Lower Ordovician Research Articles

New insights from U–Pb zircon dating of Early Ordovician magmatism on the northern Gondwana margin: The Urra Formation (SW Iberian Massif, Portugal)

The Central Iberian–Ossa-Morena transition zone (SW Iberian Massif) represents a segment of the northern Gondwana margin with a long geodynamic evolution, characterized by the superposition of Cadomian and Variscan events. The Early Ordovician is mainly represented by porphyritic felsic volcaniclastic rocks (the Urra Formation) that pass up into a siliciclastic sediments typical of the Central Iberian Zone (Lower Ordovician Armorican Quartzite Formation). The Urra Formation unconformably overlies the previously deformed and metamorphosed Ediacaran sediments of the Série Negra (with Ossa-Morena Zone paleogeographic affinity). New SHRIMP zircon data obtained from the Urra Formation volcaniclastic rocks indicate an Early Ordovician age ( 206Pb/ 238U ages ranging from 494.6 ± 6.8 Ma to 488.3 ± 5.2 Ma) for this magmatic event. The inherited zircon cores indicate the presence of multicycle protoliths with different Precambrian ages: Neoproterozoic (698–577 Ma), Paleoproterozoic (2.33 Ga) and Paleoarchean (3.2–3.3 Ga). There is a noticeable lack of Meso- to Neoarchean and Mesoproterozoic ages. The data support the hypothesis that the volcaniclastic rocks were derived by partial melting of Cadomian basement (linked to a West African Craton provenance). The Urra Formation volcaniclastic rocks have rhyolitic to dacitic compositions, are peraluminous and similar to calc-alkaline high-K series suites elsewhere. Isotopic signatures present a wide range of values ( 87Sr/ 86Sr) t = 0.7085–0.7190, more restricted εNd t (− 2.65 to − 0.35) and δ 18O = 9.63–10.34‰, compatible with magmas derived from crustal rocks, including portions of the lower crust. Some samples show disturbance of the Rb–Sr system as shown by unrealistic values for ( 87Sr/ 86Sr) t < 0.703, probably due to Variscan deformation and metamorphism. The volcaniclastic rocks with a significant sedimentary contribution (upper unit) are distinguished from the others by the lowest values of εNd t (− 5.53 to − 4.85). The geochemical data are compatible with an orogenic geodynamic environment. However, the “orogenic” signature can be considered to represent, in part, an inherited feature caused by melting of the Cadomian basement which also has calc-alkaline affinities. The Early Ordovician crustal growth and associated magmatism, represented by the Urra felsic volcaniclastic rocks and associated calc-alkaline granitoids, diorites and gabbros, can be interpreted in terms of the underplating and temporal storage of mantle-derived magmas as the potential source for the “orogenic melts” that were intruded during Early Paleozoic extension. This record of Early Ordovician magmatism has striking similarities with other correlatives from the Iberian, Bohemian and Armorican massifs that are discussed in this paper. This comparison reinforces the probable existence of a large-scale crustal melting process linked to a significant episode of extension on the northern Gondwana margin that probably resulted in the birth of the Rheic Ocean.

Estonian georesources in the European context

Estonia is situated on the southern buried slope of the Baltic Shield where the sedimentary bedrock overlies the Precambrian crystalline basement. The Cambrian section contains mainly sandstone and clay (e.g. famous blue clay), Lower Ordovician is represented by sandstone, including phosphate Obolus sandstone (shelly phosphorite), and Upper Ordovician by oil shale (kukersite). Estonia is not very rich in useful minerals, but we have some georesources sizeable in European context: oil shale (the Estonia deposit is the largest commercially exploited and best-studied oil shale deposit in the world), phosphorite (the well- studied Rakvere deposit is the largest phosphorite deposit in Europe, but not exploited), and peat (Estonia is considered as a country richest in peatlands in North Europe). Unfortunately, for more than 80 years oil shale and phosphorite have been mined and industrially used in environmentally hazardous ways, devastating large regions in northern and northeastern Estonia. The mining is mainly causing technological and technical, environmental, economic and social problems.

