Since the 1990s a plethora of titanosaur taxa have been described from the Late Cretaceous (latest Campanian-Maastrichtian) of northeastern Spain and southern France (which constituted the Ibero-Armorican island during the latest Cretaceous) as well as Transylvania, Romania, adding to the hitherto described taxa Hypselosaurus from the Provence region of southern France and Magyarosaurus from Transylvania, Romania. Until recently, however, the interrelationships of lithostrotian taxa from the Late Cretaceous of Europe was not tested within a cladistic framework, with most published cladistic studies including only Ampelosaurus and Lirainosaurus (see cited references in Díez Díaz et al. 2018). The cladistic analysis of Sallam et al. (2018) resolved Ampelosaurus, Lirainosaurus, Lohuecotitan, and Paludititan as saltasaurids closely related to Mansourasaurus from the Late Cretaceous of Egypt, implying a geographic dispersal of some lithostrotians from southern Europe into North Africa during the Late Cretaceous. Meanwhile, a cladistic analysis by Díez Díaz et al. (2018) found Ampelosaurus, Atsinganosaurus, and Lirainosaurus to form a distinct clade within Saltasauridae that they named Liranosaurinae, whereas the taxa Lohuecotitan and Paludititan were resolved as more derived than the basalmost lithostrotian Malawisaurus but cladistically more primitive than derived lithostrotians. This week, another new lithostrotian titanosaur from the latest Cretaceous of southern Europe came hot off the press, named Abditosaurus kuehnei (Vila et al. 2022), and instead of being a dwarf, insular taxon, it surprisingly happens to be a giant for its time and geographical location. Therefore, this post will discuss the implications of Abditosaurus for the paleobiogeography of lithostrotians from the latest Cretaceous of Europe.
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| Walter Georg Kühne (1911–1991), the discoverer of the holotype of Abditosaurus kuehnei |
As noted by Vila et al. (2022; supplementary material), the history of the discovery of the holotype of Abditosaurus kuehnei is rather lengthy and tortuous, hampered by funding issues and bad weather. In September 1954, German paleontologist Walter Georg Kühne, an expert on Mesozoic mammals from Europe and worldwide, prospected Cretaceous outcrops west of the village of Orcau in the Tremp Basin of Catalonia, Spain, with the goal of finding Cretaceous mammal fossils when he unexpectedly found fossils of a titanosaur sauropod. For the next two weeks (September 27 to October 5), he unearthed ten bones at the Orcau-1 site, of which two chevrons, a right tibia, and a distal left femur were collected and sent to the Instituto Lucas Mallada (ILM, now Museo Nacional de Ciencias Naturales [MNCN]) in Madrid, and other remains (e.g. proximal left femur, left humerus, two articulated dorsal vertebrate) were left at the site. Kühne returned to the Orcau-1 site and excavated a complete right femur and humerus, a distal left fibula, the anterior end of a left scapula, a chevron, an indeterminate “short” bone, a few small iliac fragments, a complete dorsal vertebra, and fragments of three dorsal ribs, which were also sent to the ILM. He requested additional excavations at the Orcau-1 site, but a lack of funds caused field work at the locality to be shelved. Lapparent and Aguirre (1956, 1957) considered the titanosaur material from the Orcau-1 site to be a probable new species of the genus Hypselosaurus Matheron, 1869 (now considered a nomen dubium; Le Loeuff 1993). In 1984 and 1986 Josep Vicenç Santafé of the Institut de Paleontologia de Sabadell in Barcelona and members of the Institut d’Estudis Ilerdencs in Lleida revisited the Orcau-1 locality; the 1986 expedition, which lasted for five days, unearthed part of a sternal plate and three dorsal ribs (of which one, MCD-6985, was collected and deposited in the Museu de la Conca Dellà). From 2012 to 2014, members from the Institut Català de Paleontologia, the Universidad de Zaragoza, and the Museu de la Conca Dellà conducted six consecutive expeditions to the Orcau-1 site, unearthing the remaining axial and appendicular remains from the locality and the remains left at the site by Kühne and Santafé, including an articulated cervicodorsal series, additional limb and pectoral girdle remains, tooth fragments, and ribs, all of which were sent to the MNCN. The chevrons (MNCN 59295, MNCN 59539, and MNCN 62760) and humerus (MNCN 79834) unearthed by Kühne were assigned to Titanosauriformes indet. and Titanosauria indet. respectively by Martín Jiménez et al. (2017). The cervicodorsal series of the holotype of Abditosaurus kuehnei constitutes the most complete neck for any titanosaur described from the latest Cretaceous of Europe given that complete or nearly complete necks have been described for very few sauropod specimens (Taylor 2022).
