3. Discussion and conclusions

The fossil record of basal iguanodonts (i.e. non-hadrosaurid members of Iguanodontia; McDonald, Reference McDonald2012a ) is well documented in the Early Cretaceous of Western Europe, with most of the material coming from England (Norman, Reference Norman and Batten2011), Belgium (Norman, Reference Norman1980), Germany (Norman, Reference Norman1987), France (Martin & Buffetaut, Reference Martin and Buffetaut1992) and Spain (for an exhaustive review of the Spanish occurrences, see Ruiz-Omeñaca, Canudo & Cuenca-Bescós, Reference Ruiz-Omeñaca, Canudo and Cuenca-Bescós1998). Until recently, almost all iguanodontian remains found in the Early Cretaceous of Europe had been assigned to the classic genus Iguanodon. At present, this group has an intricate taxonomy because of the erection of several new genera and species during the last five years: Mantellisaurus Paul, Reference Paul and Carpenter2007; Dollodon Paul, Reference Paul2008; Owenodon Galton, Reference Galton2009; Barilium Norman, Reference Norman2010 (objective senior synonym of Torilion Carpenter & Ishida, Reference Carpenter and Ishida2010); Hypselospinus Norman, Reference Norman2010 (objective senior synonym of Wadhurstia Carpenter & Ishida, Reference Carpenter and Ishida2010); Proplanicoxa Carpenter & Ishida, Reference Carpenter and Ishida2010; Sellacoxa Carpenter & Ishida, Reference Carpenter and Ishida2010; Kukufeldia McDonald, Barrett & Chapman, Reference McDonald, Barrett and Chapman2010 and Delapparentia Ruiz-Omeñaca, Reference Ruiz-Omeñaca2011. According to Norman (Reference Norman and Batten2011) and McDonald (Reference McDonald2012b ), Iguanodon bernissartensis and Mantellisaurus atherfieldensis are the only two taxa of large-bodied iguanodonts present in the Barremian–Aptian of Belgium and England, Dollodon bampingi being considered a junior synonym of the latter by these authors.

Concerning the foot anatomy of basal iguanodonts such as Iguanodon and Mantellisaurus, there are only a few detailed descriptions of associated, sometimes fully articulated, pes elements. The holotypic material of Mantellisaurus atherfieldensis (NHMUK R5764) includes almost all pes elements (Hooley, Reference Hooley1925; Norman, Reference Norman1986, Reference Norman and Batten2011). The Bernissart specimen assigned to Mantellisaurus atherfieldensis (IRNSB 1551) also provides information on the foot anatomy of this taxon. The well-preserved, articulated specimen NHMUK R1829 from the Barremian of the Isle of Wight, referable to Mantellisaurus atherfieldensis (Norman, Reference Norman and Batten2011), was originally described by Owen (Reference Owen1858a , Reference Owen b ) then re-figured by Naish & Martill (Reference Naish, Martill, Batten and Naish2001) as Iguanodon sp. Another well-preserved specimen (NHMUK R28685), also from the Barremian of Isle of Wight, was described by Hulke (Reference Hulke1882) as Iguanodon seelyi, a junior synonym of Iguanodon bernissartensis following Norman (Reference Norman and Batten2011) and McDonald (Reference McDonald2012b ). The foot anatomy of Iguanodon bernissartensis is also well-known on the basis of the material from Bernissart, especially thanks to the description of the holotype IRNSB 1534 by Norman (Reference Norman1980).

As underlined by Norman (Reference Norman and Batten2011), the hindlimb bones, including the lower leg elements, are not particularly distinctive. However, some differences can be noted between Mantellisaurus and Iguanodon regarding the size, proportion, and morphology of the various pes bones. The large species Iguanodon bernissartensis has a foot made of robust elements. The metatarsal I is reduced and laterally flattened. The length of the metatarsal III ranges from 270 to 350 mm among the specimens from Bernissart (Norman, Reference Norman1980), this length roughly corresponds to 1.5 to 2 times the value observed in the Las Hoyas specimen. The smaller taxon Mantellisaurus atherfieldensis has a more gracile foot, with more slender bones. The metatarsal I (preserved in NHMUK R1829) is narrow and has a rod-like shape, similar to the condition observed in LH-30677. In Mantellisaurus atherfieldensis, the metatarsal III is 280 mm in length. The phalangeal formula and the way in which the metatarsals II to IV are arranged in LH-30677 are the same as those in Mantellisaurus and Iguanodon. Until more material is recovered, the Las Hoyas iguanodont is tentatively assigned to Mantellisaurus atherfieldensis because of the relative proportions of the foot. However, some slight differences can be noted in the Las Hoyas specimen: the foot is more lightly built; the metatarsal IV is relatively enlarged in relation to the narrow metatarsal II; and the unguals are relatively small compared to other phalanges. Such differences may be ontogenetic, but could also suggest that the Las Hoyas iguanodont represents a new, smaller taxon.

