May 25, 2011
Everybody knows that chewing your food carefully is part of good table manners. No one told that to Shastasaurus. This 27-foot marine reptile was probably a suction feeder that slurped up little cephalopods in the Late Triassic seas.
Shastasaurus was not a dinosaur. Instead, this creature was an ichthyosaur, a member of a group of fish-shaped marine reptiles that became beautifully adapted to a life spent entirely at sea. Thanks to new specimens found in the 228- to 216-million-year-old strata of China, paleontologists P. Martin Sander, Xiaohong Chen, Long Cheng and Xiaofeng Wang have discovered that Shastasaurus differed from the rest of its family in a strange way. Whereas most other ichthyosaurs had long snouts filled with small, conical teeth suited to snatching fish and cephalopods, Shastasaurus had a shortened, toothless maw.
Sander and colleagues reported their findings in the journal PLoS One earlier this week. Although several species of Shastasaurus are already known from China, British Columbia and the western United States, the new study is based on fossils previously described under the name Guanlingsaurus liangae. These fossils, it turned out, were actually another species of Shastasaurus, and the specimens illustrate that the skull anatomy of this ichthyosaur was different than previously supposed.
In Richard Hilton’s 2003 book Dinosaurs and Other Mesozoic Reptiles of California, for example, two Shastasuaurus species were reconstructed with the long, toothy snouts typical of other ichthyosaurs. Since the complete snouts of these North American species were unknown, and partial fossils assigned to Shastasaurus from Mexico and Canada seemed to indicate they were long-snouted, the ichthyosaur was given the usual, toothy profile. As Sander and co-authors point out, though, it is now thought that those long-snouted fossils don’t belong to Shastasaurus at all, and the specimens from China indicate that Shastasaurus had a short snout devoid of teeth.
Naturally, this revised skull shape has implications for the way Shastasaurus fed. Modern-day beaked whales appear to be good analogs. Much like Shastasaurus, beaked whales have short skulls which, with the exception of one or two pairs of small teeth in the lower jaw, are functionally toothless. Rather than biting down on food, these whales rapidly retract their tongue, creating a small pocket of suction that draws in small prey. Since Shastasaurus has a generally similar skull anatomy, as well as equivalent sites for muscle attachments that would have allowed them to perform similar lingual maneuvers, Sander and colleagues propose that the ichthyosaur was adapted to be a suction feeder many, many millions of years before whales.
After revising the anatomy and habits of Shastasaurus, Sander and co-authors also suggest that the existence of multiple, suction-feeding ichthyosaur species over the course of millions of years during the Late Triassic indicates some underlying environmental cause. The scientists note that levels of atmospheric oxygen dropped during the time of Shastasaurus. Fish populations, strangled by the reduced oxygen in the seas, may have declined as a result, but cephalopods like squid—which are more tolerant of low-oxygen environments—may have proliferated. Since suction-feeding appears to be an adaptation to consuming small, quick prey, and soft-bodied cephalopods are known to have been an important part of the ichthyosaur diet, the scientists hint that the evolution of Shastasaurus might be attributable to a boom in squid which was itself caused by a decrease in ocean oxygen levels. This hypothesis is not delineated in detail and relies on assumptions about large-scale evolutionary patterns, though, and testing it will require detailed studies of the prehistoric atmosphere, Triassic cephalopods, prehistoric fish and ichthyosaurs.
Regardless of the impetus for the evolution of Shastasaurus, the recognition that this animal was a suction-feeder adds to the diversity of ichthyosaur types known to have existed during the Triassic. There were crushers, cutters and squid-suckers, all filling different ecological roles when the seas were very different. Some whale species occupy some of the same ecological roles today, and in the way they swim and feed, they are fuzzy echoes of a long-lost Triassic past.
Sander, P., Chen, X., Cheng, L., & Wang, X. (2011). Short-Snouted Toothless Ichthyosaur from China Suggests Late Triassic Diversification of Suction Feeding Ichthyosaurs PLoS ONE, 6 (5) DOI: 10.1371/journal.pone.0019480
May 10, 2011
What was Kelmayisaurus? Discovered in 1973, the lower jaw and partial upper jaw of this large, predatory dinosaur from the Early Cretaceous of China have been frustratingly difficult to interpret. Maybe Kelmayisaurus belonged to some obscure lineage of archaic theropod dinosaurs, or perhaps the fossils were simply parts of some other, already-known dinosaur. In a forthcoming Acta Palaeontologica Polonica paper, researchers Stephen Brusatte, Roger Benson and Xing Xu finally solve the mystery.
Complete, articulated dinosaur skeletons are exceptionally rare finds. Most often, paleontologists find scraps—many dinosaurs are known from only a few parts of their skeleton. Determining the identity and relationships of a partial skeleton is dependent upon knowledge of other species. Tell-tale skeletal landmarks that are peculiar to some groups, but not others, allow paleontologists to narrow down the list of possibilities for what a particular fragment or bone might represent. Thanks to a recently improved understanding of a group of large, meat-eating dinosaurs known as carcharodontosaurids, the enigmatic Kelmayisaurus could be properly placed among its extinct relatives.
