The sun distorts the woodland, drying the earth and leaving the trees with a pale, greedy countenance. It’s migration season, and the hadrosaur herd marches across the parched landscape with the might of an army. The animals themselves are primarily Hadrosaurus, which bellow to ward off unwanted attention. Then there are the nodosaurs. Nodosaurs aren’t doing very well on Laramidia during the Campanian Stage of the Late Cretaceous, but on Appalachia they thrive. Armed to the eyelids with spikes and scutes, they pose quite a challenge for predators. Lophorhothon, another type of hadrosaur, are also found within the herd, but these animals are few and far between. None of these dinosaurs, however, is as impressive as Hypsibema crassicauda. The larger of the two species of Hypsibema, H. crassicauda can attain lengths of around 50 feet, making them some of the largest hadrosaurs and the biggest dinosaurs on Appalachia. One of the most massive individuals in the herd is a fourteen year old female who measures 45 feet from beak to tail. Alongside her is another female, slightly smaller at 42 feet in length. They triumphantly lead the herd of dinosaurs across the plain, pridefully stomping across the cracked earth below. The continent of Appalachia, however, is a very dangerous place for a herbivore, and much trouble awaits the herd.
To reach a new browsing location, the herd must first cross a thin strip of beach. But there’s death in the water. The lone carcass of a Claosaurus drifts among the waves, alarming the already-weary herd. They know, however, that they must march on, and they forget about the rotting body.
A few hours pass, and the herd is almost to the forest. Suddenly, a giant Deinosuchus rugosus rushes out of the water, catching a subadult Hadrosaurus between its jaws. Another Hadrosaurus, startled at the sight of the crocodile, bellows to alarm the rest of the herd. A larger individual bellows back, trying to scare away the Deinosuchus, which pulls the juvenile hadrosaur down into the abyss.
The Hypsibema are also on guard, but for a different reason. They’ve spotted a lone male Dryptosaurus, and, although he poses little threat, they are weary of him, grunting and snorting to scare the tyrannosaur away. Undaunted, the Dryptosaurus goes about on his patrol, leaving the hadrosauroid giants to browse on the vast expanses of spruce and redwood trees which make up the forest behind them.
Hypsibema crassicauda is one of the largest known hadrosaurs. At an estimated 49.2 feet in length (Holtz and Rey, 2007), this member of the Hadrosauroidea (Horner et. al., 2004) was most likely the largest herbivore in its ecosystem. The holotype, a fragmentary specimen discovered at the King James marl pits in North Carolina, consists of a caudal vertebra, a humerus, a tibia and a metatarsal (Cope, 1869).
The material belongs to a huge hadrosaur. When we look at this Smithsonian specimen of a caudal vertebra of the animal (which is slightly smaller then the caudal vertebra included in the holotype), we find the bone’s length is approximately 10 centimeters. If we compare the bone with a large Edmontosaurus annectens caudal centrum (sadly in private hands), we realize that the Hypsibema crassicauda caudal centrum is approximately 25% larger. Knowing that large E. annectens could reach around 11.9 meters (Sues, 1997), we can estimate the size of H. crassicauda. We end up with 14.875 meters, or 48.8025 feet. Based on the estimated weight of related animals, I also estimate H. crassicauda to have weighed around 17-20 tons. That’s one large hadrosaur! Nevertheless, as Hypsibema crassicauda was a non-hadrosaurid hadrosauroid or basal hadrosaurid and E. annectens was a derived saurolophine, scaling the former from the latter is certainly a flawed approach. Holtz and Rey (2007) found H. crassicauda to be around 50 feet long and 14 tons. If Hypsibema crassicauda did attain this size, it wasn’t just a large hadrosaur, but one of the largest.
