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The nautiloid cephalopod order Ascocerida in the British Silurian
Abstract
A brief review of the unusual morphology and ontogeny of the ascoceratid shell is followed by revised description of the meagre British Silurian material assigned already to Ascoceras barrandei Salter and A. vermiforme Blake.
... Ordovician ascocerids occur in Laurentia (e.g., Sweet, 1958;Kesling, 1961, among others, see references in Aubrechtová and Meidla, 2016), Baltica (Strand, 1933;Sweet, 1958;Kröger, 2007Kröger, , 2013, and Avalonia (Evans, 1993). In the Silurian, the ascocerids had a wide geographical distribution (Holland, 1999), having been reported from more regions in Baltica and from Perunica (references cited in Aubrechtová and Meidla, 2016). The ascocerids achieved a relatively high diversity and abundance in specific regions such as Bohemia and Gotland, where they are especially well preserved, as documented in the classic studies by Barrande (1877) and Lindström (1890). ...
... Dzik (1984) refuted the suggestion of ontogenetic truncation, pointing out that postmortem truncation near the base of the body chamber would have been facilitated by the shape of the shell. Holland (1999) mentioned that if the cyrtoconic stage had remained attached to the mature shell, it would have been a disadvantage for forward movement and a serious impediment to backward propulsion. Thus, he accepted the fact of natural truncation at the mature stage, with the apex of the shell appropriately sealed off. ...
... Thus, he accepted the fact of natural truncation at the mature stage, with the apex of the shell appropriately sealed off. However, Holland (1999) is against the Cichowolski et al.-Hirnantian?-Llandovery ascocerids from Paraguay idea of a repeated truncation of the cyrtoconic portion of the shell. ...
Ascocerid cephalopods are described for the first time from high paleolatitudes of Gondwana. Studied material was collected from the Hirnantian?-Llandovery strata of the Eusebio Ayala and Vargas Peña formations, Paraná Basin, southeastern Paraguay. The specimens are poorly preserved and were questionably assigned to the sub-family Probillingsitinae Flower, 1941, being undetermined at genus and species rank because diagnostic characters are not visible. A particular feature seen in our material is the presence of both parts of the ascocerid conch (the juvenile or cyrtocone and the mature or brevicone) joined together, which is a very rare condition in the known paleonto-logical record. The specimens are interpreted as at a subadult stage of development because fully grown ascocerids would have lost the juvenile shell. A planktonic vertical migrant mode of life with a subvertical attitude is proposed for the juvenile, and a horizontal demersal nektonic mode for the adult form, as has been previously suggested. A subvertical orientation near the bottom is proposed for the subadult stage. We suggest that the immigration of asco-cerids to southwestern Gondwana was possible through ocean currents that would carry the planktonic juveniles from low to high latitudes during the end-Ordovician postglacial transgression that flooded the intracratonic basins of the region.
... The ascocerids are rarely found in the Ordovician and Silurian strata of North America (Canada, USA) and Europe (Bohemia, Estonia, Norway, Poland, Sweden, United Kingdom) (for a summary see Miller 1932;Flower 1941;Sweet 1958;Kesling 1961;Furnish & Glenister 1964;Holland 1999). The largest collections of Silurian ascocerids consisting of 190 and 130 specimens were collected and described by Barrande (1865Barrande ( , 1877 from the Prague Basin and Lindström (1890) from the Island of Gotland. ...
... Two species of the genus Ascoceras are mentioned from England by Holland (1999). Ascoceras barrandei Salter, 1858 comes from Stansbatch in Herefordshire, Whitcliffe, and A. vermiforme Blake, 1882 from Ledbury in Herefordshire, Whitcliffe, both from the Whitcliffe Formation (Ludlow Series). ...
... The ascocerids are rarely found in the Ordovician and Silurian strata of North America (Canada, USA) and Europe (Bohemia, Estonia, Norway, Poland, Sweden, United Kingdom) (for a summary see Miller 1932;Flower 1941;Sweet 1958;Kesling 1961;Furnish & Glenister 1964;Holland 1999). The largest collections of Silurian ascocerids consisting of 190 and 130 specimens were collected and described by Barrande (1865Barrande ( , 1877 from the Prague Basin and Lindström (1890) from the Island of Gotland. ...
