Pleurocoelus – A wastebasket taxon for American Sauropods

As an increasing number of bizarre forms of Sauropod are unearthed, the family tree of these huge long-necked dinosaurs is becoming more complicated.  Although, thanks to recent discoveries of more basal Sauropods and Prosauropods the evolution of these Saurischians has become a little clearer, there is still a lot of confusion over the taxonomic relationships between the various families.

To read further about this: Changing Views on Sauropods

Within the Macronaria (big noses), sub-families that include the Camarasaurids and the Brachiosaurids for example, there is still a great deal we have to learn about the relationships between these different types of Sauropod.  In particular, there is very little in the fossil record to provide clues regarding the classification of Brachiosaurs, particularly those found in late Jurassic and early Cretaceous rocks.  Even those specimens that have dominated museum exhibits for many years hold their secrets.  The mounted Brachiosaurus fossil in the Humboldt museum, Berlin is actually a composite made up of at least 5 different animals and recently some palaeontologists have argued that this type of Brachiosaur from Africa is sufficiently different enough from the North American form to warrant its own genus – Giraffatitan.

The Brachiosaur Exhibit at the Humboldt Museum

Photo: W. D. Heinal (University of Bonn)

To read more about the Brachiosaur/Giraffatitan debate: Is it Brachiosaurus or Giraffatitan?

Many fossils of North American Brachiosaurs have been classified as belonging to the genus called Pleurocoelus (name means hollow sided vertebrae).  Fossils from Brachiosaurids from all over the United States (Maryland, Texas as well as the American mid-west) have been attributed to this genus, it has become a “wastebasket taxon” for North American Brachiosaurs in a similar fashion to the genus Megalosaurus was used to describe miscellaneous Theropod remains from Europe.

Originally described by the famous American palaeontologist Ohniel C. Marsh from fossils discovered in the 1880s this dinosaur is typical of the Brachiosaurs with forelimbs bigger than hindlimbs and a long neck.  Estimates of the size of Pleurocoelus vary (depending on which fossil bones are studied), but some scientists have estimated that this dinosaur could have reached lengths in excess of 20 metres and weighed perhaps as much as 40 tonnes.

Pleurocoelus is a state fossil of Texas, an honour awarded to it to this genus in 1997.  Fossils of another dinosaur called Astrodon may actually represent the same genus.  This is unfortunate as Astrodon is the state fossil of Maryland.  Pleurocoelus may be a junior synonym for Astrodon.   In taxonomy, the earliest of several names given to an organism is considered to be senior to any subsequent names and descriptions.  As Astrodon was formerly named in 1865, it is the senior synonym to Pleurocoelus, whilst Pleurocoelus named in 1888 is the junior synonym to Astrodon.

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