A revision of the stratigraphic nomenclature of the Cambrian–Ordovician strata of the Philipsburg tectonic slice, southern Quebec

The Philipsburg tectonic slice is bounded to the west by a northeast–southwest-trending thrust fault (Logan’s Line) and preserves 10 formations of Middle (?) to Late Cambrian (Milton, Rock River, and Strites Pond formations), Early Ordovician (Wallace Creek, Morgan Corner, Hastings Creek, and Naylor Ledge formations), and early Middle Ordovician (Luke Hill, Solomons Corner, and Corey formations) age. The strata were previously assigned to the Philipsburg Group. Early correlations between the Philipsburg succession and coeval strata of the St. Lawrence Platform were mainly based on sparse macrofauna and inferred stratigraphic position. Unconformities at the Cambrian–Ordovician and Early Ordovician – Middle Ordovician boundaries occurring in autochthonous St. Lawrence Platform and the allochthonous Philipsburg succession (Philipsburg tectonic slice) highlight new stratigraphic interpretations between the inner-shelf (St. Lawrence Platform) and the outer-shelf (Philipsburg) successions. The succession in the Philipsburg tectonic slice is divided into three new groups. The Middle (?) to Upper Cambrian Missisquoi Group (new) includes the Milton, Rock River, and Strites Pond formations. The upper boundary of the Missisquoi Group is defined by the upper unconformable contact between the Upper Cambrian Strites Pond Formation and overlying Lower Ordovician Wallace Creek Formation. The Missisquoi Group correlates with the Potsdam Group of the St. Lawrence Platform. The Lower Ordovician School House Hill Group (new) includes the Wallace Creek, Morgan Corner, Hastings Creek, and Naylor Ledge formations. The upper boundary of this group is marked by a regionally extensive unconformity at the top of the Naylor Ledge Formation and correlates with the younger Beekmantown-topping unconformity. The School House Hill Group is correlative with the lower to upper part of the Beekmantown Group (Theresa Formation and the Ogdensburg Member of the Beauharnois Formation) of the St. Lawrence Platform. The Middle Ordovician Fox Hill Group (new) consists of the Luke Hill, Solomons Corner, and Corey formations. This group correlates with the uppermost part of the Beekmantown Group (Huntingdon Member of the Beauharnois Formation and the Carillon Formation).

A new, phylogenetically significant Early Ordovician asteroid (Echinodermata)

A new monospecific family of asteroids (Echinodermata) is based onEukrinaster ibexensisn. gen. and sp. from the Lower Ordovician of Utah and Nevada.Eukrinaster, Arenig in age, is one of the earliest of known asterozoans. The new, relatively well-preserved fossils yield important information on character state distribution that will be useful for the interpretation of phylogenetic relationships among the three asterozoan classes, the Somasteroidea, Ophiufoidea, and Asteroidea. In addition, overall form is suggestive of certain living asteroids: to the extent that form equates with function, similarities suggest ecologic parallels in these only distantly related asteroids inhabiting ecologically distinct worlds.

The earliest turrilepadid: a machaeridian from the Lower Ordovician of the Northwest Highlands

Synopsis A machaeridian sclerite from a thrombolite bed in the Sangomore Formation (Durness Group: Tremadoc) is the first to be described from NW Scotland. The large size, thickness and coarsely spaced rugae indicate the sclerite is that of a turrilepadid, but its morphology is distinct from any taxon described previously. Lacking a longitudinal fold and with three major inflections present along the accreting margin, it is interpreted as an outer right sclerite. It is the oldest turrilepadid yet recorded and shows that all three major groups of the problematical Machaeridia were present in the early Ordovician.

