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Dinosaur Park Blog

Mar 29

New research sheds light on Maryland's ostrich dinosaurs

Posted on March 29, 2018 at 12:31 PM by Bonnie Man

New research on fossils found at Dinosaur Park was published yesterday in the open access journal PeerJ. Author Chase Brownstein, a research associate at the Stamford Museum of Stamford, Connecticut, has found evidence that two distinct ornithomimosaur (or “ostrich dinosaur”) species were present in Early Cretaceous Maryland. 

Smithsonian paleontologist Charles Gilmore first identified ornithomimosaur remains among the fossils recovered from the Arundel Clay in 1920. Many subsequent finds have also been attributed to this group, but until now they had never been compared to one another in detail.

When Brownstein visited the Dinosaur Park collections last November, he inspected all the available fossils, and picked out those that he could confidently identify as belonging to ornithomimosaurs. These included two ends of a humerus (upper arm bone) found by Park visitors Chris C. and Jackson C., as well as a number of claws, one of which was found by visitor Rebecca W. Next, Brownstein carefully recorded every detail of the fossils, from the angle of every curve to the depth of every depression. 

By comparing this dataset to similar measurements from ornithomimosaur fossils found elsewhere, Brownstein was able to draw some interesting conclusions. The toe claws came in two shapes: some were more triangular at the base, while others were more rounded. This may sound like a subtle difference, but it’s important. The triangular claws strongly resemble those of more advanced ornithomimosaurs from the Late Cretaceous of North America, like Struthiomimus. By comparison, the round claws are similar to primitive ornithomimosaurs from Asia, like Harpymimus. Likewise, the humerus is more like primitive ornithomimosaurs than advanced ones. 

The significance of this discovery is twofold. First, two distinct types of ornithomimosaurs means that the Dinosaur Park fauna was more diverse than we previously thought. More important, however, are the implications of primitive and advanced ornithomimosaurs living side by side. It’s likely that ornithomimosaurs as a group originated in Asia, with animals like Harpymimus. At some point during the Cretaceous, they migrated to North America, along with other groups like the ceratopsians and oviraptorids. Indeed, over the course of the Cretaceous we see wholesale replacement of dinosaur groups shared with Europe (like sauropods and ankylosaurs) with Asian groups. The Atlantic Ocean was widening at this time, so it makes sense that migration between America and Europe would have been harder. Apparently, a land bridge formed with Asia around the same time, which allowed eastern dinosaurs to enter the Americas. However, the presence of primitive and advanced ornithomimids in Maryland makes things more complicated. It suggests that there may have been multiple migrations of Asian dinosaurs, separated by millions of years!

Research like this is the reason museum collections like ours exist. The process of discovery doesn’t end with finding the fossil. There’s always more to learn by studying known fossils in new ways. Museums hold important specimens and objects in the public trust, so that people can continue to learn new things from them far into the future. 

References

Brownstein, C.D. 2017. Description of Arundel Clay ornithomimosaur material and a reinterpretation of Nedcolbertia justinhofmanni as an “ostrich dinosaur.” PeerJ 5e3110.


Gilmore, C.W. 1920. Osteology of the carnivorous Dinosauria in the United States National Museum, with special reference to the genera Antrodemus (Allosaurus) and Ceratosaurus. Bulletin of the United States National Museum 60: 1-154.

JPG2014.s.195_1.36
Above: an "advanced"-type ornithomimosaur claw.
Mar 29

Dinosaurs and Flowers: Causality or Coincidence?

Posted on March 29, 2018 at 12:30 PM by Bonnie Man

Angiosperms – plants with flowers – make up more than 90% of the modern flora. From oak trees to tulips and from grass to potatoes, angiosperms are an immensely important part of the world around us. This was not always the case, however. Back in the Jurassic Period (150 million years ago), when Stegosaurus and Brachiosaurus roamed North America, angiosperms were hard to find. Instead, forests were made up of needled conifers, with ferns and cycads filling in the lower vegetation. Fast forward to the middle to late Cretaceous (100 to 66 million years ago), and suddenly flowering plants and trees are everywhere – and they have been ever since. 

What changed? Some paleontologists think plant-eating dinosaurs caused the angiosperm explosion. During the Jurassic, long-necked sauropods were the most diverse plant-eating dinosaurs. More than twenty sauropod species are known from western North America alone. Sauropods are specially adapted for eating high-growing conifers, and the rise of coniferous forests may have been what prompted the evolution of sauropods in the first place. While sauropods didn’t disappear during the Cretaceous, they became less dominant, and had to share their salad with new plant-eaters like hadrosaurs, ankylosaurs, and ceratopsians. These newcomers were by no means small, but their mouths were situated closer to the ground, and they focused their attention on lower-growing vegetation.

