R.B. Chandler

Bio: R.B. Chandler is an academic researcher from Birkbeck, University of London. The author has contributed to research in topics: Ammonite & Emileia. The author has an hindex of 1, co-authored 1 publications receiving 17 citations.

The stratigraphy of the Inferior Oolite at South Main Road Quarry, Dundry, Avon

Abstract: A new section opened on the site of the former South Main Road Quarry, Dundry, Avon, is described. The biostratigraphy of the ammonite succession has been revised through careful bed-by-bed collecting and is presented in a range chart. It is recast in the form of a sequence of discrete ammonite fauna! horizons as has become customary in the classical areas of Somerset and Dorset first described in this way by Buckman. One new ammonite faunal horizon has been identified in the Lower Bajocian: Bj-10b, Sonninia micracanthica (Buckman). This carries several of what have been regarded in the past as leading guide-fossils of the Sauzei Zone. But it is now seen to be also the type horizon of the index of the Laeviuscula Zone, Witchellia laeviuscula. It is therefore retained in the Laeviuscula Zone as its youngest horizon on grounds of nomenclatural stability. The basal boundary of the overlying Sauzei Zone is drawn above it and its lowest ammonite horizon renamed, Bj-11a, Stephanoceras kalum (Buckman). The systematics of some important species based on types from Dundry are reviewed briefly and the origins and phylogeny of some important ammonite genera of the Ovale to Sauzei Zones are discussed. The succession of forms of Witchellia is now separated as a phyletic subfamily Witchelliinae subf. nov. within the polyphyletic clade of the Sonniniidae. A new species of Emileia, E. dundriensis, is described from the Laeviuscula Zone. The nautiloids are also reviewed briefly.

Taphonomic analysis and sequence stratigraphy of the Albarracinites beds(lower Bajocian, Iberian range, Spain).An example of shallow condensed section

Abstract: Several ammonite fossil assemblages from Sierra de Albarracin (Teruel, Castilian Branch) contain abundant specimens of the Ovale and Laeviuscula zones (lower Bajocian, Middle Jurassic), including microconchs and macroconchs of the characteristic genus Albarracinites. Over 1500 ammonites from the type horizon of the species A. albarraciniensis, in the outcrop of Masada Toyuela, have been studied. Ammonites are commonly preserved as phosphatized, calcareous, concretionary internal moulds (mean size = 50.5 mm) of unflattened shells, partially or completely filled with relatively heterogeneous mudstone to wackestone sediment. Incomplete, fragmented phragmocones with calcitic septa are the dominant remains, generally bearing traces of rounding, incrustation and bioerosion. Taphonomic data, such as the predominant taphonomic populations of type 3, composed of reelaborated, relatively heterogeneous concretionary internal moulds, isolated concretionary body chambers and hollow ammonites, bearing traces of abrasion, bioerosion and encrusting organisms, are indicative of low rate of sedimentation and occasional high rate of sediment accumulation, due to sedimentary winnowing and bypassing interrupted by storm depositional events, in shallow-water marine environments. The exceptional occurrence of taphonic populations of type 2 belonging to Albarracinites and Hebetoxyites, with dominant shells of pre-adults and absence of juveniles, suggests autochthonous biogenic production of shells by indigenous populations of Sub-Mediterranean taxa at the Iberian platform system. In the outcrop of Masada Toyuela, the Albarracinites beds correspond to a condensed section, developed in shallow- water, open-marine, carbonate environments of the External Castilian platform. The sharp, irregular base of beds and the normal grading of reworked elements suggest these carbonate deposits were affected by tractive currents, scouring and redeposition. Limestone beds represent storm-related sedimentary events, whereas the local marly intervals represent background-sedimentation time-intervals of winnowing and bypassing on the seafloor. This condensed section is composed of at least four, decimetric or centimetric, expanded-deposit intervals, stacked and showing an overall thinning upward, which constitute a deepening sequence developed during two biochrons. Taphonomic results also corroborate the development of an incipient-deepening phase, which represents the first episode of a deepening half-cycle of third order, in the Albarracin area within the Castilian platform, during the Ovale and Laeviuscula biochrons (early Bajocian).

