Charlie J. Underwood

High-Resolution carbon isotope stratigraphy of the basal Silurian Stratotype (Dob's Linn, Scotland) and its global correlation

Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silurian System, the choice of Dobs Linn, Southern Scotland, has received criticism due to the difficulties of relating its well-constrained graptolite biostratigraphy to shallow-water sequences elsewhere. Kerogen samples from across the Ordovician-Silurian boundary interval at Dobs Linn have yielded carbon stable-isotope signatures consistent with those recorded elsewhere, in particular showing a clear positive δ13C excursion in the terminal Ordovician. The architecture of the δ 13C curve from Dobs Linn enables very high-resolution stratigraphic subdivision and direct correlation between the deep water Dobs Linn section and time-equivalent carbonate shelf deposits. An integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites, conodonts and shelly faunas has been constructed. This direct correlation shows that the shallow water successions, including the former stratotype candidate at Anticosti Island, are generally incomplete, with hiatuses related to the rapid sea-level changes during the Hirnantian stage. This confirms and greatly increases the global utility of Dobs Linn as a boundary stratotype.

Diversification of the Neoselachii (Chondrichthyes) during the Jurassic and Cretaceous

Abstract The Neoselachii are a monophyletic group including all of the extant sharks and rays. They underwent rapid diversification throughout the Jurassic and Cretaceous, going from low-diversity assemblages of members of extinct orders in the Late Triassic to diverse assemblages containing representatives of most extant clades by the end of the Cretaceous. The known fossil record of Mesozoic neoselachians is composed largely of isolated teeth, with articulated skeletal remains being known from a limited number of sites. The small tooth size of a large proportion of neoselachians, including almost all taxa in existence prior to the mid Cretaceous, led to very poor representation in older publications. Their state of knowledge has improved dramatically since 1970 with the increased use of bulk sampling for isolated dental remains. Despite this, the high proportion of Lazarus taxa from some stages suggests that the state of knowledge is still intermittent. Increase in assemblage diversity throughout the Jurassic and Cretaceous suggests that radiation events resulted in real and dramatic increases in diversity, and that the perceived diversification is not an artifact of poor knowledge. Cladogenesis inferred from the fossil record typically compares more favorably with divergence predicted from molecular analysis, where Batoidea form a discrete basal clade, than with divergence predicted from morphological analysis, where Batoidea are considered a derived crown group within the Squalea. The timing of diversification events is discussed in light of the known fossil record, cladistically generated divergence times, and the paleoenvironmental distribution of faunas.

Shark and ray teeth from the Hauterivian (Lower Cretaceous) of north‐east England

Sampling of hiatal horizons within the Hauterivian part of the Speeton Clay Formation of north-east England has produced teeth of several species of sharks and rays, four of which are previously unnamed. One species of shark, Cretorectolobus doylei sp. nov., and two species of rays, Spathobatis rugosus sp. nov. and Dasyatis speetonensis sp. nov., are named, whilst the presence of an indeterminate triakid shark is also noted. Synechodus dubrisiensis (Mackie) is shown to be a senior synonym of S. michaeli Thies. Although the dasyatid ray and triakid shark are by far the oldest representatives of their respective families, the overall composition of the fauna is considered to resemble more closely assemblages known from the Jurassic than those from upper parts of the Cretaceous.

High palaeolatitude (Hodh, Mauritania) recovery of graptolite faunas after the Hirnantian (end Ordovician) extinction event

Marine shales directly overlying lower Hirnantian (uppermost Ordovician) glacially related sediments in Mauritania (northwest Africa) have produced a rich graptolite fauna spanning the Ordovician–Silurian boundary in an area of high palaeolatitude. The lowermost transgressive sandy shales are barren of graptolites, but overlying shales show a sudden appearance of a diverse fauna indicative of the terminal Ordovician persculptus Zone, suggesting that with improving conditions, colonisation by a relatively cold-tolerant fauna was possible. This fauna is replaced by a low-diversity assemblage dominated by long-ranging taxa, probably representing the basal Silurian acuminatus and atavus Zones. With the extinction of the persculptus Zone fauna, conditions were still hostile to warm water Silurian graptolites, and a Normalograptus fauna was again established. A sudden influx of fairly diverse taxa marks the base of the acinaces Zone and the establishment of a typical Lower Silurian fauna with the establishment of warmer water conditions.