... Two species of the genus Ascoceras are mentioned from England by Holland (1999). Ascoceras barrandei Salter, 1858 comes from Stansbatch in Herefordshire, Whitcliffe, and A. vermiforme Blake, 1882 from Ledbury in Herefordshire, Whitcliffe, both from the Whitcliffe Formation (Ludlow Series). ...
The early Palaeozoic order Ascocerida is a group of morphologically unique and rare cephalopods known from the
Ordovician and Silurian rocks of Avalonia, Baltica, Laurentia and Perunica. The limited Estonian record of Silurian ascocerids is
complemented with a specimen from the Pähkla locality (Island of Saaremaa; Paadla Regional Stage, Ludlow Series) representing
the stratigraphically oldest known occurrence of ascocerids in the Silurian of Estonia. The strata that were formerly exposed in
Pähkla are likely correlated to the Hemse Group of the Island of Gotland (Sweden) having a remarkable record of ascocerids. The
appearance of Silurian ascocerids in Estonia is confined to a time interval when the group had the highest species diversity and the
widest geographic dispersion, reaching also outside Baltica for the first time.
... Foerste 1930, Miller 1932, Flower 1941, 1963 and Europe (e.g. Barrande 1865, 1867, 1877a, b, Lindström 1890, Dzik 1984, Holland 1999). The group comprises morphologically peculiar cephalopods, in which repeated shell truncation and morphological transitions occurred during ontogeny (see Furnish andGlenister 1964, Turek and for discussion and references; Text- fig. ...
The order Ascocerida K uhn , 1949 includes rare and morphologically unique early Palaeozoic cephalopods, in which periodic shell truncation occurred during ontogeny; mature shells subsequently became inflated, with thin sigmoidal septa and phragmocone chambers situated above the living chamber. The ascocerids are at present known mainly from North America and Baltoscandic Europe. The group was first described by J. Barrande in the mid 1800’s from the upper Silurian of Bohemia. Finds of ascocerid fossils in Bohemia are generally scarce but Barrande’s collection includes tens of well-preserved specimens. These are briefly reviewed in the present paper and additional, more recently collected material is also discussed. In Bohemia (Prague Basin), ascocerids occur in limestones of Ludlow to late Přídolí age. Their maximum diversity and abundance was reached close to the Ludlow/Přídolí boundary interval. Five out of the fourteen currently recognized Bohemian species are also known from late Silurian strata in Sweden (the island of Gotland). The ascocerids thus illustrate palaeobiogeographic relationships between the Prague Basin and Baltica during the late Silurian.
... and its descendants, akin to the development of the dorsomyarian condition that defines the Subclass Orthoceratia. The Subclass Tarphyceratia nov. is considered here to contain the orders Tarphyceratida (Furnish and Glenister 1964b;King 2014) and Ascoceratida (Holland 1999;Aubrechtová and Meidla 2016). See also d and e below The Tarphyceratida are the earliest representatives of the Subclass Tarphyceratia nov. ...
High-level classification of the nautiloid cephalopods has been largely neglected since the publication of the Russian and American treatises in the early 1960s. Although there is broad general agreement amongst specialists regarding the status of nautiloid orders, there is no real consensus or consistent approach regarding higher ranks and an array of superorders utilising various morphological features has been proposed. With work now commencing on the revision of the Treatise Part K, there is an urgent need for a methodical and standardised approach to the high-level classification of the nautiloids. The scheme proposed here utilizes the form of muscle attachment scars as a diagnostic feature at subclass level; other features (including siphuncular structures and cameral deposits) are employed at ordinal level. We recognise five subclasses of nautiloid cephalopods (Plectronoceratia, Multiceratia, Tarphyceratia nov., Orthoceratia, Nautilia) and 18 orders including the Order Rioceratida nov. which contains the new family Bactroceratidae. This scheme has the advantage of relative simplicity (it avoids the use of superorders) and presents a balanced approach which reflects the considerable morphological diversity and phylogenetic longevity of the nautiloids in comparison with the ammonoid and coleoid cephalopods. To avoid potential confusion arising in the higher levels of nautiloid classification employed in the revision of the Treatise Part K, we propose herein to replace the suffix ‘-oidea’ at subclass level with the suffix ‘-ia’. Apart from removing ambiguity and clarifying the nomenclature, this approach also brings greater consistency and affinity with modern zoological classification schemes used for cephalopods. The original Treatise Part K adopted an ‘abbreviated’ form of name for nautiloid orders using the ending ‘-cerida’ rather than ‘-ceratida’ (e.g., Order Actinocerida rather than Actinoceratida). For the revision of Treatise Part K, we propose using the ‘full’ version of the ordinal names. This approach re-employs several order names in their original form, e.g., Ellesmeroceratida, Oncoceratida, and Tarphyceratida. For reasons of consistency, we also apply the same to ordinal names created since the original Treatise Part K; therefore, Order Bisonocerida becomes Bisonoceratida.
