The Bathonian Femur of Morocco and Other "Atlasaurs"

Introduction

In 1992, Mohammed Charroud and Bouazza Fedan mentioned an uncatalogued sauropod femur from Morocco, found in fossil deposits dating to roughly 167 million years ago. In 2020, it was suggested to be a giant cousin of the sauropod Atlasaurus imelakei. However, the same source also suggested that a number of other sauropods found across the southern hemisphere were also relatives of Atlasaurus. Were these all related animals, or are their suggested relationships based on nothing more than coincidence?

The Suspects

In 1992, Mohammed Charoud and Bouazza Fedan released a preliminary study on dinosaurs found in Boulhafa, Morocco. In one section, they noted a pair of large femurs, measuring between 230 and 240 cm in length. These were dated to the Bathonian, roughly 167 million years ago and are of particular importance. Another femur from a nearby location has a length of 236 cm. Only seven years after Charoud and Fedan published their study, the sauropod Atlasaurus imelakei, found nearby, was named.

In a recently published book on sauropods, Asier Larramendi and Rubén Molina-Pérez suggested that a set of footprints, dubbed Malakhelisaurus mianwali, were made by an animal related to Atlasaurus. Found in Pakistan in association with theropod footprints, they were initially believed to belong to titanosaurs. Larramendi and Molina-Pérez's suggestion is supported, to some extent, by the lack of Jurassic sauropod fossils. Either the group hadn't evolved yet, or their fossils just haven't been found. Either way, it seems more likely that the Malakhelisaurus tracks were made by another dinosaur altogether.

Another example of a possible "atlasaur," one known from actual bones, is Lapparentosaurus madagascariensis. This species' convoluted history has led to it being classified as an early brachiosaurid, some other advanced sauropod, or something altogether more primitive. Larramendi and Molina-Pérez supported the idea that it was an advanced sauropod, albeit one which may have been a close relative of Atlasaurus.

Systematics

So, if there is a valid clade consisting of Altasaurus and its relatives, where would it fit in the sauropod family tree? Well, it would probably be among the most basal of the Neosauropoda (advanced sauropods). A phylogeny produced by Larramendi & Molina-Pérez lists Atlasaurus as a relatively basal member of the clade.

A general phylogeny of the Gravisauria (true sauropods and the Vulcanodontidae), with a possible clade including the "atlasaurs" highlighted in red. Note that a recent phylogeny suggests that Atlasaurus is in a clade alongside the Brachiosauridae, which is why there are two separate positions shown here. The idea of an "atlasaur" clade as a whole is only tentative.
Author's own work

As shown above, an alternative hypothesis is that the atlasaur clade—which I've referred to as Atlasauridae for clarity—is a member of a clade including the Brachiosauridae, or even the Brachiosauridae proper. If the latter is the case, which is very likely, then the Atlasauridae would be demoted to subfamily level. This would result in it being renamed Atlasaurinae.

Evidence Against

Now that we've covered the possible members of a clade including Atlasaurus, the giant Bathonian taxon, the Malakhelisaurus trackway producers and Lapparentosaurus madagascariensis, and the evidence in favour of this hypothesis, we must now cover the other side of the debate. What is the evidence against an "atlasaur" clade?

We'll start with L. madagascariensis. A comparison between the femur of this taxon and that of Atlasaurus shows startling differences, mainly that the latter is much longer and a little more curved than that of the former. Indeed, the femur of L. madgascariensis is much more like a typical sauropod, and aspects of its anatomy led to another recent paper classifying it as a member of the Cetiosauridae—much less advanced than Atlasaurus, assuming that it was a brachiosaurid.

Then there's Malakhelisaurus mianwali. In 2007, Sadiq Malkani identified these tracks as those of a titanosaur. M. mianwali is an even more dubious candidate than L. madagascariensis as an Atlasaurus-grade sauropod, since not a single bone has been found. It probably isn't a titanosaur at least assuming it does possess claws on its hind feet, but specifically depicting it as a giant relative of Atlasaurus, as per Larramendi & Molina-Pérez, seems to be nothing more than conjecture. 

The uncatalogued Bathonian femur seems equally as dubious as M. mianwali. There are no photographs or illustrations of it anywhere, but for it to be mentioned by Charroud and Fedan, it has to exist somewhere. I personally feel that, without a good photograph or illustration, this should be either classified as Atlasaurus proper or simply an incertae sedis (a species of uncertain placement) within the Sauropoda.

Conclusion

Though Larramendi & Molina-Pérez noted a variety of taxa apparently closely related to Atlasaurus, the identities of these forms are highly debatable. The Bathonian femur is unillustrated and thus can't be compared to Atlasaurus, Malakhelisaurus mianwali is probably something else altogether, and L. madagascariensis was a member of the Cetiosauridae. Overall, it seems that Atlasaurus is merely a divergent member of an already established clade, either the Turiasauria or the Brachiosauridae, and that there was no bizarre clade of short-necked, long-armed sauropods.

References

Charroud, M  Fedan, B; 1992. "Données préliminaires sur la découverte du gisement de Boulahfa a dinosauriens (SW de Boulemane, Moyen Atlas central)." Notes et Mémoires du Service Géologique du Maroc. 366: 448–449. Translated by Matthew C. Lamanna, University of Pennsylvania, March 2002.

Larramendi, A; Molina-Pérez, R; 2020 "Encyclopedia of Dinosaurs: The Sauropods." Natural History Museum.

Lydekker, R: 1985 "On bones of a sauropodous dinosaur from Madagascar." Quarterly Journal of the Geological Society of London. 51 (1–4): 329–336.

Malkani, MS; 2007 "Trackway evidence of sauropod dinosaurs confronted by a theropod found from Middle Jurassic Samana Suk Limestone of Pakistan." Sindh University Research Journal (Science Series). 39 (1): 1–14.

Malkani, MS; 2011 "Trackways: confrontational scenario among a Theropoda and a herd of wide-gauge titanosaurian sauropods from Middle Jurassic of Pakistan."

Mannion 2010 "A revision of the sauropod dinosaur genus ‘Bothriospondylus’ with a redescription of the type material of the Middle Jurassic form ‘B. madagascariensis'." Palaeontology.

Michel Monbaron, Dale A. Russell, Philippe Taquet, 1999 "Atlasaurus imelakei n.g., n.sp., a brachiosaurid-like sauropod from the Middle Jurassic of Morocco alen." Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science. 329 (7): 519–526.

Raveloson, MLT; Clark, ND; Rasoamiaramana, AH; 2019 "New information on the Madagascan Middle Jurassic sauropod Lapparentosaurus madagascariensis." Geosciences. 9 (12): 498.