Sharks, Rays and A Chimaeroid from the Kimmeridgian (Late Jurassic) of Ringstead, Southern England

Sampling of a lenticular concentration of vertebrate debris and associated sediments from the lower Kimmeridgian of southern England has allowed the study of a diverse and abundant assemblage of chondrichthyan remains. A number of previously undescribed species are recorded, of which three new species are named; Squatina? frequens, Synechodus plicatus and Protospinax planus. Additional diagnosis of the genus Paracestracion Koken is given to allow its identification from dental remains. Several nominal batoid species are synonymised with Spathobatis bugesiacus Thiolliere. This assemblage is considered to be typical of Middle–Late Jurassic neritic environments, and is compared to other contemporaneous selachian faunas.

Marine vertebrates from the ‘middle’ Cretaceous (early Cenomanian) of South India

ABSTRACT Vertebrate fossils have been known from South Indias Cauvery Basin since the 1840s, but records of marine vertebrates from the late Albian to Turonian Karai Formation have been limited to a single set of ichthyosaur remains. Recent surface collecting and sieving of lower Cenomanian glauconitic mudstones has yielded the first ichthyosaur material reported in India over the last 140 years, as well as a diverse and previously unrecorded shark assemblage. The ichthyosaur material, including several teeth and vertebrae, is assigned to the sole described Cretaceous genus Platypterygius and to the species P. indicus (Lydekker, 1879). Eight species of shark (one squaliformes, two hexanchiformes, and five lamniformes) are recorded. A new hexanchiform genus Gladioserratus is erected, and two new species (Gladioserratus magnus, gen. et sp. nov., and Dwardius sudindicus, sp. nov.) are named. Many of the shark genera within this largely species-level endemic fauna are known from high paleolatitudes elsewhere, with many showing an antitropical distribution, but are absent in Tethyan areas. This first description of the Karai Formation marine fauna documents the previously unappreciated diversity and unique character of Indias Cretaceous marine vertebrates, and indicates a cool-water paleoenvironment for the marine vertebrate assemblage.

Localization of Quaternary slip rates in an active rift in 105 years: An example from central Greece constrained by 234U-230Th coral dates from uplifted paleoshorelines

Mapping, dating, and modeling of paleoshorelines uplifted in the footwall of the 1981 Gulf of Corinth earthquake fault, Greece (Ms 6.9–6.7), are used to assess its slip rate history relative to other normal faults in the area and study strain localization. The 234U-230Th coral ages from Cladocora caespitosa date uplifted shoreface sediments, and paleoshorelines from glacioeustatic sea level highstands at 76, (possibly) 100, 125, 175, 200, 216, 240, and 340 ka. Uplifted Quaternary and Holocene paleoshorelines decrease in elevation toward the western tip of the fault, exhibiting larger tilt angles with age, showing that uplift is due to progressive fault slip. Since 125 ka, uplift rates varied from 0.25 to 0.52 mm/yr over a distance of 5 km away from the fault tip. Tilting was also occurring prior to 125 ka, but uplift rates were lower because the 125 ka paleoshoreline is at 77% of the elevation of the 240 ka paleoshoreline despite being nearly half its age. Comparison of paleoshoreline elevations and sedimentology with the Quaternary sea level curve shows that slip rates increased by a factor of 3.2 ± 0.2 at 175 ± 75 ka, synchronous with cessation of activity on a neighboring normal fault at 382–112 ka. We suggest that the rapid localization of up to 10–15 mm/yr of extension into the narrow gulf (∼30 km wide) resulted from synchronous fault activity on neighboring faults followed by localization rather than sequential faulting, with consequences for the mechanism controlling localization of extension.

