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The shape of the "Pachysuchus" fossil (in grey) set into a sauropodomorph dinosaur skull. Image from Barrett and Xu, 2012.

Paleontologists are naming new dinosaurs at an extremely rapid pace. This past week alone, we’ve seen the announcement of Philovenator and Ichthyovenator, and the next new dinosaur is undoubtedly only a few days from publication. But we have also lost a few dinosaurs. Some of these, such as Dracorex, Anatotitan and Torosaurus, might get folded into other genera thanks to our changing understanding of how dinosaurs grew up. And as paleontologist Bill Parker pointed out at Chinleana, creatures once thought to be dinosaurs have been recategorized as very different, distantly related sorts of archosauriforms (the major group to which dinosaurs, crocodiles and many related lineages belong). Shuvosaurus, for example, was originally described as a Triassic iteration of the “ostrich mimic” dinosaurs such as Ornithomimus but turned out to be a strange, bipedal creature that was more closely related to crocodiles. And Revueltosaurus, an animal originally cast as a dinosaur because of its teeth, is now known to be more closely related to the well-armored “armadillodile” aetosaurs.

Yet reinterpretations can go the other way. Parker points out that a paper just published in Vertebrata PalAsiatica reports that a fossil thought to represent a superficially crocodile-like animal is actually part of a dinosaur jaw.

In 1947, paleontologist Yang Zhongjian—better known to many by the name C.C. Young—mentioned a fragment of a sauropodomorph dinosaur’s snout discovered in the roughly 195-million-year-old, early Jurassic deposits near Lufeng, China. He referred the specimen to Lufengosaurus, one of the many long-necked, small-skulled dinosaur cousins of the more famous sauropods. A few years later, Young changed his mind. He redescribed the battered fragment as a piece of a phytosaur skull. These archosaurs, found in older Triassic strata, generally resembled crocodiles but were actually a different group. (The easiest way to tell the difference is that the nasal openings of phytosaurs sat far back on their snouts, near their eyes.) Young named the animal Pachysuchus imperfectus, and although heavily damaged, the fragment became an important milestone for phytosaurs. The fossil was discovered in early Jurassic rock, so it lived millions of years after phytosaurs disappeared elsewhere. Young’s phytosaur seemed to represent the last of these trap-jawed aquatic predators.

Not everyone agreed with Young’s conclusion. While some paleontologists followed Young’s phytosaur ID, others said that the fragment was too uninformative to tell exactly what kind of archosaur it belonged to. The specimen was somehow lost in the collections of China’s Institute of Vertebrate Paleontology and Paleoanthropology, hindering efforts to figure out exactly what sort of animal Pachysuchus was.

Paul Barrett and Xu Xing relocated and re-examined Pachysuchus, but they didn’t see a phytosaur. Young was much closer to the mark with his original determination. The damaged skull piece exhibits many traits never seen in phytosaurs but that closely match what paleontologists have documented among sauropodomorph dinosaurs. Exactly what species of dinosaur the jaw belonged to is impossible to say—the appropriate traits for a species identification may be missing—but the best fit is certain some variety of sauropodomorph.

There were no Jurassic phytosaurs in Asia. And the proposed occurrences of Jurassic phytosaurs elsewhere are highly questionable, at best. These creatures, which lived alongside and probably preyed on early dinosaurs, were wiped out at the end of the Triassic, just before dinosaurs rose to global dominance.

Reference:

Barrett, P. M., and X. Xu. 2012. The enigmatic reptile Pachysuchus imperfectus Young, 1951 from the lower Lufeng Formation (Lower Jurassic) of Yunnan, China. Vertebrata PalAsiatica 50:151-159

Did egg-laying spell doom for non-avian dinosaurs, such as this crispy Troodon at the San Diego Natural History Museum? Photo by the author.

