Pleurocoelus No More
Until recently, most paleontology books would have one believe that sauropods all across the Northern Hemisphere (especially North America) went extinct in the Late Jurassic. The small-brained herbivores found themselves unable to adapt to a changing world and thus gave way to the (moderately) smarter ornithopods and ankylosaurs of the Cretaceous. In truth, sauropods are a much tougher breed than people give them credit for. While they did take a hit in diversity in the Cretaceous, the group as a whole survived and kept going on, surviving and adapting to the changing conditions of the Cretaceous. In fact, over the Cretaceous some titanosaurs were becoming more efficient and gracile, a topic which I may blog about someday.
Neither did sauropods just leave North America in the hands of the iguanodonts and nodosaurs. While the diplodocids did go extinct in North America after the Late Jurassic, the continent was still overrun by titanosaurs and their kin, the little buggers. There was Sauroposeidon, essentially an Early Cretaceous version of Brachiosaurus on steroids. There was also smaller titanosaurs, like Venenosaurus. But I would like to focus your attention to the dinosaurs oftentimes described as "Pleurocoelus"
The first fossil ever to be attributed the name Pleurocoelus were some fossils found about 1859 in the Arundel Formation of Maryland. However, it was not called Pleurocoelus at all, rather it was named Astrodon. Discovered by Christopher Johnson at this time, who coined the name, Astrodon really did not enter the paleontology lexicon until 1865, when Joseph Leidy described the species and attributed a species name to it, johnstoni, after its discoverer. So why Pleurocoelus then? Well, some years later, more sauropod specimens that looked exactly like Astrodon were brought to Othniel Marsh, who rather than follow the rules and refer them to Astrodon, created his own taxon for them, Pleurocoelus. This actually happened quite often between Cope and Marsh, neither would accept names coined by each other or other paleontologists, and so made up their own names. Over the years, numerous other taxa were attributed to the genus Pleurocoelus, from places as far away as Texas and Utah. Before long Pleurocoelus became a wastebasket taxon, a taxon to which numerous different remains are attributed.
For a long time, sauropod fossils found in Texas have been referred to as Pleurocoelus. These include the famous "sauropod vs. Acrocanthosaurus" footprints of Glen Rose. However, it soon became apparent that there were some differences between the supposed "Pleurocoelus" specimens of Texas and Maryland. For example, the Texas specimens apparently lacked a claw on one of their digits, making their feet rather distinct. These were not "Pleurocoelus", they were a species all their own. In 2007, the Texas fossils of "Pleurocoelus" were redescribed as a new genus, Paluxysaurus jonesi. Like Astrodon and other former members of "Pleurocoelus", Paluxysaurus is a basal titanosauriform, a group that was quite common across North America during the Early Cretaceous.
And what of Astrodon, the "original" specimen of Pleurocoelus? Well, in a study done by Kenneth Carpenter and Virginia Tidwell, they found that Astrodon was indeed a unique species, and that the name Astrodon should be kept, per ICZN legislation. In addition, the study found that there were differences between the supposed "Pleurocoelus" teeth from Texas differed from those of Astrodon.
So what does this mean for Early Cretaceous paleobiology? Well, rather than one sauropod as expected, it now seems that there were two sauropods running around in Early Cretaceous North America (in addition to the other sauropods, such as Sauroposeidon). First there was Astrodon, a somewhat smaller coastal-dwelling sauropod. Then there was the larger Paluxysaurus, who lived more to the west and inland of the East Coast dwelling Astrodon.
Ironically, there is a final twist to our story. While most of the Texas sauropod fossils known can probably be attributed to Paluxysaurus, one; a partial skeleton from Wise County, Texas, cannot. There has not been a study done yet comparing the Paluxysaurus holotype, but there appear to be some differences in the bone. But at the same time, it is difficult to tell these Pleurocoelus remains apart from those of Paluxysaurus.
References
Carpenter, K. and Tidwell, V. 2005. Reassessment of the Early Cretaceous sauropod Astrodon johnstoni Leidy 1865 (Titanosauriformes)
Rose, Peter J. (2007). "A new titanosauriform sauropod (Dinosauria: Saurischia) from the Early Cretaceous of central Texas and its phylogenetic relationships" (web pages). Palaeontologia Electronica 10 (2). http://palaeo-electronica.org/2007_2/00063/.
