33.2: A Dinosaur by Any Other Name - Species controversies

Figure \(\PageIndex{1}\): Angler fish famously have incredible differences between males and females. If they were found as fossils, would they be classified as the same species?

When a paleontologist finds a new fossil, one of the first things they analyze is the dimensions of the fossil to compare it to existing species. If the organism is different enough, then a new description is written about it, defining the new species. Seems easy, right? There is a problem though. How does the paleontologist know how representative the organism was to its species? Imagine you are a future scientist studying human fossils. If you came upon the bones of a male basketball player, and then the bones of a female gymnast, would you think they were the same species? How about an elderly person versus a juvenile?

Figure \(\PageIndex{2}\): Jane, previously assigned to Nanotyrannus, is now believed to be a young Tyrannosaurus. (Picture from the Burpeee Museum on Natural History by Zissoudisctrucker via Wikimedia.)

Of course, part of the training of any paleontologist is studying modern anatomy and populations of organisms so you have a basis for comparison for things like sexual dimorphism (changes between the genders of a species), gerontology (changes based on age), variability within a species, or even diseases that may alter the appearance of tissues. But when presented with an extinct species, certain extrapolations and assumptions are required. This has led to several controversies in paleontology, notably within dinosaurs. In fact, a very large number of dinosaurs, even some famous ones, may be nothing but a synonym of an existing species. In biologic nomenclature, the first name published gets precedent. This is why Ultrasaurus, Nanotyrannus, and Monoclonius are now considered invalid, and some scientists feel as many as one third of dinosaur species may join them in infamy. Some other famous examples are described below.

Brontosaurus

Figure \(\PageIndex{1}\): O.C. Marsh’s Brontosaurus skeleton, with skull material that was later assigned to Brachiosaurus.

Perhaps the most famous dinosaur naming faux-pas comes from Brontosaurus. It is still one of the most famous dinosaurs ever, but it might not even exist! Toward the end of the 19th Century, scientists O.C. Marsh and Edward Cope were in a constant battle to describe and discover as many dinosaurs as possible, a rivalry known as the Bone Wars. This led to rushed, inaccurate, and sloppy science as they fought for prominence in paleontology, and Brontosaurus was one of the victims. In 1877, Marsh described a sauropod which he named Apatosaurus. Two years later, Marsh described a similar animal, but different enough in his mind to warrant a new genus. He named it Brontosaurus. One part of the skeleton was missing, however: the skull.

Figure \(\PageIndex{3}\): Modern corrected Apatosaurus skeleton. (Photo by Tadek Kurpaski of the Carnegie Museum via Wikimedia.)

In 1903, Elmer Riggs, a paleontologist from Chicago’s Field Museum, said that Apatosaurus and Brontosaurus were similar enough to be the same genus, and since Apatosaurus was named first, the term Brontosaurus became invalid as a junior synonym. However, when the first mount was made for the American Museum of Natural History (the first sauropod mount ever displayed) in 1905, the name Brontosaurus was chosen for the skeleton. Since the skull was not known, a bulky skull similar to Camarasaurus was used. This mount may at least partially explain why the term Brontosaurus has persisted, even though throughout the 20th Century, almost all scientists agreed with Riggs that Apatosaurus was the proper name that should be used. In 1909, an Apatosaurus skull was found (which was more slender, similar to another sauropod, Diplodocus), but the knowledge of this skull remained limited until it was published in the 1970s.

Figure \(\PageIndex{4}\): Infographic showing the timeline of Apatosaurus vs. Brontosaurus.

In 1998, Robert Bakker argued that different species of Apatosaurus should be split into multiple genuses. Then, in 2015, Emanuel Tschopp, Octavio Mateus, and Roger Benson released a study on Apatosaurus and related animals and their analysis also concluded that there is enough variation in the Apatosaurus’ to justify a new genus name. Rather than create a new name, they decided to resurrect the Brontosaurus name for this new genus. While not all paleontologists agree, it may have solved the problem of the popular, but potentially incorrect, Brontosaurus.

Toromorph

Figure \(\PageIndex{5}\): Range of skulls of Triceratops, with youngest on the right and oldest on the left. There are several dramatic changes in the skulls with age, including the angle of the horns and shape of the frill.

One of the more infamous controversies is called the toromorph hypothesis. Triceratops is a famous ceratopsian dinosaur from the late Cretaceous of western North America. Torosaurus is from the same general time, place, geologic formation, and habitat. It is noticeably larger than Triceratops, with a larger frill with large holes in the center of each half of the frill. Starting in 2009, John Scanella and Jack Horner proposed that Triceratops was just a juvenile of Torosaurus. Other related animals, like Nedoceratops and Ojoceratops have also been suggested as growth stages of Triceratops. Evidence to support this position includes several juvenile-like features of Triceratops, dramatic changes in Triceratops’ known young versus known ‘adults,’ and a lack of Torosaurus young.

Figure \(\PageIndex{6}\): This analysis shows more overlap in skull features between Triceratops than other species that have been proposed to be synonyms.

This idea has come under a fair amount of scrutiny, most notably by Andrew Farke and Nicholas Longrich. They have analyzed features of the skulls of each animal and noticed some differences in the skulls. Even a computer analysis of morphometric statistics showed that there is a significant difference between the species. Also, subtle but important differences between the animals have been noted. For example, Torosaurus is much more rare than Triceratops overall, Triceratops specimens that seem old enough to be mature, slight differences in age and location of each species without exact overlap, and no other known ceratopsians which gain holes in frills with age.

Stygimoloch and Dracorex

Figure \(\PageIndex{7}\): Sketch of a Pachycepholosaur from the Hell Creek Formation of South Dakota (by Jordan Mallon via Wikimedia.)

Pachycephalosaurs are dinosaurs famous for their thick skull domes. They are also some of the most poorly understood dinosaurs, as a complete skeleton has never been found of any member of the group. They are also related to the ceratopsians (within Marginocephalia) and have similar growth changes. This had led to significant species controversies.

Figure \(\PageIndex{8}\): Analysis by Horner and Goodwin showing how many features of Dracorex and Stygimoloch can be seen in Pachycephalosaurus.

Pachycephalosaurus, the ‘thick-headed lizard’ as its name describes, was first identified in 1931. In 1983, a relative of Pachycephalosaurus named Stygimoloch (“Demon of the River Styx”) was named, and in 2006, another relative named Dracorex hogwartsia (“Dragon King of Hogwarts”) was named. Just a year later, scientists like Jack Horner had started to question these other species as growth stages of Pachycephalosaurus. In 2009 a study was published by Horner and Goodwin showing that specific points on the skull of each individual species can easily be correlated, supporting the idea that Dracorex being a juvenile, Stygimoloch being a sub adult, and Pachycephalosaurus being an adult and the proper name of the genus. This had spread to other members of the group as well; other flat-headed pachycephalosaurs (like Goyocephale and Homalocephale) have been proposed as juveniles of other genuses.