Revision of the Middle Jurassic dimorphic ammonite genera Strigoceras/Cadomoceras (Strigoceratidae) and related forms

Abstract: We present a modern revision of the dimorphic ammonite pair Strigoceras QUENSTEDT, 1886/Cadomoceras MUNIER-CHALMAS, 1892 from the Middle Jurassic. The stratigraphically oldest hitherto known chronospecies of Strigoceras, S. praenuntium (BUCKMAN) appears in the Middle Aalenian as an extreme rarity. Records of the microconch genus Cadomoceras remain unknown from strata older than Early Bajocian, but otherwise its stratigraphical range corresponds well with that of the macroconchs. The genus probably became extinct in the Early Bathonian (Zigzag Zone). The youngest known Strigoceras is S. callomoni n. sp. described here from the Macrescens Subzone of the Early Bathonian. Another new strigoceratid species, Granulochetoceras oppeliisculptum n. sp., is recorded by a single specimen from the early Garantiana Zone of eastern Spain, representing an interesting phyletic link between Strigoceras QUENSTEDT, 1886 and Granulochetoceras GEYER, 1960. We include a brief review of the latter genus. By including Granulochetoceras in the Strigoceratidae the family would extend to the Late Kimmeridgian of the Late Jurassic. The palaeogeographic distribution of Strigoceras/Cadomoceras is focussed on the western Tethyan Province, but some species also occur in the northern Pacific and its adjacent shelves. A benefit of this large area of distribution is that strigoceratids may be used to correlate distant locations, but this is partly hampered by the relatively wide range of variability within succeeding chronospecies and remarkably large variation in size of adult macroconchs.

Aalenian to Lower Bajocian ammonites from the Qiangtang block (North Tibet)

Abstract: Aalenian and Bajocian ammonites from the Qiangtang block (North Tibet) are described. Comparisons with successions of similar age in Europe allow the establishment of a correlation at zonal level. The relationship between early stephanoceratids and the otoitid Emileites is discussed. Emileites callomoni sp. nov. is erected. Haplopleuroceras , Fontannesia , Witchellia and Sonninia also occur and their significance highlighted. A lectotype for Haplopleuroceras mundum Buckman is designated.

Mollistephaninae and Frebolditinae, new subfamilies of Middle Jurassic stephanoceratid Ammonoidea

Abstract: Two new Bajocian stephanoceratid subfamilies are distinguished based on morpho-structural criteria and phyletic patterns. At the Aalenian/Bajocian transition, Stephanoceratinae of the genus Albarracinites are the source of the earliest species of Mollistephanus and of the new Mollistephaninae lineage that includes three successive genera: Mollistephanus, Paramollistephanus gen. nov. and Phaulostephanus. The Mollistephaninae span across the Mediterranean-Caucasian Subrealm during the lower Bajocian, but Paramollistephanus is pandemic to both the Mediterranean-Caucasian and East Pacific subrealms during the Propinquans Zone. The Frebolditinae evolved from Paramollistephanus in the lower Bajocian, beginning with Freboldites and giving rise to diverse genera such as Parabigotites, Patrulia, Bajocia, Subcollina and Parastrenoceras that occur into the upper Bajocian of both the East Pacific and Mediterranean-Caucasian subrealms. Palaeobiogeographical and phylogenetic data of these two subfamilies support an active Bajocian Central-Atlantic Seaway, the so-called Hispanic Corridor, as a bidirectional, biodispersal route driven by changes of the relative sea level. Several bioevents of appearance, immigration and dispersal, associated with the range expansion of ammonoid taxa, were effective (Paramollistephanus in the latest Laeviuscula Zone, Subcollina in the latest Humphriesianum Zone, and Parastrenoceras in the earliest Niortense Zone). Based on life-history strategy, morpho-structural and functional criteria, the dimorphic Caumontisphinctes-Infraparkinsonia pair seems to be the origin of the Parkinsoniidae. The Mollistephaninae and Frebolditinae show small adult size, scarcity of fossils, and low stratigraphic persistence and constancy; however, they present some pandemic genera of the Tethys-Panthalassa Realm and display high resolution for time correlation between the western Tethyan and eastern Pacific marine basins of separate bioprovinces.