How did dinosaurs come to rule the Mesozoic world? No one knows for sure, but the way dinosaurs reproduced probably had something to do with it. Dinosaurs grew fast, started mating before they hit skeletal maturity, and laid clutches of multiple eggs—a life history that may have allowed dinosaurs to rapidly proliferate and diversify. And egg laying itself may have been critical to why many dinosaurs were able to attain gigantic sizes. By laying clutches of small eggs, dinosaurs may have been able to sidestep biological constraints that have limited the size of mammals.

But there was a catch. Consider a large dinosaur, such as Diplodocus. Infant Diplodocus hatched out of eggs roughly the size of a large grapefruit, and if they were lucky, the dinosaurs grew to be more than 80 feet long as adults. And the little sauropods were not just small copies of adults. Like many other dinosaurs, individual Diplodocus changed drastically during their lives, and young dinosaurs may have preferred different habitats and food sources from those of more mature individuals. As outlined by Daryl Codron and co-authors in a new Biology Letters paper, this peculiar life history may have been a consequence of laying eggs.

Codron’s group created a virtual dinosaur assemblage to see how intensely dinosaurs might have competed with one another as they grew. If all dinosaurs started off relatively small, then the largest species had to pass through a series of size classes and change their ecological role as they matured. This ramped up the pressure on young dinosaurs. Juvenile dinosaurs had to contend with other juveniles as well as dinosaurs that topped out at smaller sizes. In a diverse Late Jurassic ecosystem, for example, young Allosaurus, Torvosaurus and Ceratosaurus not only had to compete with one another, but also with smaller carnivores like Ornitholestes, Coelurus, Marshosaurus and Stokesosaurus. Dinosaurs would have faced the most competition at small size classes, and this may have driven some dinosaur lineages to become large.

The new paper also suggests that dinosaur life history may have played a role in the demise of the non-avian species. Competition at smaller size classes, Codron and colleagues suggest, drove dinosaurs to become bigger and bigger, and this created a lack of species that were small at maturity. Mammals and avian dinosaurs occupied those niches. This could have made dinosaurs more vulnerable to the intense pressures of the end-Cretaceous extinction. If the catastrophe targeted large animals, but was less severe among small animals, then non-avian dinosaurs would have been doomed. The big dinosaurs disappeared, and there were no small non-avian dinosaurs left to quickly proliferate in the aftermath.

As John Hutchinson pointed out in a Nature news story about this research, however, we’re going to need a lot more testing to see if this hypothesis holds up. The conclusion is based on a virtual model of ecosystems that we can’t study directly, and mass extinctions are frustratingly complicated phenomena.

Of course, a new dinosaur extinction scenario is irresistible journalist bait. Various news sources picked up the extinction hook (promoted in the paper’s press release) and pointed to the fact that dinosaurs laid eggs as the seeds of their undoing. But this isn’t quite right. After all, turtles, crocodylians and birds all laid eggs, too, and they survived. And mammals did not survive the end-Cretaceous extinction unscathed—several mammalian lineages disappeared or took major hits during the catastrophe. Likewise, not all dinosaurs alive during the final days of the Cretaceous were huge. Titans like Tyrannosaurus, Triceratops and Edmontosaurus are the most famous end-Cretaceous dinosaurs, but in western North America alone, there were also relatively small ceratopians, oviraptorosaurs and troodontid dinosaurs that topped out at about six feet in length. Were these dinosaurs still too big to survive? Was the threshold even lower? If it was, then the reason why medium-sized animals such as crocodylians survived, and why some mammals disappeared, becomes even more complicated. Why non-avian dinosaurs perished, and why so many other lineages survived, remains a mystery.

References:

Codron, D., Carbone, C., Muller, D., & Clauss, M. (2012). Ontogenetic niche shifts in dinosaurs influenced size, diversity and extinction in terrestrial vertebrates Biology Letters DOI: 10.1098/rsbl.2012.0240

Somewhere, out in the interstellar void, there may be a planet inhabited by hyper-advanced dinosaurs. At least, that’s what a new paper by Columbia University chemist Ronald Breslow says.

