A second specimen of the enigmatic Wealden reptile Yaverlandia

The Lower Cretaceous Wessex Formation, best known for its exposures along the south-west coast of the Isle of Wight (southern England, UK), is famous for its rich, diverse and well documented dinosaur fauna (Martill and Naish, 2001; Naish and Martill, 2007; Batten, 2011). Ornithopods, thyreophorans and sauropods are represented by substantial remains, and Wessex Formation theropods include spinosaurids, carcharodontosaurian allosauroids, tyrannosauroids and possible compsognathids and maniraptorans (Naish et al., 2001; Naish, 2002, Naish, 2011; Naish and Martill, 2002, Naish and Martill, 2007; Brusatte et al., 2008; Benson et al., 2009; Barker et al., 2021; Naish and Cau, 2022). Of the several smaller Wessex Formation theropods that have been named (Calamospondylus oweni, Thecocoelurus daviesi, Aristosuchus pusillus, Calamosaurus foxi and Ornithodesmus cluniculus), all are known from fragmentary and mostly non-overlapping remains such as cervical vertebrae (Naish and Martill, 2002) or sacral vertebrae preserved with adjacent pelvic elements (Naish, 2002). This has resulted in a convoluted history of suggested synonymy and a tradition of referring new specimens to one or other of these taxa (see Naish, 2011). The Wessex Formation has also yielded a reasonable number of non-dinosaurian reptile groups whose taxa overlap in size with dinosaurs, including testudines (e.g., Milner, 2011) and crocodyliforms (e.g., Salisbury and Naish, 2011).

Among the most enigmatic of Wessex Formation taxa within the context of this reptile diversity is Yaverlandia bitholus Galton, 1971, named for a partial skull-roof conventionally interpreted as that of an unusual pachycephalosaurian dinosaur. The Y. bitholus holotype (MIWG 1530) was discovered at Yaverland on the south-east coast of the Isle of Wight (Fig. 1) by Mr F. M. G. Abell in 1923 and reported by Watson (1930), who suggested similarities with ‘Proodon’, a presumed misspelling of Troodon, a taxon then regarded as synonymous with the pachycephalosaur Stegoceras. This suggestion was developed in full by Galton (1971) who both described and named the specimen. The holotype consists of two fused frontals, each of which has a low dome, separated along the midline by a shallow sulcus. A sub-rectangular ‘rostrum’ projects anteriorly and is formed from fused processes that emerge from each frontal. The dorsal surface possesses a covering of tiny foramina. The lateral surfaces of the ‘rostrum’ are covered by concavities, and the ventral surfaces of the frontals possess a bilobed cerebral concavity, hourglass-shaped olfactory tract and bilobed anterior concavity for the olfactory lobes (Naish, 2011). A possible fragment of orbitosphenoid is attached to the ventral surface.

Seen within the context of phylogenetic hypotheses established for pachycephalosaurs (Sereno, 2000; Williamson and Carr, 2003; Maryańska et al., 2004), Yaverlandia is an enigma: its claimed pachycephalosaurian identity was based on the presence of thickened, domed frontals with complete intrafrontal fusion (Galton, 1971), a suite of characters long considered limited to the dome-skulled clade Pachycephalosauridae (though now known to be of broader distribution due to Zavacephale rinpoche from the Aptian–Albian of Mongolia; Chinzorig et al., 2025). However, Yaverlandia is unlike all pachycephalosaurs – that is, supposedly early-diverging, flat-skulled ‘homalocephalid’ taxa as well as dome-skulled ones – in lacking a parietal contribution to the domed region of the skull, in possessing individually domed frontals, frontals that form the lateral margins to the orbits, a thin olfactory tract, small, subparallel olfactory tracts, and a short and broad cerebral concavity where the spaces for the hemispheres are separated by a median ridge (Hopson, 1979; Giffin, 1989; Sullivan, 2000, Sullivan, 2003). It should be added that evaluating the polarity and distribution of these characters is made difficult by the fact that ideas on the phylogeny and systematics of this group are controversial, with some workers arguing that flat-skulled early-diverging taxa are not early-diverging at all, but ontogenetic forms of dome-skulled taxa (Longrich et al., 2010; Schott et al., 2011). Yaverlandia's Early Cretaceous (Barremian) European occurrence is also unusual within the context of a pachycephalosaurian identity, since virtually all other members of the group are from the Upper Cretaceous of eastern Asia and western North America.

On the basis of these inconsistencies, Sullivan, 2000, Sullivan, 2003 argued that Yaverlandia should be removed from Pachycephalosauria, although he refrained from proposing an alternative identification. Overlooked until recently is that Yaverlandia possesses several characters suggestive of a theropod affinity, and specifically a coelurosaurian one: the bilobed cerebral concavity and form of the olfactory tract and concavities for the olfactory bulbs are similar to those of maniraptorans (Russell, 1969; Currie, 1985; Rauhut, 2003). Furthermore, the presence of prominent ridges that form both the lateral margins to the cerebral concavity and olfactory tract, and the concave medial margins to the dorsal parts of the orbits is seen elsewhere in coelurosaurs, including ornithomimosaurs and maniraptorans (Rauhut, 2003). MIWG 1530 also possesses a grid-like pattern of vascular impressions on the ventral surface of its cerebral depressions, a condition not unique to coelurosaurian theropods (Evans, 2005) but similar to the one present in ornithomimids, oviraptorids, dromaeosaurids and troodontids (Russell, 1972; Osmólska, 2004). In view of these similarities, Yaverlandia has been suggested to be a theropod: Naish (2011) suggested a troodontid identification. Here, we take the view that a specific identity cannot be established and that substantially better remains are required before the mystery of Yaverlandia's phylogenetic placement can be resolved.

Galton's (1971) description is cursory and does not describe or illustrate several features present in the specimen. Accordingly, a longer manuscript redescribing the holotype specimen is planned (an unpublished redescription, now in need of revision, is included in Naish, 2006, with additional comments in Naish, 2011). Here we report a second specimen, highly similar to the holotype and essentially consisting of the same section of the cranium.