I only know of two other non-dubious hadrosauroids estimated at longer then 49.2 feet in length and greater than 14 tons in weight: Shantungosaurus giganteus at an estimated 54 feet (Zhao et. al., 2007)(if Ji et. al., 2011 were right that Zuchengosaurus maximus is a junior synonym of S. giganteus) and 18 tons (Horner et. al., 2004) and Magnapaulia laticaudus at between 49.2 and 54.1 feet and with a weight of up to 25 tons (Morris, 1981). A more recent study, however, has estimated the latter dinosaur to be around 41 feet in length (Prieto-Márquez et. al., 2012). If Prieto-Márquez et. al. (2012) are correct in their size estimate of Magnapaulia, Hypsibema may be the longest known North American hadrosauroid and rivals Shantungosaurus giganteus as the biggest ornithischian known to science.
Like other hadrosaurids, H. crassicauda was designed to be an efficient eater. It has even been suggested that hadrosauroids could browse for food sources up to 4 meters above the ground (Mallon & Anderson, 2013). This would make sense for a giant hadrosauroid such as Hypsibema crassicauda, the body of which would require an ample daily food supply. I imagine H. crassicauda as the elephant of Appalachia, browsing on anything it could get its beak around. The habitat of this dinosaur would have been woodland, where it would have had a food supply to match its appetite.
The size of Hypsibema crassicauda also interests me for another reason. Along with Hypsibema crassicauda, two other species of gigantic basal hadrosaurid are known from the eastern United States. These are the gigantic Hypsibema (=Parrosaurus) missouriensis from Missouri (estimated at around 50 feet and 14 tons as well (Holtz and Rey, 2007)(although this animal seems to be smaller then H. crassicauda) and Ornithotarsus immanis from New Jersey, a possible synonym for Hadrosaurus foulkii (Prieto-Marquez et al., 2006) (which is estimated to be 39.6 feet in length and 7 tons in weight (Holtz, 2012)). The location of these animals (did I mention Hypsibema crassicauda material is reported to have been found in NJ?) shows that gigantic hadrosaurids in the 40-50 foot, 10-20 ton size range were very widespread across Appalachia during the Late Cretaceous, whereas on Laramidia, only one hadrosauroid taxon has been estimated to be that gigantic (the lambeosaurine Magnapaulia laticaudus, see above). Even these lambeosaurine giants had competition! The morphological similarities of lambeosaurine and ceratopsid dinosaurs might have been the reason that they weren’t able to coexist for long periods of time due to competitive exclusion (Mallon & Anderson, 2013).
So why were there so many gigantic hadrosauroids on Appalachia? I believe the evolution of gigantism in these animals may have to do with the lack of ceratopsids and highly derived hadrosaurids in eastern North America during the time. Ceratopsids were most likely low-level browsers, using their strong beaks to ingest tough plant matter (Mallon & Anderson, 2013), and lambeosaurines probably occupied similar regions of ecomorphospace (Mallon & Anderson, 2013). The possible scarcity of these groups on Appalachia (so far, we haven’t found any ceratopsian remains on the East Coast, and lambeosaurine remains, though present (Gallagher, 2002), are seldom found and only reported from Maastrichtian deposits) might have allowed more basal forms to thrive.
I know what you might be thinking:
So what about Saurolophinae? You said derived hadrosaurids earlier, not just lambeosaurines.
A couple of possible saurolophine dinosaurs are known from Appalachia. I have heard that the Alabama taxon Lophorhothon atopus is to be a saurolophine by some (a juvenile Prosaurolophus in particular). More recent analyses have concluded that L. atopus is a basal hadrosauroid (Prieto-Marquez & Salinas, 2010). Although some have suggested the presence of Edmontosaurus sp. at the Ellisdale site (Gallagher, 1993), a Marshalltown Formation exposure that is around 72 million years old, this dinosaur’s presence in New Jersey is tentative at best, and the reported “Edmontosaurus sp.” remains are probably better considered to be from Hadrosaurus or an indeterminate hadrosaur.
Another question might be about competitive exclusion between east coast nodosaurs and giant hadrosaurids. Mallon & Anderson (2013) also covers this for us. Their analysis concluded that ankylosaurs were the least likely to compete with other large herbivores.
