doi: 10.1371/journal.pone.0029234. Epub 2012 Jan 3.
New ophthalmosaurid ichthyosaurs from the European Lower Cretaceous demonstrate extensive ichthyosaur survival across the Jurassic-Cretaceous boundary
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- PMID:22235274
- PMCID: PMC3250416
- DOI: 10.1371/journal.pone.0029234
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New ophthalmosaurid ichthyosaurs from the European Lower Cretaceous demonstrate extensive ichthyosaur survival across the Jurassic-Cretaceous boundary
Valentin Fischer et al. PLoS One.2012.
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- PLoS One. 2012;7(1).doi:10.1371/annotation/9731f93a-c28f-4234-8fd9-c587a103b572.
Abstract
Background: Ichthyosauria is a diverse clade of marine amniotes that spanned most of the Mesozoic. Until recently, most authors interpreted the fossil record as showing that three major extinction events affected this group during its history: one during the latest Triassic, one at the Jurassic-Cretaceous boundary (JCB), and one (resulting in total extinction) at the Cenomanian-Turonian boundary. The JCB was believed to eradicate most of the peculiar morphotypes found in the Late Jurassic, in favor of apparently less specialized forms in the Cretaceous. However, the record of ichthyosaurs from the Berriasian-Barremian interval is extremely limited, and the effects of the end-Jurassic extinction event on ichthyosaurs remains poorly understood.
Methodology/principal findings: Based on new material from the Hauterivian of England and Germany and on abundant material from the Cambridge Greensand Formation, we name a new ophthalmosaurid, Acamptonectes densus gen. et sp. nov. This taxon shares numerous features with Ophthalmosaurus, a genus now restricted to the Callovian-Berriasian interval. Our phylogenetic analysis indicates that Ophthalmosauridae diverged early in its history into two markedly distinct clades, Ophthalmosaurinae and Platypterygiinae, both of which cross the JCB and persist to the late Albian at least. To evaluate the effect of the JCB extinction event on ichthyosaurs, we calculated cladogenesis, extinction, and survival rates for each stage of the Oxfordian-Barremian interval, under different scenarios. The extinction rate during the JCB never surpasses the background extinction rate for the Oxfordian-Barremian interval and the JCB records one of the highest survival rates of the interval.
Conclusions/significance: There is currently no evidence that ichthyosaurs were affected by the JCB extinction event, in contrast to many other marine groups. Ophthalmosaurid ichthyosaurs remained diverse from their rapid radiation in the Middle Jurassic to their total extinction at the beginning of the Late Cretaceous.
Conflict of interest statement
Figures
A: articulated nasals in dorsal view. B: left nasal in lateral view. C: right supratemporal in dorsal view. D–F: supraoccipital magnified two times with respect to the other bones, in posterior view (D) and in left anterolateral (otic) view (E,F). Note the lateral wing of the nasal forming an overhang on the naris, the narial process of the nasal, the long and straight squamosal facet of the supratemporal, and the weakly arched shape of the supraoccipital. Abbreviations: avsc: impression of the anterior vertical semicircular canal; pvsc: impression of the posterior vertical semicircular canal; ut: utriculus.
A–C: basisphenoid, in dorsal view (A), in lateral view (B), and in posterior view (C). The thick and claw-like shape of the basipterygoid process in lateral view in shown in B. Note the dorsal crest and the paired facialis (VII) nerve foramen. D,E: left exoccipital, in medial view (D) and ventral view (E). The arrow indicates a notch that matches a small bump on the dorsal surface of the basioccipital, suggesting a close fit of these bones (and therefore a thin cartilage layer) that we interpret as indicator of a mature age. F,G: left stapes, in anteromedial view (F) and posterolateral view (G). Note the slenderness of the shaft compared to the occipital head. H,I: left quadrate, in lateral view (H) and posterior view (I). J,K: fragmentary tooth crowns magnified four times with respect to the other bones. Note the subtle striations and the constriction at the base of the crown in K. Abbreviations: bbf: facet for basioccipital and basisphenoid; bpp: basipterygoid process; hgf: hypoglossal foramina; sof: supraoccipital facet; VII: foramen for the facialis nerve (VII).
A–F: SNHM1284-R, in posterior view (A,D), in dorsal view (B,E), and in ventral view (C,F). G–H: GLAHM132588 (holotype), in lateral view (G) and ventral view (H). Note the markedly concave extracondylar band that separates the condyle from the rest of the basioccipital and the bilobed median concavity for the foramen magnum. The arrow in E indicates the protruding anterior edge of the extracondylar area posterior to the depressed opisthotic facet. Abbreviations: eca: extracondylar area; exf: exoccipital facet; fm: median concavity for the foramen magnum.
A–C:A. densus (GLAHM 132588, holotype), in posterior view (A,B) and anterior view (C). D,E:A. densus (SNHM1284-R), in posterior view. F:Platypterygius australis (QMF14339), in posterior view redrawn from Kear . G:Sveltonectes insolitus (IRSNB R269, holotype), in posterior view, from Fischer et al. . H:Ophthalmosaurus icenicus (NHMUK R4522, mirrored), in posterior view, redrawn from Kirton . I:Ophthalmosaurus icenicus (NHMUK R2161), in posterior view, redrawn from Andrews . Abbreviations: amp: ampulla; avsc: impression of the anterior vertical semicircular canal; hsc: impression of the horizontal semicircular canal; M.a.m.e. facet: facet for attachment of musculus adductor mandibulae externus; sac: sacculus; ut: utriculus.