When cattle ranchers let their herds graze in one area for too long, the bushes and shrubs get wiped out. Fast-growing angiosperm grasses and weeds quickly take the place of the woody plants, and it takes years – even decades – for bigger plants to recover. Perhaps the new plant-eating dinosaurs did the same thing on a global scale. Low conifers, cycads, and ferns couldn’t grow back fast enough, so fast-growing angiosperms were able to take over the understory.

The problem with a scenario like this is that it is hard to test. The patchiness of the fossil record means they we can’t say for sure whether the new plant-eating dinosaurs arrived on the scene before angiosperms, or if it was the other way around. An expansion of fast-growing angiosperms could have just as easily caused new plant-eaters to evolve to exploit the new resource. Or maybe there’s no connection between dinosaurs and flowers at all, and the timing is just a coincidence. 

That’s what Butler and colleagues found in 2009, when they used statistical tests to compare the diversity of flowering plants to the diversity of plant-eating dinosaurs at various points in time. Under closer inspection, the timing just doesn’t work out. Based on currently available data, there is no clear correlation between increases in abundance or diversity between the two groups. 

Still, the spread of angiosperms didn’t come from nowhere. Perhaps insects or small forest-dwelling mammals had a role in ensuring the success of flowering plants. As always, the answer depends on finding more fossils!

References

Barrett, P.M. and Willis, K.J. 2001. Did dinosaurs invent flowers? Dinosaur-angiosperm coevolution revisited. Biological Reviews 76:3:411-447.

Butler, R.J., Barrett, P.M, Kenrick, P., and Penn, M.G. 2009. Diversity patterns amongst herbivorous dinosaurs and plants during the Cretaceous: implications for hypotheses of dinosaur/angiosperm co-evolution. Journal of Evolutionary Biology 22:3:446-459.
Mar 29

Egg-laying sauropods used osteoderms as calcium banks

Posted on March 29, 2018 at 12:30 PM by Bonnie Man

Sauropods - long-necked dinosaurs like Brontosaurus and Astrodon - were remarkable creatures. They were the largest land animals the world has ever known. Their crane-like necks were incredible feats of bioengineering. As a group, they were incredibly long-lasting, living all over the world for 140 million years. But one feature of sauropods often goes underappreciated: they were armored.

The backs and flanks of many sauropod species were studded with osteoderms. These small, teardrop-shaped bones were embedded in the animals’ skin. Traditionally, paleontologists have assumed defense was the main function of sauropod osteoderms. However, researchers at the National University of Distance Education in Madrid have proposed another possibility: osteoderms were calcium reserves for egg-laying females. 

When modern birds are preparing to lay eggs, they grow a temporary layer of densely mineralized bone on the interior surfaces of their hollow limb bones. The birds use this layer, called medullary bone, as an extra supply of calcium while eggshells are forming in their bodies. This prevents debilitating bone resorption – making eggshells requires a lot of calcium! Several dinosaur species, including Tyrannosaurus and Allosaurus, are also known to have produced medullary bone. According to Daniel Vidal and his colleagues in Madrid, sauropod osteoderms may have served a similar function in pregnant females. 

Sauropod osteoderms are generally rare, perhaps because they are not connected to the rest of the skeleton and scatter shortly after the animal dies. However, an unusually large number have been collected at Lo Hueco, a late Cretaceous fossil site in east-central Spain. Vidal and colleagues examined 17 osteoderms from Lo Hueco inside and out, using CT scans to see the interior surfaces. They found that while most osteoderms had a solid bone core, 30% of the fossils had hollow centers. In both cases, the researchers observed a dense network of neurovascular canals radiating out from the osteoderm cores. 

This suggests that, like medullary bone in birds, the osteoderms were reseviors of calcium that the animal could draw on in times of need. Notably, only a small percentage of the osteoderms had drained cores. Vidal and colleagues argue that the calcium reserves were therefore only being used by portion of the population – females preparing to lay eggs. If the calcium reserves were being drawn on during a time of environmental stress, like a drought, all the sauropods would have similarly depleted osteoderm cores.

References

Vidal, D., Ortega, F., Gascó, F., Serrano-Martinez, A., and Sanz, J.L. 2017. The internal anatomy of titanosaur osteoderms from the Upper Cretaceous of Spain is compatible with a role in oogenesis. Scientific Reports 7:42035.

Schweitzer, M. H., Wittmeyer, J. L. and Horner, J. R. 2005. Gender-Specific Reproductive Tissue in Ratites and Tyrannosaurus rex. Science 308:1456-1460.