceeded the diversity of bivalves even in the Early Ordovician, when the bivalves were rare and restricted to the Gondwana and peri-Gondwana terranes.

CONCLUSION
The genus Ribeiria is known from the late Cambrian (Australia, northwestern Argentina) to the late Katian (Bohemia).Ribeiria johni sp.nov. is restricted to the upper Katian of the Prague Basin only, whereas R. apusoides is known from the Darriwilian to the late Katian of the Prague Basin.In the Middle Ordovician the ribeirioids of Bohemia show close affinities to similar associations from Armorica and Iberia; in the Late Ordovician they share some taxa with Baltica.
The ribeirioids from Bohemia provide specimens with well-preserved muscle scars, such as anterior and posterior median retractors, and very rarely preserved side muscle scars.The function of the side muscle scars in ribeirioids seems to be very similar to that of the accessory muscle scars in bivalves, which are preserved in the umbo region or between adductor muscle scars.The function of the pegma in ribeirioids could be in supporting the anterior median retractor.
The diversity of conocardiids is almost the same during the Ordovician.The ribeirioids reached their acme of diversity during the Lower Ordovician.Rostroconchs never exceeded the diversity of bivalves even in the Lower Ordovician, when the bivalves were rare and restricted to the Gondwana and peri-Gondwana terranes.
1903 Ribeiria apusoidesSchubert & Waagen; Perner,  pl.49, figs 14-17.1976RibeiriaapusoidesSchubert&Waagen; Pojeta  & Runnegar, p. 53, pl.6, fig.13.Lectotype.(SD Pojeta & Runnegar 1976) NM L 7860, internal mould of the shell, figured by Schubert & Waagen in Perner 1903, pl.49, figs 18-20.Paralectotypes.NM L 7861, internal mould of the shell, figured by Schubert & Waagen in Perner 1903, plOrnamentation comprising fine commarginal lines, in the posterior part these lines are rippling.Anteriorly from the posterior dorsal notch posterior median muscle scar is preserved, extended almost to the umbo.Anterior median muscle scar lying on the posterior part of the pegma.In one specimen (NM L 7852) slightly inserted side muscle scars between anterior and posterior median muscle scars are present.Discussion.Ribeiria apusoides is the most similar to R. pholadiformis from the Darriwilian of Portugal, France, Morocco and Spain.The main difference in the diagnosis of the species, mentioned by Pojeta & Runnegar (1976) is lack of the prominent posterior dorsal notch in R. apusoides.This character is vague because in some specimens (NM L 7860) of R. apusoides the prominent notch is also visible.Also the outline of the shell, presence of the anterior cleft and posterior median muscle scars are very similar.The late Ordovician R. apusoides seems differ from R. pholadiformis in a concave dorsal margin.Ribeiria spinosa Babin & Distribution.Bohemia, Prague Basin, Darriwilian: Osek, Rokycany, Díly, Pětidomky, Malé Přílepy; lower and upper Katian: Bohdalec, Loděnice, Prague-Lhotka,