The preliminary results of a systematic investigation of the nautiloid faunas from the upper Silurian (Pridoli) Eggenfeld section of the Graz Paleozoic are presented. Afaunal list of the seven genera recognized to date, representing the families Oonoceratidae and Lechritrochoceratidae and subfamilies Michelinoceratinae, Kionoceratinae, Leurocycloceratinae is given and selected taxa illustrated. These genera documentfaunal exchange between the Graz Paleozoic, central Bohemia, the Carnic Alps, Sardinia, France (Montagne Noire), Spain (the Ossa Morena Zone) and Morocco during the late Silurian. The study adds a further contribution to the ongoing systematic description by diverse research groups of Silurian nautiloid cephalopods from the North Gondwana sector within a well defined biostratigraphic framework in order to elaborate their use as a tool for biostratigraphic correlation and paleobiogeographic reconstructions.
Silurian actinocerid cephalopods from the Italian side of the Carnic Alps are reported on the basis of newly collected material. Three actinocerid taxa, belonging to Armenoceratidae, Huroniidae, and Ormoceratidae families, are described and left in open nomenclature.
A nautiloid assemblage collected from Upper Silurian (Pří dolí) limestone horizons at the Rauchkofel Boden section, Carnic Alps, Austria is described. The assemblage includes 13 species that may be assigned generically to Kopaninoceras, Michelinoceras, Orthocycloceras, Sphaerorthoceras, Arionoceras, Columenoceras, Geisonoceras, and Temperoceras. The dark grey micritic fossiliferous limestones from which the nautiloid assemblage was collected form part of the Cephalopod Limestone Biofacies which may be traced all along the North Gondwana margin. The fauna from the Carnic Alps shows close affinities with both Bohemian and south-west Sardinian nautiloid faunas. The paper adds more data to support the idea of faunal exchange between these North Gondwana terranes and Baltica.
The SiO2/Al2O} and K2OM2O 3 ratios are used as proxies for grain size and sea depth changes in a more than 400 m thick succession of Telychian to Ludfordian shale and marlstone of the Priekule core. Good correlation of the increased SiO 2/Al2O3 and K2O/Al2O 3 values in the Priekule section with positive δ13C excursions recognized worldwide suggests that these sea depth changes correspond to global sea level changes. The curves composed indicate a moderate long-range sea level low stand from the Sheinwoodian to the middle Ludfordian, superimposed with larger sea level falls in the lower Sheinwoodian, upper Homerian and middle Ludfordian. The Telychian and upper Ludfordian are characterized by sea level high stands. Higher sulphur contents over 1% and higher contents of carbonates occur close to the intervals interpreted as sea level low stands.
Distribution of nautiloid cephalopods in the Silurian of England, Wales, and Scotland is reviewed stratigraphically and geographically. Genera are considered first and then species. Seven orders are present: Endocerida (one genus), Actinocerida (three), Orthocerida (17), Ascocerida (one), Oncocerida (at least ten), Discocerida (three), and Tarphycerida (seven). Wenlock and Ludlow faunas are more abundant than those of the Llandovery. Records from the Pridoli are very rare.