Jacobs, L. 1995. Lone Star Dinosaurs. Texas A&M University Press, College Station, Texas
Fraser, N. 2006. Dawn of the Dinosaurs, Life in the Triassic. Indiana University Press
Neither did sauropods just leave North America in the hands of the iguanodonts and nodosaurs. While the diplodocids did go extinct in North America after the Late Jurassic, the continent was still overrun by titanosaurs and their kin, the little buggers. There was Sauroposeidon, essentially an Early Cretaceous version of Brachiosaurus on steroids. There was also smaller titanosaurs, like Venenosaurus. But I would like to focus your attention to the dinosaurs oftentimes described as "Pleurocoelus"
The first fossil ever to be attributed the name Pleurocoelus were some fossils found about 1859 in the Arundel Formation of Maryland. However, it was not called Pleurocoelus at all, rather it was named Astrodon. Discovered by Christopher Johnson at this time, who coined the name, Astrodon really did not enter the paleontology lexicon until 1865, when Joseph Leidy described the species and attributed a species name to it, johnstoni, after its discoverer. So why Pleurocoelus then? Well, some years later, more sauropod specimens that looked exactly like Astrodon were brought to Othniel Marsh, who rather than follow the rules and refer them to Astrodon, created his own taxon for them, Pleurocoelus. This actually happened quite often between Cope and Marsh, neither would accept names coined by each other or other paleontologists, and so made up their own names. Over the years, numerous other taxa were attributed to the genus Pleurocoelus, from places as far away as Texas and Utah. Before long Pleurocoelus became a wastebasket taxon, a taxon to which numerous different remains are attributed.
For a long time, sauropod fossils found in Texas have been referred to as Pleurocoelus. These include the famous "sauropod vs. Acrocanthosaurus" footprints of Glen Rose. However, it soon became apparent that there were some differences between the supposed "Pleurocoelus" specimens of Texas and Maryland. For example, the Texas specimens apparently lacked a claw on one of their digits, making their feet rather distinct. These were not "Pleurocoelus", they were a species all their own. In 2007, the Texas fossils of "Pleurocoelus" were redescribed as a new genus, Paluxysaurus jonesi. Like Astrodon and other former members of "Pleurocoelus", Paluxysaurus is a basal titanosauriform, a group that was quite common across North America during the Early Cretaceous.
And what of Astrodon, the "original" specimen of Pleurocoelus? Well, in a study done by Kenneth Carpenter and Virginia Tidwell, they found that Astrodon was indeed a unique species, and that the name Astrodon should be kept, per ICZN legislation. In addition, the study found that there were differences between the supposed "Pleurocoelus" teeth from Texas differed from those of Astrodon.
So what does this mean for Early Cretaceous paleobiology? Well, rather than one sauropod as expected, it now seems that there were two sauropods running around in Early Cretaceous North America (in addition to the other sauropods, such as Sauroposeidon). First there was Astrodon, a somewhat smaller coastal-dwelling sauropod. Then there was the larger Paluxysaurus, who lived more to the west and inland of the East Coast dwelling Astrodon.
Ironically, there is a final twist to our story. While most of the Texas sauropod fossils known can probably be attributed to Paluxysaurus, one; a partial skeleton from Wise County, Texas, cannot. There has not been a study done yet comparing the Paluxysaurus holotype, but there appear to be some differences in the bone. But at the same time, it is difficult to tell these Pleurocoelus remains apart from those of Paluxysaurus.
References
Carpenter, K. and Tidwell, V. 2005. Reassessment of the Early Cretaceous sauropod Astrodon johnstoni Leidy 1865 (Titanosauriformes)
Rose, Peter J. (2007). "A new titanosauriform sauropod (Dinosauria: Saurischia) from the Early Cretaceous of central Texas and its phylogenetic relationships" (web pages). Palaeontologia Electronica 10 (2). http://palaeo-electronica.org/2007_2/00063/.
Jacobs, L. 1995. Lone Star Dinosaurs. Texas A&M University Press, College Station, Texas
Fraser, N. 2006. Dawn of the Dinosaurs, Life in the Triassic. Indiana University Press
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