Wednesday, April 24, 2013

Microraptor: Crow of the Cretaceous

Most of what we know about the diet of extinct animals comes from looking at their teeth, with obvious reasons. We can easily tell which animals were suited for slicing through flesh and which could easily grind vegetation, or which had teeth and jaws perfect for catching fish. There are a few instances, however, when we are lucky enough to actually find the preserved stomach contents of extinct animals. These findings, more than teeth or jaws, provide the most complete picture of what extinct animals were eating.

I've mentioned the diminuitive microraptorine Microraptor gui in a previous post. This little dromaeosaur has provided an enormous amount of information on not only the evolution of feathers and flight in non-avian dinosaurs, but it has revealed startling clues about its own habits. Like most other dromaeosaurs, M. gui was a predator, likely feeding on mammals, insects, and other small prey. In 2011, a specimen of M. gui was found with the fragments of bird bones in its stomach, indicating that it was an adept aerial hunter capable of taking out smaller, feathery prey on the wing (O'Connor et. al, 2011). This specimen revealed that Microraptor was, definitively, an arboreal hunter. However, just last week, another specimen of Microraptor revealed something even more surprising.

The new specimen of Microraptor. The preserved fish scales can be seen in the magnifying glass. Photograph by Scott Persons.
It turns out that the largest specimen of Microraptor, which also contains the remnants of its last meal, was preserved with fish scales in its stomach (Xing et. al, 2013). Did the specimen in question hunt the fish it was preserved with, or was it washed upon the shore of an ancient lake? These are questions we can't determine through fossils; however, Microraptor does not display any particular adaptations for fishing. The animal probably scavenged the fish, or managed to catch it in very shallow water. Either way, this changes our perception of the dromaeosaur in a very unexpected way. Instead of regarding Microraptor as a skilled arboreal hunter, which it most certainly was, it appears that the little glider was more of an opportunist than we imagined.

Microraptor eating a fish. Whether the specimen hunted the fish or was scavenging remains uncertain. Reconstruction by Emily Willoughby.
Between the discovery of the color of Microraptor, and clues about its generalist lifestyle, it seems as though Microraptor was the Cretaceous equivalent of a crow. Hopefully, more specimens are unearthed with stomach contents in tact. I wouldn't be surprised if we found stomach contents revealing something like this...

Microraptor gui feeding on a cycad. Reconstruction by Emily Willoughby.

O’Connor, Jingmai, Zhonghe Zhou, and Xing Xu. 2011. “Additional Specimen of Microraptor Provides Unique Evidence of Dinosaurs Preying on Birds.” Proceedings of the National Academy of Sciences (November 21). doi:10.1073/pnas.1117727108.

Xing, Lida, W. Scott Persons, Phil R. Bell, Xing Xu, Jianping Zhang, Tetsuto Miyashita, Fengping Wang, and Philip J. Currie. 2013. “Piscivory in the Feathered Dinosaur Microraptor.” Evolution: n/a–n/a. doi:10.1111/evo.12119.

Monday, April 15, 2013

The extreme diversity of Accipitridae

In classification, families range from being pretty well-defined to diverse beyond belief. The family Accipitridae falls into the latter category. Accipitridae includes most species birds of prey, collectively known as raptors. At first glance, the diversity of the family appears to be too great to be supported by anything but morphological; however, karyotypic data has supported the theory that the accipitrids are monophyletic (that is, sharing a common ancestor). Though most of the birds of prey are accipitrids, not all species are included: the secretary bird, owls, New World vultures, and the osprey are all included in families of their own due to genetic differences.

At first glance, raptors may appear to be very similar. To demonstrate just how diverse this family is, I've chosen a representative from each of the subfamilies of Accipitridae. A very special "thank you" to Gareth Monger, who created the following visual. All raptors are to scale, although some of them may be skewed due to odd angles.

Click to embiggen.

1. Bearded vulture (subfamily Gypaetinae) - Also known as the lammergeyer, it feeds on marrow by dropping bones from great heights. It also uses this technique to feed on tortoises.

2. Snail kite (subfamily Milvinae) - The long upper jaw of this raptor allows it to feed almost exclusively on snails, though it will also eat crustaceans and fish as well.

3. Dark chanting-goshawk (subfamily Melieraxinae) - Couldn't find anything particularly unusual about this species. Sorry, folks.