If the lack of ceratopsids and derived hadrosaurs is what allowed the large basal hadrosaurids of the East Coast to thrive, then the presence of these groups might have also caused the downfall of hadrosaurian giants like Hypsibema crassicauda. The Navesink Formation, a Maastrichtian-age deposit, bears both the remains of (possible) lambeosaurines (Gallagher, 2002) and smaller, basal hadrosaurids, such as Hadrosaurus (Gallagher et. al., 1986), found on Appalachia prior to the closing of the Western Interior Seaway (AMNH 7626, attributed to Hadrosaurus, from the Campanian, for example). It seems to me that the immigration of west-originating dinosaur groups to the eastern US is correlated with the disappearance of large basal hadrosaurids on Appalachia. The giant basal hadrosaurids of Appalachia may have simply been out-competed. If lambeosaurines were indeed migrating to the eastern US, it might also have been the case that native eastern US forms headed westward. The end result would be the odd giant basal hadrosaur in a western Maastrichtian deposit, like Hell Creek. Others have also speculated about the presence of these animals in the west. These must have been very rare occurrences, though, as I do not know of any dryptosaur and/or giant basal hadrosaurid remains reported from Hell Creek or any other western Maastrichtian deposits. If ACDs (Alien Coast Dinosaurs is what I will call these migrants from now on) existed in both the west and the east, we would be looking at a cross-continental dinosaur migration occurring in the time range of 2-3 million years. I will talk more about this concept in later installments of Antediluvian Beasts, so for now, keep this idea in mind.
Many other types of animals uncommon or absent in the west coexisted with Hypsibema crassicauda. On Appalachia, the dryptosaurs were among the top terrestrial predators, whereas in the west, derived tyrannosaurids ruled the roost. We know H. crassicauda existed alongside dryptosaurs due to the presence of H. crassicauda at Ellisdale (Grandstaff et. al., 1992) alongside Dryptosaurus sp. (Grandstaff et. al.,1992). It’s possible that the gargantuan hadrosaur might have even fallen prey to the dryptosaurs present in its environment.
So what were the dynamics of Hypsibema crassicauda’s environment, exactly? In the southern part of H. crassicauda’s known range, it would have co-existed with other hadrosaurs, such as “Hadrosaurus minor” (Lull & Wright, 1942), and theropods such as “Ornithomimus“ antiquus (Baird & Horner, 1979). At Ellisdale, the gigantic hadrosauroid would have shared the woodland with Hadrosaurus (Grandstaff et. al, 1992), “Ornithomimus” (=Coelosaurus)(Denton & Gallagher, 1989), and the predatory theropod Dryptosaurus (Grandstaff et. al., 1992). The variation of sizes among the hadrosaurs present in the environments mentioned above (Hypsibema crassicauda grew to 48.8 feet in length, whereas a Hadrosaurus reached 26 feet in length (Holtz, 2012)) suggests that niche partitioning might have evolved between these dinosaur genera. Hadrosaurus could have been the low-browsing deer to Hypsibema’s variety-browsing elephant. So many hadrosaur species of varying sizes from the Late Campanian and Early Maastrichtian of the East Coast are known (i.e. Hadrosaurus foulkii, Hadrosaurus cavatus, Ornithotarsus immanis, Hadrosaurus minor, etc.) that it is hard to imagine the absence of niche partitioning among these dinosaurs.
Hypsibema crassicauda‘s kind would survive until the end of the Cretaceous, with forms like Hadrosaurus minor surviving into the Maastrichtian and their remains deposited in the Navesink Formation (Gallagher et. al., 1986). The giant hadrosauroids, however, would decrease in number, being replaced by the lambeosaurines and saurolophines which arrived from the west as the Western Interior Seaway had closed. The KT extinction event finally sealed the giant hadrosaurs’ fate, their bones left to dry in the hot, cracking dirt.
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EDIT: The remains of a leptoceratopsid have been reported from the Tar Heel Formation of Cretaceous North Carolina. This shows that ceratopsians were present on Appalachia. However, the presence of leptoceratopsids on Appalachia does not automatically imply that the generally larger ceratopsids were present as well.