Category Archives: Dinosaurs

Argentine Sauropods Were Big

But even more interesting, other animals grew really big, too.

This summer, researchers report a new fossil find in Argentina, yet another gigantic animal (more than 15 meters nose to tail, weighing something like 7 tons), this one from the late Triassic [1].  The fossil looks a lot like sauropods, but dates earlier than the well know giant dinosaurs of the Jurassic, such as Diplodocus.

This find is interesting for several reasons.  It shows that dinosaurs grew very large even earlier than suspected, indeed, before the die off at the end of the Triassic.  And then, other species grew very large again, independently, during the Jurassic.  So we seem to have two independent cases of giganticism.

This second case forces reexamination of theories of the development of giganticism, what leads to what, and what is necessary and what is accidental in the evolution of really large land animals.

“The evolution from small bipedal to giant quadrupedal sau- ropodomorphs involved numerous anatomical changes,” ([1], p. 1)

The new species is as large as later cousins, but  has anatomical differences from them.  The researchers report that the neck is not elongated and the front limbs were not columnar, traits found in later sauropods.  These differences show that “that these fea- tures were not strictly necessary for the acquisition of gigantic body size.” ([1], p. 4)

The fossil remains suggest that the animal had a adaptation of avian like lungs and air sacs in the skeleton, unlike later suaropods.  In birds, this contributes to efficient respiration and also makes the bones lighter.  In this case, lighter bones would make larger bones easier to support.

These cavities may also play a role in thermoregulation.  Large animals need ot keep cool, so pneumatic cavities might have made larger size feasible.

In addition to the respiratory system, examination of the growth rings in the bones reveals evidence of “cyclical and remarkably high growth rates.”   One way to get big is to grow fast, which seems to be what these animals did.

Clearly, there is more than one way for a species to evolve extreme size. Future work will have to try to sift out more understanding the necessary and accidental features of these two cases, and what environmental and other factors were involved.  (What did they eat?  What ate them?  and so on)

  1. Cecilia Apaldetti, Ricardo N. Martínez, Ignacio A. Cerda, Diego Pol, and Oscar Alcober, An early trend towards gigantism in Triassic sauropodomorph dinosaurs. Nature Ecology & Evolution, 2018/07/09 2018.
  2. Helen Briggs, Fossil of ‘first giant’ dinosaur discovered in Argentina, in BBC News – Science & Environment. 2018.

Feathered Fossil Revises Family Tree of Birds

After the the Chicxulub impact there was a mass extinction, followed by a huge explosion of new species.  In recent decades we have learned more and more about these events, and as usually happens, the story gets more complicated the more we know.

One of the remarkable facts is how the great variety of not that different bird-like dinosaurs was sifted out, with only the modern-day birds surviving and spreading across the planet. What is the history and family tree of birds?  Where did different lines emerge, and how did they spread over the last 60 million years?

This summer, a new analysis of a fossil uncovered in Wyoming in 1982 found it to be a close relative of the ancestors of current day “banana eaters”, which live only in Africa [2].  The Wyoming fossil is reliably dated to 50 million years ago, so it looks like these birds lived in America at that time as well as Africa.  This group must have branched from other birds before that time, and spread from Africa to North America.  However they may have been distributed then, at some point, these birds died out everywhere except Africa.

nternational Turaco Society (From [1]_
This finding upends previous thinking about when and where this family of birds arose.  In large part this is because the study of living species focusses on the relatively abundant physical, geographical, and genetic data available, with little attention to the sparse and difficult fossil record of the ancestors of living animals.  These efforts derive putative family trees and evolutionary timelines even without reference to fossils.

The researchers point out that the new findings not only suggest a revised family tree, but offer “a valuable calibration point for neornithine molecular divergence dating analyses.”  There is nothing like a solidly documented specimen to constrain otherwise freewheeling data analytics!  The paper notes just how misleading results based solely on extant species can be, when compared to the same analysis with the fossil record included.

“[O[ur results serve as a cautionary warning against overreliance on extant data and high statistical support values when studying evolutionary processes that are fundamentally historical in nature.”


  1. Helen Briggs, Bird family tree shaken by discovery of feathered fossil, in BBC News – Science & Environment. 2018.
  2. Daniel J. Field and Allison Y. Hsiang, A North American stem turaco, and the complex biogeographic history of modern birds. BMC Evolutionary Biology, 18 (1):102, 2018/06/25 2018.

Dinosaur Dandruff?

Headline writers had fun with headlines like, Dinosaur dandruff reveals first evidence of skin shedding[1], but it’s a real find, and really important.

There has been much discussion about the outer covering of dinosaurs, which may have featured scales and/or feathers. But little is known about dinosaur skin, which is an integral part of this complex.