A:A. densus (GLAHM 132588, holotype), in posterolateral view showing the lateral ridge. B,C:A. densus (SNHM1284-R), in ventral view showing the peculiar thickened lateral foot. D,E:A. densus (GLAHM 132588, holotype), in otic (anteromedial) view. F,G:A. densus (SNHM1284-R), in otic view. H:Ophthalmosaurus icenicus (NHMUK R4523), in otic view, redrawn from Kirton . I:Platypterygius australis (AM F98273), in otic view, redrawn from Kear . J:Mollesaurus periallus (MOZ 2282, holotype), in otic view, redrawn from Fernández . Abbreviations: amp: ampulla; bof: basioccipital facet; hsc: impression of the horizontal semicircular canal; pvsc: impression of the posterior vertical semicircular canal; sac: sacculus; ut: utriculus; vag: vagus foramen.
A: anterior dorsal centrum and associated neural spine of SNHM1284-R, showing the large size of the neural spine with respect to the centrum height. B: series of posterior postflexural centra of SNHM1284-R showing their square shape (H/L≈1). C: series of posterior dorsal centra of GLAHM 132588 (holotype), showing the markedly curved profile of the posterolateral lamella. D: cross-section of a rib of NHMUK R11185 showing their robust morphology and the minute groove occurring on one side only.
Both specimens have incomplete vertebral series and the centra were therefore superposed manually in each region, using the centrum shape and relative position of the apophyses. Each obvious gap in the vertebral column is represented by a single void in the series of SNHM1284-R. The x-axis refers to the centrum count, not its actual position within the vertebral column. The regionalization of the vertebral column is quite weak, in between that ofSveltonectes insolitus andOphthalmosaurus icenicus.
A: coracoids and scapulae of SNHM1284-R in ventral view. B: Outline of the cross-section of the scapula, showing its flattened shape. C: right scapula of GLAHM 132588 (holotype), in ventral view. D: lateral surface of the right coracoid of SNHM1284-R. E,F: comparison of the medial surface of the right scapula of SNMM1284-R (E) and of GLAHM 132588 (holotype; F).
A–D: right humerus of NBM1284-R, in dorsal view (A), proximal view (B), distal view (C), and posterior view (D). E,F: right humerus of GLAHM 132588 (holotype), in ventral view with the associated ulna (E) and in distal view (F). The arrow points at the concave and edge-like posterior surface of the ulna, diagnostic of ophthalmosaurine ophthalmosaurs. G: proximal phalange of GLAHM 132588 (holotype). Abbreviations: aae facet: facet for the anterior accessory element; dpc: deltopectoral crest; td: trochanter dorsalis.
A–C: basioccipital (CAMSM B57962) in posterior view (A), dorsal view (B), and posteroventral view (C), showing the important extracondylar area (delimited by the broken line). The specimen is the only one of the Cambridge Greensand Formation to exhibit a bilobed concavity for the foramen magnum. D,E; basisphenoid (NHMUK R2341) in lateral (view (D) and posterior view (E), showing the facialis nerve foramen posterior to the basipterygoid process and the pentagonal posterior surface because of the dorsal crest that is characteristic for the genus. Abbreviation: bpp: basipterygoid process; eca: extracondylar area; fm: bilobed concavity for the foramen magnum; VII: foramen for the facialis nerve (VII).
A,B: basioccipital (LEICT G3.2001.001), in anterodorsal view (A) and posteroventral view (B). The basioccipital is sheared flat, giving the impression of a ‘Liassic-grade’ basioccipital. C,D: left humerus (LEICT G1.2001.016), in dorsal view (C) and ventral view (D). Abbreviation: aae facet: facet for the anterior accessory element; dpc: deltopectoral crest; eca: extracondylar area; exf: exoccipital fact; fm: median concavity for the foramen magnum.
A: the single most parsimonious tree (length = 101 steps; consistency index = 0.54; retention index = 0.66) in unambiguous optimization. Black circles represent non-homoplastic synapomorphies, open circles represent homoplastic synapomorphies, and numbers above and below circles represent character number and character state, respectively. Ophthalmosauridae rapidly separated in two markedly distinct clades: Ophthalmosaurinae and Platypterygiinae. B: stratigraphy-calibrated phylogeny and bremer support for each branch (white number in plain black circle). The black box represents the stratigraphic range of the species and the gray box expresses the additional range of the corresponding genus. Plurispecific genera (Ophthalmosaurus andPlatypterygius, which may not be monophyletic, see Results–Phylogeny) have their range indicated at the level of one of their species only (O. icenicus andP. hercynicus, respectively). Many genera and lineages cross the JCB, represented by the vertical orange line. Abbreviations: Aal: Aalenian; Baj: Bajocian; Bar: Barremian; Bat: Bathonian; Ber, Berriasian; Cal: Callovian; Cen: Cenomanian; Hau: Hauterivian; Het: Hettangian; Kim: Kimmeridgian; Nor: Norian; Oxf: Oxfordian; Pli: Pliensbachian; Rha: Rhaetian; Sin: Sinemurian; Tit: Tithonian.
A: survival (positive) and extinction (negative) rates in the conservative scenario, in which post-Jurassic occurrences of bothOphthalmosaurus andBrachypterygius are ignored. B: survival (positive) and extinction (negative) rates in the total evidence scenario. C: cladogenesis rate. Light color represents phylogenetic lineages and dark color represents genera. Rates for the JCB are colored in orange. The JCB records high survival rates and low to null extinction rates, suggesting that no extinction took place amongst ichthyosaurs at the end of the Jurassic. Abbreviations: Apt: Aptian; Bar: Barremian; Ber, Berriasian; Hau: Hauterivian; Kim: Kimmeridgian; Oxf: Oxfordian; Tit: Tithonian.
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