Sphooceras truncatum (Barrande, 1860), a Silurian straight-shelled cephalopod with a short finger-shaped shell, is one of a few cephalopods in which natural truncation of the apical part of the phragmocone from the rest of the conch is con-firmed. Periodic natural removal of the apical part of the shell (4 to 5 phragmocone chambers) preceded formation of a terminal callus and a calcareous plug closing the septal foramen. The apical callus probably originated by fusion of the truncation septum with episeptal deposits. These structures temporarily formed the new apex on which two additional calcareous layers had been secreted. A unique specimen preserves a colour pattern in the convex apical region, which proves that the shell in Sphooceras was temporarily completely surrounded by mantle extending from the body chamber, i.e. the cephalopod was at least temporarily endocochleate. The co-occurrence of different growth stages of S. truncatum together with one type of short juvenile orthoceracone shell, with a maximum of eight phragmocone chambers and a very small subglobular initial chamber indicates that these embryonic shells may belong to Sphooceras. Two other gen-era are discussed, both previously included in the family Sphooceratidae: Disjunctoceras Gnoli in Kiselev, 1992 and Andigenoceras Gnoli in Kiselev, 1992. The newly discovered thickening of the apex in "Disjunctoceras" disjunctum, the type species of Disjunctoceras, indicates that this species does not differ substantially from Sphooceras and should be reassigned to this genus. Similarly, representatives of Andigenoceras also possess characteristic features of Sphooceras. Sphooceras has many features characteristic for modern cephalopods: short, thin-walled, semi-internal shell; phragmocone reduced to only a few chambers; uncalcified connecting rings; apical callus (a structure analogous to the belemnite rostrum); retractor muscle scars situated dorsally; very small protoconch without cicatrix. In some excep-tionally well-preserved cephalopods with orthoceracone shell radula with seven rows of teeth were observed. All these features support the thesis that some straight-shelled cephalopods are evolutionarily closer to coleoids than nautiloids and their separation from nautiloids is legitimate. Vascular imprints on the surface of the cameral deposits provide fur-ther support for their primary origin and the existence of a cameral mantle. The character of cameral deposits in Sphooceras demonstrates that the systematic value of these structures in other straight-shelled cephalopods, a subject of controversy, has limited value. The morphology of Sphooceras also demonstrates that the boundary between endo-cochleate and ectocochleate cephalopods is not sharp, i.e. internalisation of the shell in cephalopods occurred repeatedly.
Silurian nautiloid-bearing strata (Llandovery–Přídolí) from the Cellon Section of the Carnic Alps (Austria) are compared using the innovative perspective of the color of the enclosing sediment of these fossiliferous beds with coeval regional occurrences along the North Gondwana Margin (Prague Basin, Sardinia, Morocco). Particular attention was placed on distinguishing and comparing the colors of these levels and their general nautiloid faunal composition in order to identify the prevailing features of their occurrence at precise stratigraphic intervals. Red-brown strata usually reflect time-rich or slowly accumulated deposits from a diverse in-situ fauna while black-gray strata are related to single event beds due to episodic deposition probably influenced by currents. Other single event beds of distinctive pink (Gorstian in age) and purple strata (Ludfordian in age) may prove useful as marker horizons.The overall data set available in relation to these diversely colored nautiloid-bearing horizons at the Cellon Section at precise time slices when compared with global eustatic conditions and biotic events has highlighted that these deposits reflect both local phenomena within the depositional basin and regional depositional episodes. These include apparent mass mortality events and the cephalopod limestone biofacies that may be considered as time-specific facies.
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The Midland Valley of Scotland (MVS) forms a rift valley or graben, defined by the parallel Highland Boundary Fault (HBF) and the Southern Upland Fault (SUF), and floored mainly by ORS and Carboniferous sediments and volcanics. Towards its southern margin, and a little distant from the SUF lies a chain of Lower Palaeozoic inliers, extending from Girvan in the west to the Pentland Hills, near Edinburgh, (Fig. 1) and forming areas of high ground. These inliers have been studied now for a very long time, but it is only within the last few years that we have come to understand their history in the context of their tectonic setting, and why they are there.