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| Phylogenetic analysis of Abditosaurus kuehnei and other derived lithostrotian taxa (after Vila et al. 2022) |
In their phylogenetic analysis of Titanosauria, Vila et al. recover Abditosaurus as the sister taxon of the Paralititan from the middle Cretaceous (Cenomanian) of Egypt within Saltasaurinae sensu Gorcsak & O'Connor (2019), while Ampelosaurus, Lirainosaurus, Lohuecotitan, and Paludititan are recovered within Opisthocoelicaudiinae and Atsinganosaurus is placed within Lognkosauria as the sister taxon of Notocolossus. Because Sallam et al. (2018) recover all lithostrotian taxa from the Late Cretaceous of Europe except Atsinganosaurus within the same clade as Mansourasaurus, the cladistic analysis by Vila et al. (2022) suggests that more than large/medium-size lithostrotian clade dispersed into Europe from Africa, as Paludititan and Mansourasaurus are recovered by Vila et al. (2022) in a basal position to a clade formed by Ampelosaurus, Lirainosaurus, Lohuecotitan, and the South American taxa Baurutitan, Dreadnoughtus, and Pellegrinisaurus within Opisthocoelicaudiinae sensu Gorcsak & O'Connor. The notion of some Gondwanan tetrapod groups dispersing into the Europe during the late Cretaceous is not new; the assignment of the crocodyliform Doratodon carcharidens to the predominantly Gondwanan clade Notosuchia shows that some notosuchians immigrated to Europe from North Africa (Rabi and Sebők 2015), while abelisauroids are represented in the Late Cretaceous of France by Arcovenator and Tarascosaurus (see Tortosa et al. 2014). Since Opisthocoelicauda is resolved as more closely related to some of the taxa included in Lirainosaurinae by Díez Díaz et al. (2018) than to either Paludititan or Mansourasaurus, and Wang et al. (2021) recover the East Asian taxa Abdarainurus and Huabeisaurus as sister to the basal titanosaur Andesaurus from the middle Cretaceous (Cenomanian) of Argentina, it's reasonable to assume that two lineages of large/medium-bodied lithostrotians dispersed into Europe from Gondwana during Late Cretaceous because Normanniasaurus from the middle Cretaceous (Albian) of northern France and Volgatitan from the Early Cretaceous (Hauterivian) of European Russia are recovered as sister to Colossosauria by Averianov & Efimov (2018) and Mannion et al. (2019), while Mocho et al. (2019) recover two indeterminate titanosaur specimens from the late Aptian-Cenomanian of Spain and Italy as more closely related to Rapetosaurus than to Colossosauria.
When comparing the results of the cladistic analysis by Vila et al. (2022) to the phylogenetic results obtained by Díez Díaz et al. (2018), I should emphasize that the difference between the two studies in terms of the phylogenetic placement of Lohuecotitan and Paludititan relative to the taxa included in Lirainosaurinae by Díez Díaz et al. is best explained by the huge amount of missing data for the cranial and cervical vertebral characters for several Late Cretaceous titanosaur taxa from Europe in the data matrix employed for the Díez Díaz et al. (2018) analysis, the number of non-European lithostrotian taxa selected for the cladistic analysis, and the fact that the recovery of Nemegtosaurus as the sister taxon to Rapetosaurus in several phylogenies (e.g. Wilson 2002) is now known to be artificial because more than one clade of lithostrotians had a Rapetosaurus-like skull. Given that the Abditosaurus kuehnei holotype preserves the only complete or near-complete neck for any Late Cretaceous titanosaur from the western Tethyan archipelago, and unpublished cladistic results concur with Vila et al. (2022) in recovering Lohuecotitan and Paludititan as part of the same lithostrotian clade as Mansourasaurus and Opisthocoelicauda, it is possible that currently undescribed titanosaur specimens from the Lo Hueco locality could shed new light on the cervical morphology of taxa assigned to Lirainosaurinae. For one thing, Atsinganosaurus is recovered as sister to Ampelosaurus and Lirainosaurus by Díez Díaz et al. (2018) but falls as sister to Colossosauria in the Vila et al. (2022), and because Díez Díaz et al. (2018) note that material referred to Atsinganosaurus by Garcia et al. (2010) was not found articulated or associated with the A. velauciensis holotype (comprising four posterior dorsal vertebrae), it is possible that some of the specimens referred to this taxon might instead belong to Abditosaurus.
In summary, more than one clade of large- and medium-sized lithostrotian titanosaurs immigrated to Europe from Gondwana during the Late Cretaceous, gradually supplanting dwarf, insular genera like Magyarosaurus by the latest Maastrichtian. Although insular dwarfism is prevalent among some lithostrotian taxa from the Late Cretaceous of Europe, the size estimates and phylogenetic position for Abditosaurus demonstrate that the environment of the Ibero-Armorican Island most likely did not pose an ecological obstacle to giant titanosaurs dispersing from Gondwana because titanosaur specimens from the middle Cretaceous of Italy and Spain and phylogenetic affinities of Normanniasaurus to colossosaurians indicate that multiple lithostrotian clades dispersed into Europe by the Albian. Future discoveries and re-evaluation of lithostrotian titanosaur specimens currently seen as indeterminate may further shed light the palebiogeographic patterns of lithostrotians from the Late Cretaceous of Europe.
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