Comments are provided on a published paper on Middle Jurassic Laevaptychus from central Mexico C.Esquivel-Macías, P.Zell, J.A.Moreno-Bedmar and K.Flores-Castro, Giant Middle Jurassic (Bathonian) cf. ...Laevaptychus sp. of the Aztlán section, Hidalgo State, central Mexico, Journal of South American Earth Sciences, 110, 103302. This article describes an interesting finding of large-sized ammonite lower jaws (aptychi referred to Laevaptychus paragenus), claimed as the largest Jurassic aptychi ever known. However, the age of these specimens was erroneously defined due to misidentification of an associated ammonite specimen as Bathonian Procerites. Although poorly preserved, this ammonite shows typical features of the Kimmeridgian genus Idoceras. The Kimmeridgian age of these occurrences is in agreement with findings of Laevaptychus, as this is one of few aptychi formal genera, which belongs to a single ammonite family (Aspidoceratidae). Aspidoceratids appeared in the late Callovian and during the evolution of this lineage maximum sizes of adult specimens and the relative whorl height gradually increased up to Kimmeridgian - Tithonian; only prior to their extinction in early Berriasian, aspidoceratids became uncommon and smaller in size. Laevaptychi are thick-valved aptychi, which have high preservation potential while compared with other aptychi of Jurassic ammonites and their host shells. Giant laevaptychi reported in previous publications (the largest of which reaches 35 cm in length) are briefly reviewed. In adult aspidoceratids the maximum length of aptychi is slightly less than the maximum whorl height. Thus, taking into account the size of the largest aspidoceratid ammonites (up to 85 cm in diameter), the estimated length of the largest laevaptychi can be expected to be ~35–40 cm, which is close to their known record.
•Age of Laevaptychus findings in Mexico revised.•Review of shell and aptychi size oscillations through evolution of aspidoceratid ammonites is provided.
Acanthohoplites aschiltaensis, a late Aptian ammonite originally described from the Caucasus of Dagestan, is recognised from the British Isles for the first time. A study of the full ontogenetic ...development of this species is enabled owing to the preservation of a mature body-chamber, not previously known. Stratigraphically, it confirms the correlation of the Fullers' Earth deposits within the Parahoplites nutfieldiensis Zone at Baulking, Oxfordshire, with strata of the Parahoplites melchioris Zone at the type locality and elsewhere. Palaeobiogeographically, it underlines the strong relationship between the Boreal fauna of western Europe and that of the Mediterranean-Caucasian Subrealm of the Tethyan realm. Additionally, a small Hypacanthoplites spp. fauna is described from the H. rubricosus Subzone of the Hypcanthoplites jacobi Zone at the Aptian/Albian boundary interval exposed in the Shanklin area, Isle of Wight. From this locality, otherwise almost devoid of ammonite remains, we present fairly well-preserved specimens of ‘Hypacanthoplites’ elegans, H. plesiotypicus, H. pricei, H. tenuiformis, H. rubricosus, H. sigmoidalis and H. sp. 1 indet., only known thanks to some decades of intensive collecting. In addition to the other members of this genus previously described from the British Isles, the occurrence of ‘H.’ elegans and H. plesiotypicus further supports the existence of narrow sea connections eastwards onto the European continent.
The sequence exposed at Waaney (South—western Somalia) consists, from bottom to top, of the following horizons: A) Blackish marly limestones, thickness 20 m; B) Grey and greenish marls and shales, ...thickness 10 m; C) Vacuolar yellowish—grey limestone, thickness 6 m; D) and F) Bioclastic, dark grey limestones in beds up to 1 m thick, thickness 54 m; E) and G) Yellowish—brown or Pink limestones with nodular or brecciated structure, with two levels rich in Ammonites, thickness over 45 m. Sedimentological features give evidences that the environment evolved from restricted lagoon with fine terrigenous supply and probable evaporitic episodes to shallow sea with varying energy conditions and eventually to deeper but partially restricted sea with oligotypic fauna. Fossils collected in the E) and G) levels include about 40 specimens of Ammonites of size varying from 6 to- 15 cm, fossilized in a yellow—grey limestone. The Ammonites of the E) level may be referred to the genus Protogrammoceras (?) and to the species Protogrammoceras madagascariense; the Ammonites of the G) level to the Protogrammoceras madagascariense and to the genus Hildaites. The Ammonite association of. the E) level seems to indicate a Lowermost Toarcian age; the association of the G) level an Early Toarcian age, on the ground of biostntigraphic investigations in the Ethiopian—Indo—Malagasy region and in areas outside this Province.