Graptolites from the Lower Ordovician (Tremadoc) Mungok Formation in Danyang area, Korea

Lower Ordovician (Upper Tremadoc) graptolites were found for the first time at the two sections of the Mungok Formation in Danyang area: the Seokgyori and Daejeonri sections. The graptolites from these sections comprise two species of two genera: Adelograptus brevibrachiatus and Psigraptus jacksoni. The graptolite faunas of the sections in the area show simple composition relative to Yeongwol area; they appear to be correlated to: (1) the Adelograptus cf tenellus Zone of Yeongwol area, (2) the Adelograptus cf. tenellus Zone of Yukon, Canada, (3) the Psigraptus Zone of Tasmania, Australia, and (4) the Psigraptus jacksoni Zone of Jilin, China. The age of graptolite beds of the Mungok Formation at Danyang appears to be early Upper Tremadoc of the Lower Ordovician.

Upper Tremadoc–Lower Arenig? Anisograptid–Dichograptid fauna from the Cabitza Formation (Lower Ordovician, SW Sardinia, Italy)

The discovery of the anisograptid graptolite Araneograptus murrayi (Hall, 1865), the dichograptids Clonograptus ( Clonograptus) cf. rigidus (Hall, 1858) and Clonograptus ( Clonograptus) cf. multiplex (Nicholson, 1868) and of two unassigned species of Didymograptus in the Late Tremadoc–Early Arenig? sediments of the Fluminese area (SW Sardinia) adds new data on the biostratigraphy and palaeoecological setting of the Cabitza Formation, as well as on the paleobiogeographical distribution of these taxa.

Life habit and spatial distribution of siphonotretid brachiopods in the Lower Ordovician of the Prague Basin, Czech Republic

ABSTRACTSiphonotretid brachiopod distribution in the Lower Ordovician of the Prague Basin is described and discussed; the new species Alichovia cometa is established. The preservation and distribution of the group indicate a preference for shallow-water offshore, high-energy environments: siphonotretids are abundant shelly fossils in the shallower-water, mostly marginal sites of the basin but are absent in the deeper parts. Their environmental limits were complementary to those of the graptolites; siphonotretids co-occur with epiplanktonic graptolites at only a few localities. Deeper offshore deposits with biramose dichograptids and diverse, generally delicate benthic dendroids lack siphonotretids. Indeed, siphonotretids were intolerant to dysoxia and preferred a well-aerated siliciclastic, firm sea floor in the shallower parts of the basin. They were tolerant to suspended coarser detritus, using their spines which functioned as an effective sieve. The density and ramification of spines indicate three sifting strategies. Suggested life styles above the seabed on elevated surfaces (algae, sponges, dendroids) are not supported by direct evidence. They more likely lived within more supple and soft matrices (sponges, tufts of algae) stabilised by their spines, with the spinose basket maintaining free space for inhalant and exhalant currents.

New specimens of Lingulocystis Thoral, 1935 (Eocrinoidea, Blastozoa) from the Arenig (Lower Ordovician) of Montagne noire (southern France): Intraspecific morphological variability, stratigraphic, and palaeoecological implications

The genus Lingulocystis Thoral, 1935 is a preponderant component of the echinoderm fauna of the Lower Ordovician of Montagne noire (France), as suggested by its abundant remains. Its morphology is characterized by a flattened theca, composed of a frame enclosing numerous small polygonal central plates. The new specimens have been determined partly as Lingulocystis elongata Thoral, 1935 and partly as Lingulocystis aff. deani Ubaghs, 1994. This study is the first report of Lingulocystis in the Foulon Formation. New stratigraphic ranges have been defined for both species: L. elongata from the early Tremadoc (Saint-Chinian Formation) to the middle Arenig (lower part of the Landeyran Formation), L. aff. deani from the early Arenig (Cluse de l’Orb Formation and Foulon Formation), and L. deani from the middle Arenig (upper part of the Landeyran Formation). The new specimens of L. elongata highlight its intraspecific variability, mainly concerning the size and arrangement of the central plates, the size of the theca, and the type of stem columnal articulations. Sedimentological studies in Montagne noire currently use the presence of Lingulocystis remains as taphonomic and palaeoecological indicators. The presence of two types of columnal articulations enhances the palaeoecological use. The development of the synostosial articulation seems to be correlated with a proximal platform palaeoenvironment, whereas development of the symplexial type seems to be related to a deeper palaeoenvironment or relatively high hydrodynamic conditions.