This morning, friend and fellow science writer David Dobbs forwarded me an American Chemical Society press release titled “Could ‘advanced’ dinosaurs rule other planets?” Since I was still a little bleary-eyed at the early hour, I thought I had read that wrong. But I saw it right the first time. “New scientific research raises the possibility that advanced versions of T. rex and other dinosaurs—monstrous creatures with the intelligence and cunning of humans—may be the life forms that evolved on other planets in the universe,” the item explained.

I couldn’t help but wonder if the pronouncement was inspired Planet of Dinosaurs—the awful 1978 film about a futuristic space crew stranded on a planet stuck in the dinosaurian heyday of the Mesozoic. But the paper itself suggests a different origin for what is ultimately a fossil-based non sequitur.

Breslow’s paper is primarily concerned with why the biochemical signature of life on earth is so consistent. Molecules such as amino acids, sugars, DNA and RNA exist in one of two possible orientations, left-handed or right-handed. Instead of showing a mixture of both forms, biomolecules typically come in only one form: Most sugars have a right-handed orientation, while most amino acids exhibit a left-handed orientation. Why life on earth should exhibit these particular arrangements and not the other possible orientations is a mystery that goes back to the origin of life itself.

One idea, favored by Breslow, is that meteorites carried specific types of amino acids and other organic flotsam to earth around 4 billion years ago. This is an extension of the idea that life here was “seeded” by comets, asteroids or meteorites. The origin and subsequent evolution of our planet’s flora and fauna would be constrained by the characteristics of the biomolecules that gave life a jump-start.

None of this has anything to do with dinosaurs. (The first dinosaurs, as far as we know, originated a scant 230 million years ago.) Yet, in closing, Breslow briefly speculates on what alien creatures might look like—perhaps possessing the opposite biochemical orientations of life on earth. “Such life forms could well be advanced versions of dinosaurs,” Breslow writes, “if mammals did not have the good fortune to have the dinosaurs wiped out by an asteroidal collision.” Whatever such space dinosaurs might look like, though, “We would be better off not meeting them,” Breslow warns.

As much as I’m charmed by the idea of alien dinosaurs, Breslow’s conjecture makes my brain ache. Our planet’s fossil record has intricately detailed the fact that evolution is not a linear march of progress from one predestined waypoint to another. Dinosaurs were never destined to be. The history of life on earth has been greatly influenced by chance and contingency, and dinosaurs are a perfect example of this fact.

Prior to 250 million years ago, the synapsids—our ancestors and relatives—were the dominant creatures on land. But the apocalyptic extinction at the end of the Permian Period eliminated most synapsid lineages, in addition to many other forms of life. This clearing of the ecological slate is what allowed a different group of creatures to proliferate. Early archosaurs, or “ruling reptiles,” included the archaic forerunners of crocodiles, pterosaurs and dinosaurs, in addition to various groups now extinct, and these creatures dominated the Triassic.

Despite what has been traditionally told, though, the dinosaurian branch of the greater archosaur family tree didn’t immediately out-compete its neighbors. Eoraptor and Herrerasaurus were not the Triassic terrors they were cast as during the mid-1990s. For the most part, Triassic dinosaurs were small, rare, marginal parts of the ecosystems they inhabited. It was only after another mass extinction at the end of the Triassic, around 200 million years ago, that the competitors of early dinosaurs were removed and the reign of the dinosaurs truly began. “[T]here was nothing predestined or superior about dinosaurs when they first arose,” paleontologist Stephen Brusatte and colleagues wrote in a massive review of dinosaur origins, “and without the contingency of various earth-history events during the early Mesozoic, the Age of Dinosaurs might have never happened.”