The 6th Kilian Group meeting of the IUGS Lower Cretaceous Ammonite Working Group (the Kilian Group) was held in Vienna, Austria, 20th August 2017. The Group mainly discussed the standard zonation ...that is suitable for and based mainly on ammonite data of the Mediterranean Province of the Mediterranean–Caucasian Subrealm (Tethyan Realm). Some changes have been made on three stages. The uppermost part of the Berriasian is now characterised by the Tirnovella alpillensis Zone, that is subdivided into a lower T. alpillensis Subzone and an upper “Thurmanniceras” otopeta Subzone; the T. alpillensis (Sub-)Zone is here defined by the first appearance datum (FAD) of the index-species. For the Valanginian, the Neocomites premolicus Subzone is introduced in the lower part of the “Thurmanniceras” pertransiens Zone. The FAD of N. premolicus occurs at the base of the “T.” pertransiens Zone. So this subzone starts at the base of the “T.” pertransiens Zone and provisionally ends at the last occurrence of N. premolicus as the upper part of the zone is not characterised by a subzone for the moment. For the Barremian, the former Holcodiscus fallax, Nicklesia didayana, Heinzia communis, Subtorcapella defayae, Coronites darsi, Heinzia caicedi and Anglesites puzosianum horizons have been abandoned. In agreement with systematic positions, the term “auctorum” is deleted for Taveraidiscus hugii, the index-species of the first zone of this stage. The Kotetishvilia compressissima Zone is subdivided into a lower Holcodiscus fallax Subzone and an upper Holcodiscus caillaudianus Subzone. The Barrancyloceras barremense Subzone (upper subzone of the Toxancyloceras vandenheckii Zone) is replaced by the Gassendiceras alpinum Subzone. The Imerites giraudi Zone is kept but its base should be now defined by the FAD of I. dichotomus, which marks the inception of the genus Imerites. For certain stratigraphic intervals the Group discussed and in some cases strengthened the previous correlations between the standard Mediterranean zonation with the different ammonite zonal schemes of other provinces and realms. Some members of the Group presented the possibility of using regional stages as secondary standards, in accordance with the recommendations of the International Commission on Stratigraphy. The report concludes with an appreciation of the French ammonitologist Jean-Pierre Thieuloy deceased in 2017, the nomination of a vice-chair, membership, future work and next meeting of the Kilian Group.
•Summarizing the current state of the Mediterranean standard ammonite zonation.•Outlines the outstanding biostratigraphic and palaeobiogeographic problems.•The standard zonation was modified for the Berriasian, Valanginian, and Barremian.•The standard zonation was compared with the zonal schemes of other realms.•A tribute to the French ammonitologist J-P Thieuloy (deceased in 2017) is made.
Results of clumped isotope, oxygen isotope and elemental (Mg/Ca, Sr/Ca) analyses of exceptionally well-preserved belemnite rostra and ammonite shells from the uppermost Callovian–Upper Kimmeridgian ...(Lamberti–Mutabilis zones) of the Russian Platform are presented. Despite a significant decrease in belemnite δ18O values across the Upper Oxfordian–Lower Kimmeridgian, the clumped isotope data show a constant seawater temperature (ca. 16 °C) in the studied interval. The decrease in belemnite δ18O values and lower δ18O values measured from ammonite shells are interpreted as a result of the salinity decline of the Middle Russian Sea of ca. 12‰, and salinity stratification of the water column, respectively. The postulated secular palaeoenvironmental changes are linked to the inflow of subtropical, saline waters from the Tethys Ocean during a sea-level highstand at the Middle–Late Jurassic transition, and progressive isolation and freshening of the Middle Russian Sea during the Late Oxfordian–Kimmeridgian.
The obtained clumped isotope data demonstrate relative stability of the Late Jurassic climate and a paramount effect of local palaeoceanographic conditions on carbonate δ18O record of shallow epeiric seas belonging to the Subboreal Province. Variations in Mg/Ca and Sr/Ca ratios of cylindroteuthid belemnite rostra, which are regarded by some authors as temperature proxies, are, in turn, interpreted to be primarily dependent on global changes in seawater chemistry.
The paleoenvironmental variations deduced from clumped and oxygen isotope records of the Russian Platform correspond well with changes in local cephalopod and microfossil faunas, which show increasing provincialism during the Late Oxfordian and the Early Kimmeridgian. Based on the review of literature data it is suggested that the observed salinity decrease and restriction of Subboreal basins during the Late Jurassic played a major role in the formation of periodic bottom water anoxia and sedimentation of organic rich facies.
•Clumped, oxygen and elemental analyses of well-preserved carbonate fossils•Constant seawater temperature (16 °C) across the latest Callovian–Kimmeridgian•Salinity decline of Subboreal basins during the Late Jurassic•Increasing provincialism of marine faunas of the Subboreal Province•Palaeoenvironmental changes stimulated local bottom water anoxia.