First report of the mitrate Peltocystis cornuta Thoral (Echinodermata, Stylophora) in the Lower Ordovician of central Anti-Atlas (Morocco)

The mitrate Peltocystis cornuta is one the best known and most abundant stylophoran echinoderms in the Lower Ordovician (upper Tremadoc - lower Arenig) of Montagne Noire (southern France). It is here documented outside this region for the first time, in coeval deposits of central Anti-Atlas (Morocco). This report confirms the strong faunal affinities between Montagne Noire and Moroccan assemblages in the Ordovician. Twenty-eight individuals of P. cornuta are described from two distinct localities of the Zagora region. Their overall morphology is very comparable to that of Montagne Noire specimens. However, the Moroccan Peltocystis differ from the French ones in their smaller mean size, and in the absence of abnormalities in thecal plating ( situs inversus is frequent in the Montagne Noire specimens). These differences are possibly related to the distinct palaeoenvironmental conditions, in which the Moroccan and French assemblages were living: in Montagne Noire, P. cornuta was found in deeper, quieter (less stressful) deposits than in the Anti-Atlas. Finally, the periproct is clearly identified for the first time in Peltocystis, in a more anterior position than previously thought.

ABNORMALITIES IN THE LOWER ORDOVICIAN CORNUTE PHYLLOCYSTIS THORAL, 1935 (ECHINODERMATA: STYLOPHORA) FROM MONTAGNE NOIRE (SOUTHERN FRANCE): IMPLICATIONS FOR PLATE HOMOLOGY AND PHYLOGENY

Two specimens of the cornute Phyllocystis crassimarginataThoral, 1935 from the Lower Ordovician of Southern Montagne Noire (Languedoc, France) showing abnormal numbers of marginal plates are described. The first one (UM 310) is characterized by the insertion of one extra plate between the fourth right marginal and the zygal plate Z. Comparison with a new, undescribed Tremadoc boot-shaped cornute suggests that 1) the extra plate of UM 310 is equivalent to M4, and 2) the three right marginals located between M1 and Z in “normal” specimens of Phyllocystis can be identified as M2, Mc, and M3. Plate configurations of the right thecal margin are thus identical in “normal” Phyllocystis, chauvelicystids, and cothurnocystids. This result indicates that Phyllocystis is probably more closely related to these two clades than to scotiaecystids. Consequently, the identification of the four marginals forming the left thecal margin of Phyllocystis (M′2, M′3, M′5, and M5) is deduced from a comparison with the cothurnocystid plate pattern. The second abnormal specimen of P. crassimarginata (R64546) is characterized by the loss of M′3. It confirms that reduction in the number of marginals is a general trend in boot-shaped cornutes.