The specimen LH-30677 was collected in situ from a taphonomically controlled area named ‘Magenta’. The fossil was apparently isolated and was showing the top part of the metatarsal III embedded in a thick (~7 cm) slab. Although no additional material has been recovered from around this specimen to date, the rest of the skeleton might be preserved in the still unexcavated part of the ‘Magenta’ area. This hindlimb was probably separated from the fleshy carcass and was buried before it could be scattered. Complete to sub-complete isolated limbs remaining in articulation and preserved more or less far away from the rest of the skeleton (or inversely a nearly complete articulated skeleton missing some limbs, e.g. the crocodyliform Susisuchus from the Cretaceous Crato Formation in Brazil; Salisbury et al. Reference Salisbury, Frey, Martill and Buchy2003) are relatively frequent in the fossil record. This kind of preservation, corresponding to the taphonomic subclass IIB defined by Holz & Barberena (Reference Holz and Barberena1994), often occurs when subaerial processes lead to mummification (i.e. dehydrated carcasses). Mummification is not unusual at Las Hoyas (e.g. the albanerpetontid Celtedens; McGowan & Evans, Reference McGowan and Evans1995) and that phenomenon might be linked to the occurrence of some extreme temperatures during the warm and dry seasons (Buscalioni & Fregenal-Martínez, Reference Buscalioni and Fregenal-Martínez2010).

More or less well-preserved iguanodont remains are not rare in lower stratigraphic sequences (Rambla de Las Cruces I) of the La Huérguina Formation (Buscalioni et al. Reference Buscalioni, Fregenal, Bravo, Poyato-Ariza, Sanchíz, Báez, Cambra-Moo, Martín-Closas, Evans and Marugán-Lobón2008). Some isolated vertebrae and a femur were reported from the nearby locality of Buenache de la Sierra (see Ruiz-Omeñaca, Canudo & Cuenca-Bescós, Reference Ruiz-Omeñaca, Canudo and Cuenca-Bescós1998). LH-30677 represents the first discovery of an ornithopod dinosaur in the Las Hoyas limestone and the third non-avian dinosaur body fossil. As mentioned above, the only occurrences of non-avian dinosaurs at Las Hoyas correspond to a few medium-sized dinosaur footprints (15 to 35 cm in length) and two well-preserved skeletons, corresponding to 2–2.5 m (Pelecanimimus) and 6 m (Concavenator) long animals. The Las Hoyas iguanodont would correspond to an individual about 5.5 to 6 m long. This bias (i.e. the very low relative abundance of medium to large-sized animals) has also been noted for the Late Jurassic locality of Guimarota (Martin, Reference Martin, Martin and Krebs2000), suggesting that heavy dinosaurs probably avoided the soft substrate of such a coastal palaeoenvironment. The Las Hoyas deposits have been interpreted as a rather isolated carbonate shallow subtropical lake located in a complex wetland system (Buscalioni & Fregenal-Martínez, Reference Buscalioni and Fregenal-Martínez2010). Thus, a similar hypothesis can be applied for the muddy palaeoenvironment of Las Hoyas. This rareness supports the notion of facultative to incidental categories, i.e. organisms that may be found in wetlands and in the terrestrial surrounding environments, or only occasionally found, respectively (Buscalioni & Fregenal-Martínez, Reference Buscalioni and Fregenal-Martínez2010). Thus, considering the Las Hoyas iguanodont as a facultative organism is not surprising considering the large number of terrestrial plants (e.g. ferns and conifers mostly represented by Weichselia and Frenelopsis, respectively) present in that palaeoecosystem (Gomez et al. Reference Gomez, Martín-Closas, Méon, Thévenard and Barale2001). Lastly, an ornithopod dinosaur such as Mantellisaurus may have represented a valuable prey item for the top predator Concavenator, thus fulfilling the reconstruction of the trophic chain of this ancient ecosystem.