A new study reports on the analysis of fossils that contain flakes of skin from Cretaceous animals—essentially dinosaur dandruff [2]. <<link>> These fossils come from the wonderful fossil beds of North East China, which have yielded so many important views of the soft tissues of dinosaurs and early birds.

The remarkable fossils provide a rare view of dinosaur skin, “preserved with remarkable nanoscale fidelity”.  These samples reveal that these animals shed skin in small flakes, which reveal the cellular structure of the skin!  Who knew we could ever find such fossils?  Cool!

Skin and whatever covers it are, of course, a very import part of the thermoregulation of animals.  Consequently, these traces of skin tell us about the physiology of the animal.

First of all, shedding skin in small flakes itself is important. Reptiles shed in large molts, while birds shed small flakes continuously. This reflects continuous growth of birds, and suggests that these ancient birds and dinosaurs grew like birds, not snakes.

The skin also shows evidence of structures characteristic of modern feathered birds. This would indicate that avian skin and feathers co-evolved from early ancestors, before the development of flight.

However, the skin cells are distinctly different from modern birds. The structure is denser, with no fat, consistent with relatively little evaporative cooling compared to avians.  The researchers characterize this as “distinctly non- avian” and  indicating that “that feathered dinosaurs and early birds had a unique integumentary anatomy and physiology transitional between that of modern birds and non-feathered dinosaurs”.  This is consistent with the hypothesis that these ancient ancestors of birds had feathers but were not adapted for powered flight.

It’s really cool to find this kind of “missing link” for the physiology of the animals.  Who would have imagined we could actually find evidence that animals might have been “in between” the physiology of reptiles and birds?

But the coolest thing about this study was that they actually looked with their microscope!  Who would have thought that there could be intact, microscopic, fossil dandruff?  Who knows what other fossils may have similar micro scale remains that no one has looked for?  Get out your microscopes!

Very, very cool.

  1. Matt McGrath, Dinosaur dandruff reveals first evidence of skin shedding, in BBC News – Science & Environment. 2018.
  2. Maria E. McNamara, Fucheng Zhang, Stuart L. Kearns, Patrick J. Orr, André Toulouse, Tara Foley, David W. E. Hone, Chris S. Rogers, Michael J. Benton, Diane Johnson, Xing Xu, and Zhonghe Zhou, Fossilized skin reveals coevolution with feathers and metabolism in feathered dinosaurs and early birds. Nature Communications, 9 (1):2072, 2018/05/25 2018.

Cretaceous Birds

The great extinction event attributed to the Chicxulub impact wiped out most species of dinosaurs, as well as other life on land and sea.  (See chapter 9 of Brusatte for a dramatic reconstruction of the catastrophe.) But out of that global disaster, some species survived—frogs, mammals, and avian dinosaurs, AKA birds. These survivors eventually thrived, radiated, and repopulated the earth.

This has been an enduring mystery. Why did some species survive, while so many died off?  And, above all, why did the ancestors of today’s birds survive, when so many similar species died out at the time?

“[Q[uestions remain regarding the mechanisms underlying the survival of the deepest lineages within crown birds across the K-Pg boundary, particularly since this global catastrophe eliminated even the closest stem-group relatives of Neornithes” ([2] p. 1)

A new study examines this question using statistical analyses (Ancestral ecological reconstructions (AERs)) to impute the primary habitat of ancestors of contemporary birds [2].  The technique uses statistically derived taxonomy which identifies common ancestors (i.e., branches in the family tree). The ancestors are examined to classify them as “arboreal” or “non-arboreal”, i.e., adapted to living in trees or on the ground.

The study also constructed a timeline of the destruction and recovery of forests at this time. Using fossil traces of plants and pollen, they find a timeline of “disaster flora” (predominantly a spike of ferns), then a gradual repopulation with relatively simple forests, and after a million years, new complex forests.

The study concludes that the species that survived the impact were primarily non-arboreal, and arboreal species did not survive. The survivors then evolved to repopulate the regenerating forests, “reinventing” tree dwelling.

“Our AERs identify several extant clades with inferred transitions to arboreality early in the Cenozoic.”  ([2], p. 1)

This research suggests that the effects of the impact wiped out forests, eliminating the habitat of tree dwelling species. Ground dwelling species were better able to survive in the rubble.  Later, as forests regrew, the ground dwellers evolved to occupy the new arboreal spaces [1].

This hypothesis makes sense, and generally fits the facts.

But I have to reserve at least a bit of judgement about this result.  The conclusions are based on a complicated web of inferences, connected by statistical methods.  It is difficult to know just what kind of uncertainty exists in, say, the classification as “arboreal” or “non-arboreal”, or in the dating of the various common ancestors in the taxonomy.