Stratigraphy and facies of Cambrian to Lower Ordovician strata in Saskatchewan

Abstract Cambrian to Lower Ordovician strata in Saskatchewan are present only in the subsurface and are mostly siliciclastic. They are divided into three units which, in ascending order, are the informally named Basal Sandstone Unit overlain by the Earlie Formation and Deadwood Formation. Strata of the Basal Sandstone Unit and Earlie Formation are Middle Cambrian and those of the Deadwood Formation range from the Upper Cambrian to Lower Ordovician. Sedimentary facies indicate that all units were deposited in environments ranging from shoreface to shallow-water shelf. Two major transgressive-regressive sequences are identified in these strata; the older is comprised of the Basal Sandstone Unit and most of the Earlie Formation, and the younger one is mostly the Deadwood Formation. The boundary between the two sequences shows very little evidence of erosion but the younger sequence overlaps the older sequence in an easterly direction. The upper boundary of the younger sequence is a major erosional surface. Facies trends in the sequences indicate an eastern to northeastern source of the siliciclastic sediment. The Saskatchewan succession is comparable to similar age strata in north-central and western USA, and the Interior Plains of Western Canada and the Northwest Territories. Many sandstone beds in the succession are porous and would be suitable petroleum reservoirs but the source rock potential of the associated shale appears poor. However, the reported presence of Gloeocapsomorpha sp., a fossil with known hydrocarbon-producing potential, could be a positive indicator if it can be found concentrated in organic-rich laminae or thin beds.

The earliest known estuarine bivalve assemblage, Lower Ordovician of northwestern Argentina

The oldest known estuarine bivalve assemblage is documented from the Lower Ordovician (upper Arenig-lower Llanvirn) Alto del Cóndor Formation, which crops out in the Cordillera Oriental of northwestern Argentina. This unit displays most of the diagnostic sedimentary attributes of estuarine environments. Biotic components include low-diversity trace fossils and a peculiar bivalve fauna consisting of the new genera Konduria, Pseudoredonia, and Pucamya, and the new species Redonia condorensis. This constitutes the earliest known occurrence of bivalves in brackish waters, suggesting that the capability of this clade to colonize estuarine environments developed early in their radiation.

Volcanology, geochemistry and structure of the Ordovician Cargo Volcanics in the Cargo – Walli region, central New South Wales

The Middle to lower Upper Ordovician Cargo Volcanics occur in a structural outlier immediately west of the contiguous Molong Volcanic Belt of the Ordovician Macquarie Arc. The sequence of basaltic to dacitic lavas, lava breccias and associated volcaniclastic rocks with medium-K calc-alkaline affinities has been interpreted as the subaqueous portion of a major intra-oceanic arc stratovolcano. The oldest part of the succession consists of massive to pillowed, poorly vesicular, aphyric to moderately plagioclase + clinopyxroxene-phyric basalt and andesite lavas and hypabyssal sills. The main part of the interpreted volcanic edifice is composed of massive and pillowed vesicular andesite and dacite, flanked by lenses of hyaloclastite, pillow breccias and interbedded with debris-flow and turbiditic deposits of crystal-rich and pebbly volcanic sandstones, siltstone and minor conglomerate. Uplift and erosion of the edifice prior to deposition of the overlying Eastonian Barrajin Group limestones is indicated by local aprons of thickly bedded subaqueous volcanic conglomerate and pebbly sandstone which appear to be separated from both the underlying andesite-dominant pile and overlying limestones by angular unconformities. The Cargo Volcanics, and the neighbouring Walli Volcanics further south, are clearly distinguished from other Macquarie Arc lavas by their high (>25) Zr/Nb values. However, the Walli Volcanics are readily differentiated from the Cargo Volcanics by their higher P2O5 contents and likely high-K calc-alkaline to shoshonitic affinities. The Cargo Volcanics are intruded by Cu – Au mineralised, plagioclase + hornblende + quartz-phyric dacites with medium-K calc-alkaline affinities. These dacites have lower TiO2 and higher MgO contents at any SiO2 level compared to the Cargo Volcanics, and are compositionally similar to Late Ordovician to Early Silurian (453 – 441 Ma) dacites at Copper Hill and in the Narromine Complex. However, at Cargo, clasts derived from the dacites are locally abundant in volcaniclastic conglomerate near the top of the Cargo Volcanics, indicating that the dacites were intruded and exhumed prior to deposition of the Barrajin Group limestones, which commenced at ca 454 Ma in this area. The dacites at Cargo are intruded by small, apparently unmineralised monzonitic intrusions with shoshonitic affinities, which also appear to pre-date deposition of the Barrajin Group. LA-ICPMS U – Pb dating of detrital zircons from the Ranch Member at the base of the Daylesford Limestone (basal Barrajin Group) revealed a dominant population with an average age of 453.0 ± 4.1 Ma (identical within error to the mid-Ea1 age for the host sediments based on fossil assemblages) and subordinate populations with average ages of 480 Ma and 505 Ma. The 453 Ma zircons were most probably derived from either the intrusive dacites or monzonites, suggesting that the Cargo region was rapidly exhumed following emplacement of the felsic intrusions. The angular unconformities near and at the top of the Cargo Volcanics and the broadly coincident change in magma chemistry suggest that this sector of the Macquarie Arc underwent major tectonic upheaval commencing in the latest Gisbornian – early Eastonian. The Cargo Volcanics are interpreted to have undergone major eastward translation at this time (from an original position now buried beneath the Cowra Trough). Their present juxtaposition with the contiguous Molong Volcanic Belt probably did not occur until the late Early Silurian during the final stages of the Benambran Orogeny.

