Ordovician Trilobites

The illaenid trilobite Vysocania is widely represented in the Upper Ordovician of the Czech Republic, Portugal and Spain, and is one of the most characteristic taxa in the high-latitude peri-Gondwana palaeobiogeographical region to which it is endemic. Three species of the genus that were formerly assigned to Stenopareia and Zetillaenus, from the Czech Republic and Spain respectively, are herein revised, and a new species is described from the Czech Republic. The genus Octillaenus, of which a new species is described from the Upper Ordovician of Morocco, is considered to have arisen from Vysocania through paedomorphic (neotenic) processes. The relationships between Vysocania, Octillaenus, Stenopareia and Zetillaenus are reviewed and the criteria used in the definition of these taxa discussed. Species assigned to Vysocania are V. vaneki (type-species), V. iberica, V. moraveci sp. nov. and V. panderi; species assigned to Octillaenus are O. hisingeri (type-species) and O. marocanus sp. nov. Vysocania appears for the first time in the lower Berounian (Sandbian) and has its last undoubted record in the upper Berounian (upper Katian); Octillaenus is presently restricted to the Kralodvorian (upper Katian). •

From the Saalian glacial deposit of the Laerheide (county of Osnabrück, NW-Germany) 620 glacial erratic boulders are investigated on their trilobite remains. The boulders originate from the Ordovician of Baltoscandia. Many of them mostly consisting of different kinds of carbonates are assigned to 16 various typs of glacial erratics. Exoskeletal remains of 82 genera and 15 subgenera of trilobites are recorded from the boulders. About 61% of the genera occur in glacial erratics of the Lower and Middle Ordovician, the remaining genera in boulders of the Upper Ordovician. A few long-lived genera are found both in glacial erratics of the Middle and of the Upper Ordovician. Some remarks are made concerning remarkable specimens and exoskeletal parts of rare trilobites.

Recent revision of the Ölandian trilobite faunas in Baltoscandia and the Ural Mountains throws new light on the devel- opment of the trilobite faunas in Baltica and possible migration links to surrounding terranes. The trilobite assemblages of 104 genera on the Uralian side of Baltica show different development patterns for the south and north through the Tremadocian to the Darriwilian. The oldest Uralian trilobites – disputably of latest Cambrian or earliest Ordovician age – arrived probably from the Siberian and Kazakh terranes being represented by mostly endemic genera such as Kujandaspis and Jdyia, but also with pandemic Akoldinioidia and Micragnostus. The following Kidryasian, Kolnabukian and Kuagachian faunas change gradually to show increasing difference between the sections in the South Urals and those in the northern Polar Urals or Pay-Khoy. In Kidryasian the olenids dominate in the South Urals as they do in many other regions during the early Tremadocian. The Kolnabukian trilobites represent the most diverse trilobite association in the region, and are comparable to the Ceratopyge fauna. The Kuagachian fauna contains a few additional elements, increasing the difference between south and north but with reduced generic diversity. The routes of faunal ex- change are modified too. Thus during the Early Ordovician migration between the Uralian side of Baltica and the Baltoscandia, Kazakh and Altai-Sayan terranes becomes more important than that between the Uralian side of Baltica and the Siberia, North and South China plates. The Darriwilian Karakoľ-Mikhailovskian faunal association shows a clear separation between north and south Urals with the former region, as in Baltoscandia, dominated by asaphids, while in south a reefal illaenid-cheirurid association of Laurentian genera occurs. This is rather intriguing given the widely ac- cepted palaeogeographical disparate position and latitude of Laurentia at the time.

Trilobites were one of the most successful groups of marine arthropods during the Palaeozoic era, yet their soft-part anatomy is only known from a few exceptionally-preserved specimens found in a handful of localities from the Cambrian to the Devonian. This is because, even if the sclerotized appendages were not destroyed during early taphonomic stages, they are often overprinted by the three-dimensional, mineralised exoskeleton. Inferences about the ventral anatomy and behavioural activities of trilobites can also be derived from the ichnological record, which suggests that most Cruziana and Rusophycus trace fossils were possibly produced by the actions of trilobites. Three specimens of the asaphid trilobite Megistaspis (Ekeraspis) hammondi, have been discovered in the Lower Ordovician Fezouata Konservat-Lagerstätte of southern Morocco, preserving appendages and digestive tract. The digestive structures include a crop with digestive caeca, while the appendages display exopodal seta...

Trilobites were one of the most successful groups of marine arthropods during the Palaeozoic era, yet their soft-part anatomy is only known from a few exceptionally-preserved specimens found in a handful of localities from the Cambrian to the Devonian. This is because, even if the sclerotized appendages were not destroyed during early taphonomic stages, they are often overprinted by the three-dimensional, mineralised exoskeleton. Inferences about the ventral anatomy and behavioural activities of trilobites can also be derived from the ichnological record, which suggests that most Cruziana and Rusophycus trace fossils were possibly produced by the actions of trilobites. Three specimens of the asaphid trilobite Megistaspis (Ekeraspis) hammondi, have been discovered in the Lower Ordovician Fezouata Konservat-Lagerstätte of southern Morocco, preserving appendages and digestive tract. The digestive structures include a crop with digestive caeca, while the appendages display exopodal setae and slight heteropody (cephalic endopods larger and more spinose than thoracic and pygidial ones). The combination of these digestive structures and the heteropody has never been described together among trilobites, and the latter could assist in the understanding of the production of certain comb-like traces of the Cruziana rugosa group, which are extraordinarily abundant on the shallow marine shelves around Gondwana. The Moroccan Fezouata Biota is the most important Ordovician Burgess Shale-type Lagerstätte known to date, having produced a very diverse marine assemblage, which includes not only articulated mineralized taxa, but also soft-bodied or lightly sclerotized organisms (e.g, annelids, priapulids, lobopodians) 1,2. Preservation usually involves pyritization and subsequent weathering to iron oxides, thus the reddish colours of the fossils. The assemblage has been dated as late Tremadocian (~478 Ma), from relatively shallow waters, at or just above the storm-wave base 3. Among the fossil arthropods, preserved appendages are frequent in the following groups: anomalocaridids, marrellomorphs, xyphosurans, aglaspidids, etc. 2,4. Additionally, fossilized digestive tracts have been described in machaeridian annelids 5. Trilobites are common constituents of the Fezouata Biota, found as complete carcasses as well as putative articulated exuviae. However, soft-body preservation is extremely rare, with only antennae and some distal appendage remains recognized in Bavarilla 3 and the nileid Symphysurus, where a single specimen of the latter exceptionally showed " preserved antenna, walking legs and midgut glands " 1. Here we present the first occurrence of the preserved gut with associated digestive structures, plus a complete set of endopods and exopods in specimens of the asaphid trilobite Megistaspis (Ekeraspis) hammondi. They come from site " F-4 " 6 , which is