Crucially, this study is imputing behavior (living in trees) from indirect evidence (primarily, sparse skeletal remains), which is always error prone.  Even though the data hangs together and seems to make sense, that doesn’t mean the explanation is correct.  All it will take is a handful of new fossils and the whole taxonomy and timeline will come tumbling down.

Another worry is that the study suggest a single, world wide process.  All the forests died, and then regrew. The tree dwellers died out everywhere, and the ground dwellers survived to invade unoccupied niches everywhere.

The Chicxulub event clearly had widespread effects. But that doesn’t mean the effects were the same everywhere.  If some areas retained minimally livable forests for a time, or started recovering earlier, then the selection and repopulation would be patchier and would involve geographic factors and migration.

Finally, the simple explanation—the trees were wiped out, along with tree dwellers—is almost certainly too simple.  There are lots of kinds of trees, and niches related to trees, and lots of “arboreal” species.  Why could tree dwellers not adapt?  What about species that lived on the ground and also in trees?  What about all the non-arboreal species that died out?

For that matter, what about all the mammals, frogs, fish, and everything else that survived the impact?  Many of the surviving ancestors might well have been “non-arboreal”, but that can’t be the whole story.  It certainly isn’t even part of the story for aquatic species.

Overall, this is an interesting hypothesis, and a moderately convincing collection of data to support it.  But I have to doubt that this is the whole story.

  1. Helen Briggs, How ancestors of living birds survived asteroid strike, in BBC News – Science & Environment. 2018.
  2. Daniel J. Field, Antoine Bercovici, Jacob S. Berv, Regan Dunn, David E. Fastovsky, Tyler R. Lyson, Vivi Vajda, and Jacques A. Gauthier, Early Evolution of Modern Birds Structured by Global Forest Collapse at the End-Cretaceous Mass Extinction. Current Biology,

Sitting on Dinosaur Nests

It has long been known that many species of dinosaurs laid eggs. Eggs and eggshells have been found, sometimes with identifiable embryos.  And nests have been found with strong evidence of parental minding.  We don’t know all the different ways dinosaurs may have built nests and tended their eggs, but we know that some made nests on the ground and apparently sat on the eggs similar to modern birds.

Of course, modern birds are relatively small and light compared to their theropod ancestors. Sitting on eggs is a delicate matter, so how would a half-ton mama manage it?

This month an international team reports a study of dinosaur nests which suggests that many species sat on their nests, even very large animals [2].  The trick is how the nest is arranged.

The study examined well preserved fossil nests of several related species ranging from approximately 40 kg to 1500 kg in body weight.  For comparison, a contemporary ostrich will weigh about 100 kg, so these animals ranged much larger than birds.

All the species in this family laid eggs in a circle, with a mound in the middle and a ditch around the outside. The nests in this study were mostly open, i.e., were not buried as with modern crocodiles. As might be expected, the larger species laid larger eggs which probably had thicker and stronger shells. However, eggshells cannot be too strong, or the baby can’t get out, so even the bigger eggs would be fragile.

Regardless of nest size, all eggs are inclined and arranged in a radial pattern within a ring-shaped clutch “ ([2], p.3)

The key finding is that the outer ditch and center mound were larger in the larger species. That is, larger animals laid their (generally larger) eggs in a larger diameter circle, with more room in the middle.  This makes sense, assuming that the center area was a place where the parent could stand and sit.

Image caption Illustration: nesting behaviours of large and small oviraptorosaurs Image credit: Masato Hattori. [From BBC]
The authors note that this adaptation is not seen in contemporary birds. They speculate that this arrangement may have meant less contact with the eggs for the larger animals, which may be less advantageous than other styles, including the nesting styles of contemporary birds.  Perhaps this factor selects for relatively small body size accompanied by sitting directly on eggs in the next.

By the way, other investigations suggest that these eggs were probably blue-green.

  1. Mary Halton, Dinosaur parenting: How the ‘chickens from hell’ nested, in BBC News -Science & Environment. 2018.
  2. Kohei Tanaka, Darla K. Zelenitsky, Junchang Lü, Christopher L. DeBuhr, Laiping Yi, Songhai Jia, Fang Ding, Mengli Xia, Di Liu, Caizhi Shen, and Rongjun Chen, Incubation behaviours of oviraptorosaur dinosaurs in relation to body size. Biology Letters, 14 (5) 2018.

Reconstruction of an Ancient Bird’s Beak

One of the intriguing questions of evolutionary biology is just how birds emerged from other dinosaurs.  In recent years we have seen a flood of feathered dinosaurs and early birds, all from approximately the same time.

These animals are clearly related to each other and are ancestors of both contemporary birds and other lines of dinosaurs that died out.  In fact, the new evidence has made the picture even more complicated, because there seem to be anatomical and behavioral convergences in these species.  It’s more of a family bramble than a family tree.