Eustatic sea-level changes of the Cretaceous are reevaluated based on a synthesis of global stratigraphic data. A new terminology for local/regional or relative sea-level changes (eurybatic shifts) ...is proposed to distinguish them from global (eustatic) sea-level changes, with the observation that all measures of sea-level change in any given location are eurybatic, even when they include a strong global signal. Solid-earth factors that influence inherited regional topography and thus modify physical measures of amplitude of the sea-level rises and falls locally are reviewed. One of these factors, dynamic topography (surface expression of mass flow in the upper mantle on land- and seascapes), is considered most pertinent in altering local measures of amplitude of sea-level events on third-order time scales (0.5–3.0Myr). Insights gained from these models have led to the reconciliation of variance between amplitude estimates of eurybatic shifts in any given region and global measures of eustatic changes. Global estimates of third-order events can only be guesstimated at best by averaging the eurybatic data from widely distributed time-synchronous events. Revised curves for both long-term and short-term sea-level variations are presented for the Cretaceous Period. The curve representing the long-term envelope shows that average sea levels throughout the Cretaceous remained higher than the present day mean sea level (75–250m above PDMSL). Sea level reached a trough in mid Valanginian (~75m above PDMSL), followed by two high points, the first in early Barremian (~160–170m above PDMSL) and the second, the highest peak of the Cretaceous, in earliest Turonian (~240–250m above PDMSL). The curve also displays two ~20Myr-long periods of relatively high and stable sea levels (Aptian through early Albian and Coniacian through Campanian). The short-term curve identifies 58 third-order eustatic events in the Cretaceous, most have been documented in several basins, while a smaller number are included provisionally as eustatic, awaiting confirmation. The amplitude of sea-level falls varies from a minimum of ~20m to a maximum of just over 100m and the duration varies between 0.5 and 3Myr. The causes for these relatively rapid, and at times large amplitude, sea-level falls in the Cretaceous remain unresolved, although based mainly on oxygen-isotopic data, the presence of transient ice cover on Antarctica as the driver remains in vogue as an explanation. This idea has, however, suffered a recent setback following the discovery of pristine foraminiferal tests in the Turonian of Tanzania whose oxygen-isotopic values show little variation, implying absence of glacioeustasy at least in the Turonian. The prevalence of 4th-order (~400Kyr) cyclicity through most of the Cretaceous (and elsewhere in the Paleozoic, Jurassic and Cenozoic) implies that the periodicity on this time scale, presumably driven by long-term orbital eccentricity, may be a fundamental feature of depositional sequences throughout the Phanerozoic.
The present contribution describes a new rich ammonite material from the Aptian-type area, Vaucluse, France, made accessible by the expansion of the wine cellar of Domaine de Château-Blanc at ...Roussillon, located at ca. 8 km northwest of Apt. This temporary outcrop exposed a few meters of sandstones and sandy, glauconitic marls, characterising the fifth unit of the Aptian-type series. The ammonite specimens are preserved as crushed, internal moulds with phosphatised shells, and are assigned to the Acanthohoplitidae Egoianiceras angulatum, Egoianiceras lautum, ?Egoianiceras exiquecostatum, and Acanthohoplites sp., together with poorly-preserved Parahoplitidae (Parahoplites sp.). This assemblage is coeval to the Fallot black shale episodes occurring in the nearby Vocontian Basin, and correlates to the uppermost Epicheloniceras martini Zone of Dutour (2005), or the lower Colombiceras tobleri Zone of Dauphin (2002), an age which challenges the younger ammonite-age calibration previously established for the fifth unit.
atterns of uranium-molybdenum covariation in marine sediments have the potential to provide insights re- garding depositional conditions and processes in paleoceanographic systems. Specifically, such ...patterns can be used to assess bottom water redox conditions, the operation of metal-oxyhydroxide particulate shuttles in the water column, and the degree of water mass restriction. The utility of this paleoenvironmental proxy is due to the differential geochemical behavior of U and Mo: (1) uptake of authigenic U by marine sediments begins at the Fe(II)-Fe(III) redox boundary (i.e., suboxic conditions), whereas authigenic Mo enrichment re- quires the presence of H2S (i.e., euxinic conditions), and (2) transfer of aqueous Mo to the sediment may be enhanced through particulate shuttles, whereas aqueous U is unaffected by this process. In the present study, we examine U-Mo covariation in organic-rich sediments deposited mostly in the western Tethyan region during oceanic anoxic events (OAEs) of Early Jurassic to Late Cretaceous age. Our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regard- ing water mass restriction and the operation of particulate shuttles in depositional systems. These insights will help to address contentious issues pertaining to the character and origin of Mesozoic OAEs, such as the degree to which regional paleoceanographic factors controlled the development of the OAEs.
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