Even if we ignore all the major evolutionary events prior to 250 million years ago, the fossil record demonstrates that the origin and rise of the dinosaurs were heavily influenced by two catastrophic extinction events. Had the Permian or Triassic extinctions not happened, there is no indication that dinosaurs would have evolved or come to rule the world—unforeseen events drastically shaped evolutionary history. Why on earth would we expect such patterns to be played out in just the right sequence on another planet? To say that there are dinosaurs on alien worlds presupposes that there is an irresistible direction that all life follows, and that dinosaurs are an inevitable actors in the drawn-out drama. There is no evidence that this is so.

The strange thing is that Breslow acknowledges the role of mass extinctions in evolutionary history. His speculative space dinosaurs are supposedly “advanced” creatures which were spared from oblivion. Other writers have toyed with this concept before, the most famous example being Dougal Dixon’s The New Dinosaurs. Sadly, though, Breslow did not include any illustrations or offer specific details about the sort of uber-dinosaurs he has in mind.

Yet, what we know of the history of life on earth dispenses with the need to imagine such fantastic, alien creatures. Dinosaurs still exist—birds are a surviving dinosaur lineage that has exploded into a beautifully array of disparate forms. And some birds, such as ravens, are quite intelligent, so we don’t have to wonder about what an especially smart dinosaur would have looked like. The reign of the dinosaurs may have ended 66 million years ago, but their 230-million-year-old legacy continues to this day. A simple shift in our understanding of dinosaur evolution has rescued the beloved creatures from extinction. I deeply doubt that there are dinosaurs in space, but I am glad that at least one variety of feathered dinosaur remains with us here.

References:

Breslow, R. (2012). Evidence for the Likely Origin of Homochirality in Amino Acids, Sugars, and Nucleosides on Prebiotic Earth Journal of the American Chemical Society DOI: 10.1021/ja3012897

Brusatte, S., Nesbitt, S., Irmis, R., Butler, R., Benton, M., & Norell, M. (2010). The origin and early radiation of dinosaurs Earth-Science Reviews, 101 (1-2), 68-100 DOI: 10.1016/j.earscirev.2010.04.001

There is no shortage of ideas about why the non-avian dinosaurs went extinct about 66 million years ago. There are so many hypotheses, in fact, that I have seen some museums put up signs warning visitors of various ideas with no evidence (ice age, disease, aliens) rather than list all the other contenders that have been taken seriously. Even now, the mass extinction that wiped out Triceratops and its kin is mysterious. We know an asteroid struck the earth at a critical time, there were massive volcanic eruptions among prehistoric India’s Deccan Traps, sea levels were dropping, and habitats were changing, but exactly how these various factors translated into one of the world’s worst environmental catastrophes is still being debated.

The animated short “Snow Day” by Kim Hazel takes a different view. Dinosaurs simply couldn’t bundle up in time. The efforts of Hazel’s doomed dinosaur to put on a sock remind me of something that might appear on “T-Rex Trying… .” I seriously doubt that dinosaurs pulled on socks, though. If dinosaurs slipped into any warm footwear, would they not have preferred slippers made from the fuzzy mammals that were always underfoot?

A restoration of the Cretaceous snake Sanajeh about to gulp down a baby sauropod. Model by Tyler Keillor, photographed by Ximena Erickson.

In 1925, when Yale University paleontologist George Wieland published a paper titled “Dinosaur Extinction,” no one knew why the great archosaurs had disappeared. The fact that the extinction of the dinosaurs was even worth explaining was a new idea. From the time dinosaurs were initially described in the early 19th century through the beginning of the 20th, their existence and disappearance simply seemed to be part of a grand progression of life that required no special attention or explanation. Even when paleontologists began to puzzle over why the dinosaurs vanished, many thought that dinosaurs were inevitably doomed by strange, internal growth factors that made them so large, stupid and ornate that they could not possibly adapt to a changing world.