Lower Ordovician (Chewtonian to Castlemainian) Radiolarians of Spitsbergen

Synopsis Diverse and well‐preserved radiolarian faunas are found in a biostratigraphically precisely dated succession in the Lower Ordovician of Spitsbergen. The faunas are from the Didymograptellus bifidus and Isograptus victoriae lunatus graptolite Biozones. They document the transition from the Upper Cambrian to Lower Ordovician spicular dominated to spumellarian dominated Middle and Upper Ordovician faunas and yield important data for a more precise biostratigraphical use of Ordovician radiolarians. The faunas are dominated by early spumellarians of the Antygoporidae and Aspiculumidae, while the Protoentactiniidae and Palaeospiculumidae are rare elements surviving from the earlier Palaeozoic. Members of the Echidninidae have not been found. The precise definition of a number of important structural characters of early Palaeozoic radiolarians is discussed. The early spumellarians of the family Antygoporidae possess a distinct microsphere formed from a number of curved bars and provided on the outside with a variable number of outer spines, representing the oldest known type of microsphere. The development of the spherical skeletons of spumellarian and entactinarian radiolarians is developed in a number of different ways that are important for phylogen‐etic considerations. The following taxa are new: Svalbardospiculum gen. nov., Svalbardospiculum arenigium sp. nov., Protoentactinia bifurcata sp. nov., Sphaeroentactiniidae fam. nov.,Sphaeroentac‐tinia gen. nov., S. hexaspinosa sp. nov., S. bispinosa sp. nov., S. integrata sp. nov., Nyfrieslandia gen. nov., N. sphaeroidea sp. nov., N. complicata sp. nov., Parechidnina densa sp. nov., Antygoporidae fam. nov., Antygopora bella sp. nov., A. compacta sp. nov., A. microspina sp. nov., A. labyrinthina sp. nov., Labyrinthia gen. nov., Labyrinthia inexpectata sp. nov.

Eobucania (Mollusca) from the Furongian (late Cambrian) of Sweden

The isometrically coiled mollusc Eobucania Kobayashi, 1955 is described from the Peltura scarabaeoides Zone (Furongian; late Cambrian) of Kinnekulle, southern Sweden, representing the oldest record of the genus and the first from the Cambrian of Sweden. While originally described from the Lower Ordovician strata within the McKay Group of British Columbia, Canada, additional species of Eobucania have been reported from the Lower Ordovician of Mexico, France and the Czech Republic, and ostensibly from the Lower Devonian of China. Eobucania has been widely interpreted as a bellerophontoidean gastropod, but placement within the monoplacophoran Class Tergomya is possible.

