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| Istiorachis macarthurae Lockwood, Martill & Maidment, 2025 Original artwork by James Brown. |
Abstract
The Wealden Group of southern England was deposited during the late Berriasian to early Aptian interval. It records a critical time in the development of iguanodontian dinosaur diversity, which increased from low levels during the Jurassic to higher levels in the Aptian and Albian. A new iguanodontian dinosaur, Istiorachis macarthurae gen. et sp. nov. from the Wessex Formation (Wealden Group) of the Isle of Wight, exhibits hyperelongation of the dorsal and caudal neural spines, suggesting that it possessed a possible sail structure. Ancestral state reconstruction for the relative height of dorsal neural spines in iguanodontians demonstrates that modest elongation began with Ankylopollexia in the Late Jurassic and elongation became established during the Berriasian stage of the Early Cretaceous, albeit with widely disparate values. Hyperelongation of neural spines occurred more sporadically throughout the Cretaceous, being recorded most often in the Barremian and early Aptian. Possible explanations for neural spine elongation in Ankylopollexia include biomechanical advantage, perhaps related to greater mass and a locomotory shift towards quadrupedalism, and visual signalling driven either by sexual selection or species recognition, or both. The function of elongate neural spines was probably pluralistic and differed in different taxa. No single explanation fully supports the variation seen throughout the Cretaceous.
Keywords: Iguanodontia, diversity, sexual signalling, Isle of Wight, ossified tendons, Barremian
SYSTEMATIC PALAEONTOLOGY
DINOSAURIA Owen 1842
ORNITHISCHIA Seeley 1888
ORNITHOPODA Marsh 1881
IGUANODONTIA Baur 1891
ANKYLOPOLLEXIA Sereno 1986
STYRACOSTERNA Sereno 1986
Genus Istiorachis nov.
Derivation of name: The name is derived from the Ancient Greek words ἱστίον (istion), meaning a sail, and ῥάχις (rachis), the spine or backbone. It refers to the probable sail-back appearance of the dinosaur.
Istiorachis macarthurae sp. nov.
Derivation of name: The species name honours Dame Ellen MacArthur, an English sailor who in 2005 set a world record for the fastest solo non-stop voyage around the world on her first attempt and who also founded the Ellen MacArthur Cancer Trust for young people on the Isle of Wight.
Holotype: MIWG 6643 is a partial skeleton composed of the following elements: one cervical vertebra, eight dorsal vertebrae, three dorsal rib heads, a partial sacrum, seven caudal vertebrae, both pubes and both ischia.
Location & horizon: MIWG 6643 was excavated by the late Mr Nicholas Chase from the ‘Black Band’ (bed L11 in Stewart 1978), a plant debris bed cropping out c. 100 m east of Grange Chine, lying above the Grange Chine Sandstone in the Wessex Formation. The site is a c. 1.5-m-thick bed that has occasionally yielded dinosaur remains, including IWCMS 1997.550, the holotype specimen of the tyrannosauroid theropod Eotyrannus lengi (Hutt et al. 2001; Naish & Cau 2022). Unfortunately the excavation site was poached and an unknown amount of the skeleton was taken before collection could be completed.
Diagnosis: Istiorachis macarthurae differs from all other iguanodontians by possessing one autapomorphy, namely two anterior parasagittal tuberosities present on the ventral surface of a posterior dorsal vertebra, marking a change from vertebrae with a ventral keel to a flat surface. A posterior cervical vertebra has a damaged anteroventral process at the base of the neural spine, potentially representing a second autapomorphy. Istiorachis macarthurae also possesses the following features, which, although not unique to the taxon, occur in a unique character combination. An interpostzygapophyseal fossa and tubular cavity is located between the origin of the postzygapophyses and above the neural canal: a similar fossa is also seen in ...
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| Istiorachis macarthurae gen. et sp. nov. (MIWG 6643). Life restoration. Original artwork by James Brown. |
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| Istiorachis macarthurae gen. et sp. nov. holotype (MIWG 6643). Skeletal reconstruction. Scale bar represents 500 mm. |
Jeremy A. F. Lockwood, David M. Martill and Susannah C. R. Maidment. 2025. The Origins of Neural Spine Elongation in iguanodontian dinosaurs and the Osteology of A New sail-back styracosternan (Dinosauria, Ornithischia) from the Lower Cretaceous Wealden Group of England. Papers in Palaeontology. DOI: doi.org/10.1002/spp2.70034 [21 August 2025]