Unraveling the identity of Kelmayisaurus required two steps. First, Brusatte and co-authors had to determine whether the dinosaur could be distinguished as a unique species. Contrary to the idea that the remains were too scrappy to make such a determination, Kelmayisaurus had a distinctive groove on the front portion of the outside of the lower jaw. Kelmayisaurus is a valid dinosaur name, after all.
With the first question resolved, the paleontologists set about determining what sort of dinosaur Kelmayisaurus was. The best-supported hypothesis was that Kelmayisaurus was a carcharodontosaurid, related to Giganotosaurus, Acrocanthosaurus and others. Aspects of the Kelmayisaurus toothrow, such as the depth of some of the bone around the teeth, and the absence of features often seen in other groups of predatory dinosaurs placed the dinosaur among the “shark-toothed” predators. Though not as large as some of the largest dinosaurs in this group, Kelmayisaurus was still pretty big—about the size of its distant cousin Allosaurus.
But Kelmayisaurus was not the first dinosaur of its kind to be found in China. In 2009, Brusatte, Benson, Xu and several co-authors described another carcharodontosaurid from another long-neglected partial skull they called Shaochilong. This means that there are now two of these large predators known from a “dark period” spanning 140 to 90 million years ago in the history of Asia’s dinosaurs. The two known species were separated by at least 8 million years, further supporting the identification of Kelmayisaurus as a distinct species, but the recognition of these large predators in China hints that there are likely other carcharodontosaurids waiting to be found. Perhaps they already have been, and are waiting in museum collections to be redescribed like their relatives have been.
Brusatte, S., Benson, R., Xu, X. (2011). A reassessment of Kelmayisaurus petrolicus, a large theropod dinosaur from the Early Cretaceous of China Acta Palaeontologica Polonica DOI: 10.4202/app.2010.0125
January 21, 2011
Just a few hours after yesterday’s post on dinosaur embryos went up, another major egg-based discovery was announced, in the journal Science.
In October of 2009, paleontologists first described the flying reptile Darwinopterus, a pterosaur that lived in what is now China over 160 million years ago. Since then, multiple other specimens have been found, including a well-preserved specimen purchased by the Zhejiang Museum of Natural History from a local farmer. This slab, given the designation M8802, preserves the nearly complete skeleton of this pterosaur, but what makes it truly remarkable is that it also contains a pterosaur egg.
The egg can be seen directly behind the pelvis of the Darwinopterus specimen—confirming that this individual was a female—but this pterosaur was not fossilized in the act of egg-laying. Her skeleton tells of a more tragic end. Based upon a break in the skeleton and the way in which the slab formed, the scientists state:
We suppose that this individual experienced a violent accident that fractured the forearm, rendering the pterosaur incapable of flight and precipitating her into a water body. After this, she drowned, her carcass became waterlogged, sank to the bottom, and, as decay processes began, the egg was expelled from her body.
It is rare that we can glean such stories from the fossil record, but the real significance of this discovery is that it gives us a few new insights into the biology of these long-dead animals. For one thing, this find may allow paleontologists to determine the sex of these pterosaurs. The hips of M8802 and another specimen from a different museum (YH-2000) are wide and have a relatively large canal that would have allowed the egg to pass through. Other specimens, which may be males, have narrower hips, and this is consistent with the idea that females would require larger hips to lay eggs.
The difference between the sexes may be apparent in the headgear of these pterosaurs, as well. Both of the female specimens lacked crests on their heads, while the narrow-hipped, possibly male specimens had crests. A larger sample size will be needed to test this idea, but the presence of crests does seem to be associated with the more narrow-hipped specimens.
As for the egg itself, close examination showed that it had a leathery shell. It did not have a hard, mineralized outer coating like a chicken or dinosaur egg. Rather than being closed off from the outside world, then, the paleontologists state that the pterosaur egg would have increased in volume by taking up water after being laid.
This same mode of reproduction is seen among lizards and snakes today, and the authors of the new paper argue that, like these reptiles, pterosaurs did not provide care for their newly-hatched young. Rare pterosaur hatchlings show that they were well-developed very early on and may have been able to fly out of the nest quickly, although these inferences about behavior can only be approached indirectly for the moment. Further discoveries will inform our still-growing understanding of how these flying reptiles reproduced.
Lu, J., Unwin, D., Deeming, D., Jin, X., Liu, Y., & Ji, Q. (2011). An Egg-Adult Association, Gender, and Reproduction in Pterosaurs Science, 331 (6015), 321-324 DOI: 10.1126/science.1197323
November 15, 2010
Paleontologists are discovering dinosaur species at a dizzying pace. These days it seems that a new species is announced almost every other week. Many of these new dinosaurs are being found in China, and two different teams of scientists have recently described a pair of unique species from two locations within the country.