In addition to the obvious questions about the origins of feathers and flight, there is a longstanding mystery of the bird’s beak. Most dinosaurs (and reptiles and mammals) have teeth, most birds don’t.  How (and why) did birds evolve beaks? How did they lose their teeth?

This question has only been complicated by fossil finds of early species that look like birds with teeth, as well as dinosaurs and other animals with beak-like mouths.  For that matter, beaks are part of a characteristic avian skull and brain case, which is quite different from skull of other animals.

Well preserved “missing links” have been, well, missing.

This summer researchers from several institutions report on a study of Ichthyornis dispar a toothed bird from the late Cretaceous [2]. The new work is based on an unusually well-preserved specimen recently found, plus other specimens from collections, including several “undescribed” fragments, i.e., specimens that have been gathering dust in some back room.  The skulls were CT scanned and the data combined to create a fairly complete 3D model of I. dispar cranium.

The reconstruction shows a considerable number of features that are clearly similar to birds.  The merest glance catches the beak and eyes of a bird.  The CT scans also reveal that the brain was extremely birdlike as well.  At the same time, I. dispar also had teeth and robust jaw muscles of other dinosaurs.

Figure 1 | Skull of the bird Ichthyornis dispar. Field et al.1 report the reconstruction of the skull of an extinct species. Their reconstruction fills in some structures missing from previously available fossils, thereby illuminating the transition between the loss of ancient dinosaur features and the evolution of characteristics found in present-day birds. The sections in yellow are newly identified fossil material, whereas the grey structures have been described previously. a, A side view of the skull. b, A view from above the skull (beak positioned on the left) showing cross-sections in two focal planes. The section above the black line is closer to the top of the skull than the region below the black line. Scale bar, 1 centimetre. (Adapted from Extended Data Fig. 2 of ref. 1.) (From [3])

In short, this species turns out to be a classic “missing link”, clearly suggesting the evolutionary history of the heads and beaks of birds was gradual and piecemeal.

This finding is particularly satisfying because Ichthyornis was discovered a long time ago, and has been subject to speculation about the connection between dinosaurs and birds since Darwin’s day [3]. The new reconstruction shows that this “missing link” was there from the very beginning!

Right under our noses this whole time was an amazing, transitional bird,” said Dr Bhart-Anjan Bhullar. (quoted in [1])

  1. Helen Briggs, How birds got their beaks – new fossil evidence, in BBC News – Science & Environment. 2018.
  2. Daniel J. Field, Michael Hanson, David Burnham, Laura E. Wilson, Kristopher Super, Dana Ehret, Jun A. Ebersole, and Bhart-Anjan S. Bhullar, Complete Ichthyornis skull illuminates mosaic assembly of the avian head. Nature, 557 (7703):96-100, 2018/05/01 2018.
  3. Kevin Padian, Evolutionary insights from an ancient bird. Nature, 557 (7703):36-37, 2018/05/01 2018.


Dinosaur Tracks on Skye

Moat of what we know about dinosaurs comes from fossil remains, mainly skeletons.  But in the last century we have discovered other traces, including footprints [3].  Tracks are particularly interesting because they capture actual behavior in a particular time and place.

This spring Scottish researchers report on a new find of tracks from large sauropods that lived in middle Jurassic times [2].

The Sauropod footprints were left in a muddy, shallow lagoon (BBC, Jon Hoad)


Despite the epic Hollywood titles, there is relatively little known about the Jurassic period because there are relatively few areas with rocks from that era that might harbor fossils.  One place that does have Jurassic fossils is the Isle of Skye in Scotland.  In fact, the area has yielded many trackways, including the newly described find.

The remains are consistent with a shallow lagoon where wading animals left footprints on the soft bottom.  The tracks include 49 trackways that appear to be sauropods (lunch?), and some tridactyl tracks that are thought to be theropods (hunters?).

Unfortunately, it is difficult to know exactly what animals made the tracks, and the preservation is imperfect, as well.

With the overall collection of remains, it is clear that this area was inhabited by a variety of species, including dinosaurs.

At the very least, this study suggests that Skye will be an important source of information about the relatively unknown Jurassic era.

  1. BBC, Dinosaur tracks on Skye ‘globally important’, in BBC News -Scotland. 2018.
  2. Paige E. dePolo, Stephen L. Brusatte, Thomas J. Challands, Davide Foffa, Dugald A. Ross, Mark Wilkinson, and Hong-yu Yi, A sauropod-dominated tracksite from Rubha nam Brathairean (Brothers’ Point), Isle of Skye, Scotland. Scottish Journal of Geology, 2018.
  3. Martin Lockley, Tracking Dinosaurs: A New Look At An Ancient World, Cambridge, Cambridge University Press, 1991.