Paraconodont Westergaardodina in the Lower Ordovician of the Prague Basin, Czech Republic

Paraconodont Westergaardodina in the Lower Ordovician of the Prague Basin, Czech Republic

Palynostratigraphy and palaeogeography of the Cambro-Ordovician strata in southwest of Shahrud City (Kuh-e-Kharbash, near Deh-Molla), Central Alborz Range, northern Iran

A total of 66 surface samples from the Mila, Lashkarak and Ghelli formations of Kuh-e-Kharbash (near Deh-Molla) were palynologically investigated, to determine the geological age of the rock units. This study was also undertaken to assess the palaeogeographic relationships of the study area to Southern and Northern Hemispheres during the Palaeozoic interval represented by these formations. 52 palynomorphs (51 acritarchs and one algal body) were recorded, which permit the recognition of six acritarch-based biostratigraphic zones. Acritarch assemblage zone I is Late Cambrian in age and occurs in the upper part of the Mila Formation. Assemblages zones II through IV are present in the Lashkarak Formation and suggest Lower Ordovician (Tremadoc–Arenig). Assemblage zones V and VI are present in the Ghelli Formation and indicate Upper Ordovician (Caradoc–Ashgill) for this formation. Based on palaeontological data, two hiatuses are present within the studied stratigraphical column. The first hiatus occurs between the Lashkarak and Ghelli formations and encompasses the Middle Ordovician strata. The second hiatus is present between the Ghelli and Geirud formations and includes the whole Silurian and Lower–Middle Devonian deposits. The above-mentioned hiatuses possibly correspond to the Caledonian Orogeny. Diverse acritarch assemblages in the Late Cambrian (Mila Formation), Lower and Upper Ordovician (Lashkarak and Ghelli formations) indicate that a marine environment continued through the entire succession. Comparison of acritarch taxa recorded in Kuh-e-Kharbash (near Deh-Molla) with those reported from other parts of the world, suggests that the Alborz Mountain Ranges have been part of peri-Gondwanan palaeoprovince during the Ordovician.

AnOistodus venustus-like conodont species from the Middle Ordovician of Baltoscandia

Ordovician conodont specimens resembling Oistodus venustus STAUFFER, 1935 have been reported from many areas. There is increasing evidence, however, that several lineages with homeomorphic conodont elements have erroneously been referred to one and the same species. I have investigated Baltoscandian conodont elements of this kind in order to find out about their origins and phylogenetic relationships with morphologically comparable elements from other areas. A natural grouping of finds from the Middle and lower Upper Ordovician of Baltoscandia is here described as belonging to a new species of a new genus, Venoistodus balticus n. gen., n. sp. The new species probably evolved paedomorphically from Drepanoistodus forceps (LINDSTROM, 1955) in the Early Ordovician. (Less)

SHELL HETEROSTROPHY IN EARLY ORDOVICIAN MACLURITELLA KIRK, 1927 AND ITS IMPLICATIONS FOR PHYLOGENY AND CLASSIFICATION OF MACLURITOIDEA (GASTROPODA)

Study of the oldest macluritid gastropod, Macluritella stantoni Kirk, 1927 from the Lower Ordovician of Colorado, has revealed that its early whorls are openly and dextrally coiled, in contrast to those in later teleoconchs which are sinistrally coiled. This is the first documentation of heterostrophic coiling in members of the Macluritoidea, which have been considered to be dextrally hyperstrophic. Juvenile M. stantoni may be interpreted as dextrally orthostrophic and, thus, it had the same type of soft-body-shell arrangement as the vast majority of living and fossil gastropods. This intepretation also suggests that the Macluritoidea evolved from the dextrally orthostrophic gastropods, and their dextral hyperstrophy is derived and not a primary feature. In addition, occurrence of shell heterostrophy in M. stantoni brings additional evidence that the Macluritoidea and Onychochiloidea are not closely related taxa. Relationships between the Macluritoidea and Euomphaloidea are still uncertain. This study provides the oldest evidence (Early Ordovician) for shell heterostrophy in the class Gastropoda.