But Wieland took a slightly different view. While his paper was more opinion than science—there was nothing measured, quantified or tested in the article—Wieland believed that he had perhaps identified some of the “invisible influences” that triggered the demise of the dinosaurs. Egg-eaters were of primary concern.

Wieland was not the first to suggest that the destruction of dinosaur eggs led to the group’s extinction. As pointed out by Wieland himself, paleontologists Charles Immanuel Forsyth Major and Edward Drinker Cope had previously speculated that small mammals may have raided dinosaur nests so frequently that Triceratops and its Mesozoic ilk were incapable of reproducing successfully. This hypothesis seemed plausible in general, but Wieland disagreed about mammals being the primary culprits. Small Mesozoic mammals seemed too weak to break open tough dinosaur eggs, and the most voracious modern-day nest thieves seemed to be those reptiles capable of swallowing eggs whole. “The potent feeders on dinosaur eggs and young must be sought for amongst the dinosaurians themselves,” Wieland remarked, “and perchance, amongst the earliest varanids [monitor lizards] and boids [boa snakes].”

Wieland believed that egg-eating must have been rampant during the age of the dinosaurs. In fact, he thought that a diet of eggs may have even led to the evolution of some of the largest of all predatory dinosaurs. Considering the giant Tyrannosaurus, Wieland wrote, “What more likely than the immediate ancestors of this dinosaur got their first impulse toward gigantism on a diet of sauropod eggs, and that, aside from the varanids, the theropod dinosaurs were the great egg-eaters of all time?” The cruel irony of this idea was that the immense predatory dinosaurs also reproduced by laying eggs, and Wieland considered it “quite inferable” that their nests, in turn, would have been raided by smaller monitor lizards and snakes.

Dinosaurs were not entirely defenseless against such attacks. Though dinosaurs were often thought in the 1920s to be reptiles write large, Wieland speculated that dinosaurs would have provided some parental care, were probably more active than living lizards and crocodiles and, among the egg-eating varieties, may have even sought out unprotected nests in coordinated “droves.” “With such active and powerful beasts at the jungle-edge,” Wieland wrote, “life was varied and sanguinary, be it within scientific dignity to say so.” Unfortunately, an active and varied existence could not save the dinosaurs. Both ecological factors and the supposed inability of dinosaurs to change sealed the fate of the dinosaurs, Wieland concluded; the great loss of eggs and the “racial senility” of dinosaurs ultimately ushered the group into extinction.

When Wieland wrote his paper, he could only speculate about predation on dinosaur eggs and babies. In the decades since, however, paleontologists have turned up rare fossil evidence that small predators truly did snap up young dinosaurs in various stages of development. In 2010, paleontologists announced the discovery of Sanajeh, a late-Cretaceous snake that may have fed on the eggs of sauropod dinosaurs. Several years before that, a different team of paleontologists found several baby Psittacosaurus skeletons in the fossilized stomach contents of the opossum-sized mammal Repenomamus, and in a 1994 paper, paleontologist James Kirkland suggested that small crocodyliforms like the slender Fruitachampsa may have also gobbled up eggs and little dinosaurs since their bones are sometimes found in association with dinosaur nests.

Despite these recent discoveries and hypotheses, however, there is no indication whatsoever that dinosaurs were driven to extinction by egg-eaters, reptilian or otherwise. Perhaps such a view was tenable when only a few dinosaur genera were known and we understood very little about their ecology, but not now. We have a greatly revised understanding of what happened at the end of the Cretaceous—a mass extinction that wiped out not only the dinosaurs, but a vast swath of life forms on land and sea. There is no hint of a run on dinosaur eggs in the fossil record, and the various types of supposed egg predators lived alongside dinosaurs for millions of years without killing off their egg-laying food supply. Dinosaur eggs certainly were a nutritious resource that were undoubtedly exploited by carnivores and omnivores, but such depredations were not the keys to dinosaur extinction.

References:

Wieland, G. 1925. Dinosaur Extinction. American Naturalist. 59 (665): 557-565