Day of the Diplodocids

Welcome, true believers, to a Very Special installment of Noah's Ravens. As you have no doubt seen while scrolling through Facebook or Twitter or while idly surfing the web, yesterday marked the appearance of a new (very long) paper reviewing the diplodocid sauropods, 'A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda)' by Emanuel Tschopp, Octávio Mateus, and Roger B J Benson, published open-access in PeerJ. Although I'm certain you've all had time to read it, pore lovingly over the cladograms, savour each and every word about each and every specimen, and completely lose yourself in the figures depicting small chunks of bone, I thought it might be nice to have a brief(ish) round-up post that covers some of the big ideas of the paper in a condensed(ish), hopefully-easy(ish) read.

It was nice knowing you, Diplodocus longus, but all good things
must come to an end. Read on to find out why. Legs belonging to
The Sauropod Formerly Known As Diplodocus longus at centre.
[photograph by the author]

Tschopp, Mateo, and Benson's Paper and What It Means For Your Diplodocids and You

Diplodocids are a group of sauropods, the long-necked, long-tailed, big-bodied plant eating dinosaurs. Diplodocids are famous for being very long with whiplike tails, while also having fairly short legs and being (mostly) pretty slim for their great size. They're mostly from the Late Jurassic, although one known taxon, the recently-described Leinkupal laticauda, made it to the Early Cretaceous. Geographically, the American West is a hotspot of diplodocid diversity, but they are also known from Portugal, Tanzania, and Argentina, with possible diplodocid fossils additionally known from Asia and other parts of Europe and East Africa.

The taxonomic history of Diplodocidae is a bit convoluted because the group was described in the 'Bone Wars,' a huge palaeontological rivalry between Othniel Charles Marsh and Edward Drinker Cope in the late 1800s. The two scientists were competing to describe the most new dinosaurs, so many rather hasty and hazy taxonomic decisions were made. Diplodocidae was created by Marsh on the basis of his Diplodocus longus. It contains many of the best-known sauropods; however, the relationships between the many genera and species of diplodocids are not well understood and until this paper no specimen-based review of the whole group had ever been undertaken. 

What did this in-depth examination of Diplodocidae reveal? In addition to giving a new understanding of the group's relationships (for example, Tornieria africana, formerly considered congeneric with Barosaurus lentus, is actually pretty distant from it among the diplodocine diplodocids), here are the major wham lines and wow factors of the new paper:

• The Portuguese Dinheirosaurus lourinhanensis is congeneric with the giant Supersaurus vivianae of Colorado, becoming Supersaurus lourinhanensis. This makes Supersaurus the only diplodocid genus known from more than one continent.
• Diplodocus longus, the type species (that is, the first species described and the basis for the whole genus) of Diplodocus, is based on material that is non-diagnostic, meaning that it cannot reliably be shown to be from a unique taxon, and is considered a nomen dubium (doubtful name). This not only puts Diplodocus, arguably the best-known sauropod, in jeopardy, but also the taxonomic groups named for it: Diplodocoidea, Diplodocidae, Diplodocinae. This is the bad news.
• But this is the good news. Although Diplodocus longus is a nomen dubium, there is hope for Diplodocus! The authors put in a petition to the ICZN (International Commission on Zoological Nomenclature), the body who dictate the rules for scientifically naming animals, to have Diplodocus longus suppressed as the type species, and have Diplodocus carnegii, which is based on diagnostic remains, made the type. This way, the popular and, more importantly, widely-published and scientifically important name Diplodocus remains valid. 
•  But what about all those poor bones that were referred to Diplodocus longus? Don't worry, there's a name available for them. The dinosaur described as Seismosaurus halli (later emended to Seismosaurus hallorum) in 1991 and hailed as The Longest Dinosaur Of Them All, Ever (it wasn't, but that's another story for another time) was, round about 2004, decided to be a species of Diplodocus, and became D. hallorum for a while. Shortly afterward, it was referred to D. longus because it was considered to be too similar to warrant separation. Tschopp et al agree that D. longus and D. hallorum are the same species, but since they also find that the type specimen for D. longus is non-diagnostic, that name can't be used. However, better specimens referred to D. longus are diagnostic and are not the same as D. carnegii, so they need a name, and lo and behold, there is one available: Diplodocus hallorum. This means that all dinosaurs formerly contained in Diplodocus longus, such as the impressive legs at the Beneski, should now be called Diplodocus hallorum.
• Still dealing with species of Diplodocus, it seems that Diplodocus hayi isn't really Diplodocus at all! It appears to be its own genus, for which the authors establish a new name, Galeamopus, which means 'William,' in honour of the two Williams, William H Utterback and William J Holland, who were involved in collection and description of the Diplodocus hayi type material. Additionally, the authors find some other diplodocid fossils to be referable to Galeamopus, although since it cannot yet be said whether they are the same species as G. hayi or a new species, they are for now called Galeamopus sp. 
• Some material from the Field Museum once referred to Apatosaurus actually represents a new genus of apatosaurine diplodocid, as yet unnamed and undescribed.
• Here's the kicker, the big one, the headliner: Brontosaurus is back! The famous and evocative genus name Brontosaurus was initially established by Marsh in 1879 with the species B. excelsus. In 1903, it was decided that Brontosaurus was the same animal as Apatosaurus, which was the name that had priority. This decision only began to permeate the popular sphere in the 1970s and still isn't firmly established, as any child of the dinosaur renaissance (*coughcough*) will tell you. Well, Tschopp and colleagues find that the decision to synonymise Bronotosaurus with Apatosaurus was not a good one, as the species referred to Apatosaurus do not form a natural group with one another. Thus, the beautiful old name Brontosaurus was resurrected. Apatosaurus excelsus and Apatosaurus parvus are now Brontosaurus excelsus and Bronotsaurus parvus (or, er, are Brontosaurus excelsus AGAIN, in that case); additionally, the not-well-known Eobrontosaurus yahnahpin is renamed Brontosaurus yahnahpin. This leaves Apatosaurus with two species, Apatosaurus ajax and Apatosaurus louisae. 
• What this DOESN'T mean, however, is that the old, boxy-headed, tail-dragging, swamp monster Brontosaurus of Charles R Knight's early-20th-century palaeoart is back. Although made iconic by the works of Knight, Zallinger, and others, this Brontosaurus is not accurate. Brontosaurus, like all sauropods, was a land animal, with a body built like an elephant's, well suited for life on dry ground. Like all diplodocids, Brontosaurus is known to have had a low, rectangular skull, not a tall, boxy one like the old paintings show. It's just the name that's back, not the old style; as it turns out, Brontosaurus was a real, unique dinosaur, but the creature in those pictures never existed at all.

Charles R Knight's depiction of Brontosaurus is, like always, terrific art.
It is not, however, scientifically accurate. Sauropods were not water beasts,
and Brontosaurus did not have a head shaped like that.
[art by Charles R Knight, in the public domain]

References:

Tschopp E, Mateus O, Benson RBJ. (2015) A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3:e857. https://dx.doi.org/10.7717/peerj.857