As described by paleontologists Nicholas Longrich, Philip Currie and Dong Zhi-Ming in the journal Palaeontology, the dinosaur Machairasaurus leptonychus was discovered in the 84- to 75-million-year-old rock near the village of Bayan Mandahu in Inner Mongolia (itself a part of northern China bordering the country of Mongolia). There was not much left of this dinosaur. All that remained was a partial right forelimb, parts of the left arm, and a few toe bones, but the lower arm bones, fingers and claws were distinctive enough to identify this as a new type of oviraptorid dinosaur.
More specifically, Machairasaurus appears to have been a small animal most similar to a subgroup of the oviraptorids called the Ingeniinae, and according to Longrich and co-authors, Machairasaurus and its close relatives had relatively robust hands that were not well-suited to grasping. Instead, the forelimbs of this dinosaur appear to have been better suited to “scratching, tearing or, conceivably, digging” than grabbing prey, and the anatomy of their mouths hints that they may have included a large amount of plant food into their diets. As paleontologists have discovered through the study of other Cretaceous dinosaurs, theropod dinosaurs can no longer be cast as a group of entirely carnivorous dinosaurs—multiple lineages of theropods switched over to plant-eating during the Cretaceous.
The second new theropod was described by a team of Chinese scientists led by Junchang Lü in Acta Palaeontologica Polonica. Named Xixiasaurus henanensis, this small animal was a troodontid dinosaur found in the circa 83-million-year-old strata of China’s Henan Province. Represented by a partial skull, lower jaw fragment and a few other bits from its lower arms, Xixiasaurus resembled other troodontids, such as Byronosaurus, in having a set of unserrated teeth which were small and closely-packed in the front of the jaw but larger and recurved in the back of the jaw. As with the forelimb specializations of Machairasaurus, the unique teeth of Xixiasaurus, Byronosaurus, and their closest relatives, Lü and colleagues suggest, may be related to a more cosmopolitan diet that included plants, but more than tooth anatomy alone will be required to investigate this hypothesis.
LONGRICH, N., CURRIE, P., & ZHI-MING, D. (2010). A new oviraptorid (Dinosauria: Theropoda) from the Upper Cretaceous of Bayan Mandahu, Inner Mongolia Palaeontology, 53 (5), 945-960 DOI: 10.1111/j.1475-4983.2010.00968.x
Lü, J., Xu, L., Liu, Y., Zhang, X., Jia, S., & Ji, Q. (2010). A New Troodontid Theropod from the Late Cretaceous of Central China, and the Radiation of Asian Troodontids Acta Palaeontologica Polonica, 55 (3), 381-388 DOI: 10.4202/app.2009.0047
November 11, 2010
In many of the books about dinosaurs I read as a child, the evolution of horned dinosaurs (ceratopsians) looked pretty straightforward. Early, lanky forms such as Psittacosaurus were succeeded by a miniature precursor of later types—Protoceratops—before generating the array of large, spiky ceratopsid dinosaurs such as Triceratops and Styracosaurus. Yet, as more discoveries have been made in Asia and elsewhere, paleontologists now understand that there was not a straight-line march of ceratopsian evolution but multiple radiations of different forms. A newly-described form from China highlights the complicated history of these dinosaurs.
The new species, named Zhuchengceratops inexpectus, was discovered in a Cretaceous-age bone bed rich in the remains of the large hadrosaur Shantungosaurus and located in China’s Shandong Province. Ribs, vertebrae, parts of the lower jaw and portions of the skull represent what we presently know about this animal, and there is enough of it to discern that it belonged to a peculiar group of horned dinosaurs called leptoceratopsids. These dinosaurs were relatively small—many were about six feet in total length—and, while they had short frills, they lacked the impressive horns of the ceratopsid dinosaurs. At one time they were thought to represent a rare and relatively short-lived part of horned dinosaur diversity, but recent discoveries and reanalysis of old data has shown that the leptoceratopsids were a relatively prolific and widespread group.
The evolutionary and environmental context of Zhuchengceratops underscores our changing view of the leptoceratopsids. Though it may have looked primitive compared to the ceratopsid dinosaurs, Zhuchengceratops and its close relatives overlapped in time and space with many of their larger, better-ornamented cousins. (Specifically, the newly described ceratopsid Sinoceratops was found just three miles away from Zhuchengceratops in rock of the same age, meaning that these dinosaurs were probably contemporaries.) And, while it is true that the relationships among the leptoceratopsids are still a little shaky, scientists have found so many forms in both North America and Asia that these animals now appear to have been members of a diverse, long-lived group which coexisted with the major radiation of ceratopsid species. Horned dinosaur evolution was not a matter of one stage giving way to the next, but is better understood as a wildly branching tree in which small, archaic types flourished alongside their well-decorated cousins.
Xu, X., Wang, K., Zhao, X., Sullivan, C., & Chen, S. (2010). A New Leptoceratopsid (Ornithischia: Ceratopsia) from the Upper Cretaceous of Shandong, China and Its Implications for Neoceratopsian Evolution PLoS ONE, 5 (11) DOI: 10.1371/journal.pone.0013835