Discovery of Late Ordovician scolecodonts from the Qusaiba-1 drill core, Arabian Peninsula, and its palaeobiogeographical implications
Résumé
Scolecodonts, the jaws of eunicidan polychaetes, constitute one of the most common groups of acid-resistant microfossils in the Ordovician and Silurian deposits. They are rather well known from Baltica and Laurentia, but only few data are available from other palaeocontinents. The global picture of palaeobiogeography of scolecodonts is therefore biased and every new account from the hitherto poorly known regions deserves full attention. In the course of chitinozoan study of the Qusaiba-1 drill core, central Saudi Arabia, a rich assemblage of scolecodonts was recovered from the Late Ordovician Quwarah Formation. This interval belongs to the Armoricochitina nigerica chitinozoan zone, which is correlated with the upper part of the Katian Global Stage. The bulk of material was obtained by re-picking the sample residues but specimens scattered in the chitinozoan slides were also studied. The entire collection contains more than 150 well-preserved scolecodonts, including some fused jaws and a compound jaw apparatus. In sample from 497.8 ft 76 posterior maxillae were counted from 10 g making this one of the richest samples ever studied for scolecodonts. Taxonomically the assemblage is not particularly diverse containing two species of Oenonites, two species of "Atraktoprion", Kettnerites sp., possibly Kalloprion, an unidentified placognath and a new genus that cannot be confidently attributed to any known family. The latter taxon bears mixed characters of polychaetaspids, kalloprionids and ramphoprionids and accounts for the majority of the specimens in the collection. Similar forms have been reported from the Late Ordovician of North American Mid-continent region, but are unknown from Baltica so far. The low percentage of placognaths is also noteworthy and possibly indicates biogeographical relationships with Laurentia rather than Baltica. Moreover, Kettnerites, being also recorded in Bohemia, is a characteristic component of the Late Ordovician polychaete faunas of Laurentia. In Baltica, paulinitids are very rare in the Ordovician though becoming common in the Silurian. In consequence, this discovery of scolecodonts from the Arabian plate allows, for the first time, to assess biogeographical links of Ordovician jawed polychaetes of Gondwana. It shows that Gondwanan polychaetes were related to Laurentian rather than Baltic faunas. The high abundance of scolecodonts indicates also their potential in stratigraphy and biofacies analysis, but indeed more data is needed for that