A comparison of 103–105 year uplift rates on the South Alkyonides Fault, central Greece: Holocene climate stability and the formation of coastal notches

[1] Measurements of Holocene coastal notch sequences exposed in the footwall of the active South Alkyonides normal fault, Greece, reveal 3 Holocene paleoshorelines near the lateral fault tip, rising in elevation eastward toward the center of the fault, where a 4th paleoshoreline appears. The implied eastward increase in Holocene uplift rate mirrors that for an uplifted Quaternary marine terrace (0.29 mm/yr–0.55 mm/yr from west to east). Assuming these uplift rates were constant through the Holocene, notch elevations predict ages of 650, 1900, 3700 and 6500 years B.P., comparable with published 14 C ages on notch fauna, and well correlated with periods of relatively stable Holocene climate. We propose that the notch sequences formed when post-glacial sea level rise became outpaced by the coastal uplift rate, whilst individual notches formed when stable climate facilitated sustained erosion. The parity of the Holocene and Quaternary uplift rates suggests that notch sequences could be used to characterize long-term patterns of uplift, slip-rate and seismic hazards on active normal faults, if 6500 years is long enough to fully characterize temporal variation in the seismic cycle. Citation: Cooper, F. J., G. P. Roberts, and C. J. Underwood (2007), A comparison of 10 3 –1 0 5 year uplift rates on the South Alkyonides Fault, central Greece: Holocene climate stability and the formation of coastal notches, Geophys. Res. Lett., 34, L14310,

Environmental controls on the distribution of neoselachian sharks and rays within the British Bathonian (Middle Jurassic).

Extensive sampling from a range of facies within the Bathonian (Middle Jurassic) of southern England has allowed the palaeoenvironmental distribution of a number of taxa of neoselachian sharks and rays to be assessed. Faunas were collected from a number of recurrent facies, with different assemblages being characteristic of particular palaeoenvironments. Palaeoenvironmental specificity occurred at both ordinal and specific level. Samples from offshore facies contain high diversity faunas containing members of all neoselachian groups known to have been present in the Middle Jurassic. Shallower water assemblages contain lower diversity faunas lacking Synechodontiformes and Hexanchiformes. Samples from lagoonal facies contain low diversity faunas typically comprising different species from open marine settings. The presence of different taxa within different palaeoenvironments suggests that by the Bathonian neoselachians had differentiated into a wide range of niches and ecologically more diverse than has previously been recognised. Implications for early neoselachian palaeoecology, salinity tolerance and diversification are discussed.

An ancient dental gene set governs development and continuous regeneration of teeth in sharks

The evolution of oral teeth is considered a major contributor to the overall success of jawed vertebrates. This is especially apparent in cartilaginous fishes including sharks and rays, which develop elaborate arrays of highly specialized teeth, organized in rows and retain the capacity for life-long regeneration. Perpetual regeneration of oral teeth has been either lost or highly reduced in many other lineages including important developmental model species, so cartilaginous fishes are uniquely suited for deep comparative analyses of tooth development and regeneration. Additionally, sharks and rays can offer crucial insights into the characters of the dentition in the ancestor of all jawed vertebrates. Despite this, tooth development and regeneration in chondrichthyans is poorly understood and remains virtually uncharacterized from a developmental genetic standpoint. Using the emerging chondrichthyan model, the catshark (Scyliorhinus spp.), we characterized the expression of genes homologous to those known to be expressed during stages of early dental competence, tooth initiation, morphogenesis, and regeneration in bony vertebrates. We have found that expression patterns of several genes from Hh, Wnt/β-catenin, Bmp and Fgf signalling pathways indicate deep conservation over ~450 million years of tooth development and regeneration. We describe how these genes participate in the initial emergence of the shark dentition and how they are redeployed during regeneration of successive tooth generations. We suggest that at the dawn of the vertebrate lineage, teeth (i) were most likely continuously regenerative structures, and (ii) utilised a core set of genes from members of key developmental signalling pathways that were instrumental in creating a dental legacy redeployed throughout vertebrate evolution. These data lay the foundation for further experimental investigations utilizing the unique regenerative capacity of chondrichthyan models to answer evolutionary, developmental, and regenerative biological questions that are impossible to explore in classical models.