4. Harris hawk (subfamily Buteoninae) - The only raptor to actively hunt in familiar packs.

5. Pearl kite (subfamily Elaninae) - The second-smallest raptor in the world. Preys mainly on Anolis sp. lizards.

6. Black baza (subfamily Perninae) - Captures insects on the wing, or plucks them off of leaves. It has also been observed eating palm fruit.

7. Philippine eagle (subfamily Circaetinae) - Also known as the monkey-eating eagle. It also preys on anything from monitor lizards to hornbills.

8. Steller's sea eagle (subfamily Haliaeetinae) - Feeds mainly on fish, though also hunts waterfowl, gulls, herons, and cranes.

9. African harrier-hawk (subfamily Polyboroidinae) - Its "double-jointed" legs can bend forwards and backwards, allowing it to raid the nest cavities of smaller birds for fledglings.

10. Cinereous vulture (subfamily Aegyptiinae) - The world's largest true raptor. Reported to fly very close to herds of wild sheep, perhaps in order to pick out potential prey.

11. Harpy eagle (subfaimly Harpiinae) - Hunts mostly sloths, though large monkeys are also eaten.

12. Shikra (subfamily Accipitrinae) - Feeds on everything from termites to bats to birds. It often uses metal wire as part of its nest.

13. Verreaux's eagle (subfamily Aquilinae) - A hyrax specialist, though it will also hunt small antelope and other animals.

Monday, April 8, 2013

Out of Africa: The diversity and extinction of Africa's oldest penguins

Not many people realize how diverse penguins are. Usually associated with the frigid wastes of Antarctica, penguins are actually spread throughout all continents in the southern hemisphere, and range in habitat from pack ice to sandy beaches to fern forests. Out of all penguins, one sticks out from the rest, purely due to its range and habitat: the African penguin, Spheniscus demersus. This little penguin, about 2ft tall, nests among boulders on none other than the sunny, sandy shores of South Africa and Namibia.

Modern African penguins in their natural habitat.

It may seem odd to us that a penguin could find a suitable place to nest somewhere so warm, compared to the cooler climes of other species. However, penguins have been present in Africa for much longer than previously thought. New fossils from the Saldanha Steel formation near Cape Town are not only the oldest evidence of African penguins, they also show just how diverse the area was.

During the mid- to late Miocene, some 10-12Ma, sea levels were much higher than they are today: in the heyday of the African penguin, the sea was a full 90m higher than present levels. This not only provided much more swimming space, but it created a small archipelago. Penguins, being flightless birds and rather clumsy on land, prefer nesting in isolated areas such as islands; this eliminates the threat from large terrestrial predators. The ample nesting space could support a variety of species with no competition between them.

Modern sea level at Saldanha Steel (A) and during the early Pliocene (B), when the sea was 90m higher than today. The scattered islands in B would have been critical nesting grounds for penguins. From Roberts et. al, 2011.
Clearly, the higher sea levels were a blessing for the ancient penguins, for not one, not two, but four new species were uncovered from Saldanha Steel. The largest of these was as tall as the modern king penguin, the second-largest living species. The smallest was about as large as the smallest living species, the little blue penguin. The other two were comparable in size to the modern African penguin. The species were already very "modern" in appearance; penguins are among the oldest families of birds, and quickly evolved into recognizable species.

A brief history of African penguins. The four newly-discovered species are silhouetted in gray. From Thomas & Ksepka, 2013.

I imagine the scene at Saldanha Steel would have been very similar to modern nesting colonies of penguins, which would have been a site to behold, especially when you realize that scenes like this one would have taken place in Africa.

A colony of king penguins on the island of South Georgia. Perhaps Saldanha Steel would have boasted a much similar scene, with even more diversity, some 10Ma. Photograph by Arthur Morris.

As sea levels receded to their modern levels, the islands on which the penguins nested connected with the mainland, allowing predators to invade. Could predation have led to the extinction of these penguins? Or was it the disappearance of crucial nesting space? It's likely that the penguins were more and more pressed to find adequate nesting grounds, and the curtain was drawn with the arrival of large terrestrial predators. No matter what the cause of their extinction, it is fascinating to picture the diversity of penguins which once roamed South Africa, surely the last place we would readily associated with those clumsy, comical birds.