In the west are the two main inliers of the Girvan district, rich in Ordovician and Silurian fossils, and renowned since the time of Charles Lapworth (1882), Nicholson & Etheridge (1878–80), Peach & Horne (1899) and more recently Williams (1962), Stone & Smellie (1988), and others for their outstanding geological interest. The central inliers of Lesmahagow and the Hagshaw Hills are noted for their exquisite upper Silurian fishes and arthropods (Rolfe 1961, 1992), and now their sedimentary history is becoming clear (Lovelock 1998). The Silurian Carmichael Inlier is poorly exposed and imperfectly known (Rolfe 1960); it is not greatly fossiliferous. To the east lie the three Silurian inliers of the Pentland Hills, and of these it is the large North Esk Inlier (NEI) (Fig. 1) that forms the principal subject of this essay. It has . . .
The nautiloid cephalopod fauna of the Silurian rocks of the Pentland Hills, Scotland is described. Though fragmentary, the available material permits assignment to 14 genera, for 10 of which specific names are given. Nearly all of the material is from the Wether Law Linn Formation and mostly from its lower part. Implications as to stratigraphical age are discussed. The fauna of the Wether Law Linn Formation has something of a Wenlock flavour.
Well-preserved shells of Ashgill (Katian-Hirnantian) and Llandovery nautiloids from Anticosti Island include the type species of 10 genera. From a diversity high of some 40 described nektic and nektobenthic species during the deeper water, distal shelf facies of the Vaureal Formation (late Katian, Richmondian), the succeeding Hirnantian shows a decline to ca. 19 species in the shallower water Ellis Bay Formation. A number of Katian species possessed very large orthoconchs (>1 m in length), but Hirnantian species were less than half that size. The initial earliest Silurian (Rhuddanian, Becscie Formation) recovery nautiloid fauna is impoverished, with low diversity (one sp.?), and generally dwarfed, to shells with diameters of <1 cm. Nautiloid diversity expanded and progressed to some 22 species in the late Aeronian through Telychian Jupiter and Chicotte formations, with an apparent peak in the Goeland Member of the Jupiter Formation. Three new species, Actinoceras lesperancei, Eridites barnesi, and Diestoceras macgilvrayoceras, are described. The distribution of shells within the succession defines the fauna's stratigraphic and paleobiogeographic significance, with a changing mix of Baltic and Laurentian taxa.
Altogether 25 taxa of nautiloids are recognised from the Silurian of Ireland. Of these, less than half can be assigned to genera with any certainty. The fauna contains representatives of the Actinocerida, Orthocerida, and Oncocerida. These are distributed from the Llandovery to the Ludlow Series. The faunal affinities of the Llandovery Series cephalopods from north of the Iapetus suture are unclear. Wenlock and Ludlow Series faunas from south of the suture suggest connections with Scandinavia and the northern Urals, and with central Europe. A number of Ludlow Series species are identical with previously described forms from Wales and the Welsh Borderlands. -from Author
This unique fossil is figured in the ‘Silurian System,’ pl. 21. f. 23. The anomaly presented by it, as a Cephalopod shell having apparently two distinct siphuncular tubes perforating the septa, has always appeared very great; since, however, so many species with large lateral siphuncles are known (Cameroceras and Endoceras), the analogy with these has always offered itself for consideration, and prevented inquiry as to which was the true siphuncle, and what might be the nature of the supplementary tube.
I think, however, that the structure of another and very rare group of Orthocerata will explain the peculiarities of this fossil.
That the remarkable lateral tube is not produced by the intrusion of any smaller Orthoceras into the cavity of a larger one—a circumstance very common indeed among these fossils, and one which has been often commented upend, is evident from the fact that the edges of the septa (a, a) where they abut upon the large tube are decurrent upon it ; so that the perforation is a natural one. Nor is it comparable with the large lateral siphuncle of Cameroceras, seeing that there exists another and a true siphnncle close to it, and the tube itself is not at all annulated as in the shelly siphuncle of species of the group last mentioned.
There is one form indeed of Orthoceras (fig. 4) doubtfully referable to the genus (and which Mr. S. P. Woodward considers an evolute form of Discites§, M'Coy, or Nautiloceras, D'Orbigny) which offers a triangular section
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