What’s new(s): 22/03/14

We’ve got a range of stories for you in this week’s What’s News including evolving cetaceans, pygmy dinosaurs, and aquatic ground sloths.

A long-mandibled porpoise

Cetaceans are fairly weird animals to begin with: secondarily aquatic even-toed ungulates (like cows and sheep) that are grouped with hippos in the colourfully-named group, the Whippomorpha.  They’ve lost their back legs, have adapted their front ones to form flippers, and have developed dorsal and tail fins, and this morphology has proved extremely successful as they have radiated to fill many niches in the oceans, including filter feeding, suction feeding and giant squid eating.  A new fossil porpoise has recently been described from the Pliocene of California that adds to this diverse array of morphologies with it’s bizarrely shaped lower jaw.  The lower jaw of Semirostrum cerutti extends well past its upper jaw to form a prognathous projection, which was apparently well nourished by arteries.  This morphology is most similar to the mandibular morphology of skimmer birds, which fly low and close to the water while using their long and highly sensitive lower beaks to probe for underwater prey.  The authors argue that the properties of Semirostrum’s mandible suggest it did a similar thing in sediment at the bottom of the sea.

skimmer porpoise

A reconstruction of a ‘skimming’ Semirostrum and his joyous friend.  (From Racitot et al)

Evolving echolocation

Continuing on the whale-y theme, modern whales are split into two groups: the Odontoceti, or toothed whales, and the Mysticeti, or baleen whales.  The baleen whales filter feed small animals with their baleen, while the toothed whales (which include Semirostrum, who we just met) use echolocation to locate their larger prey, which involves emitting calls and listening to their echoes like a sort of biological sonar. These echolocating  vocalisations are associated with a unique array of features in odontocete skulls, that serve to amplify and  receive the sound.  The newly described Oligocene whale fossil Cotylocara macei is argued to possess these features, including dense snout bones argued to act as an acoustic reflector and a lot of room for the anchoring of a muscle associated with echolocation, the maxillonasolabialis (yep, apparently that’s a thing).  This brings the evolution of echolocation firmly down the odontocete stem, meaning that it evolved shortly after they diverged from mysticetes about 34-30 million years ago.

Whale phylo

If Cotylodira did echolocate, it suggests an origin where the arrow is pointing.  From Geisler et al.

Swimming sloths

As well as the familiar cetaceans, many other mammal groups have famously made the move to an aquatic life including manatees, seals and otters.  Common adaptations to this aquatic life are pachyostosis, the swelling of the solid outside layer of bone, and osteoschlerosis, the densification of bone, adaptations which are both argued to have evolved to reduce the animals buoyancy.   Were you to visit the Miocene-Plocene of Peru you might meet a less well-known aquatic mammal with these adaptations: the ground sloth genus, Thalassocnus.  While modern sloths seem happy to go for the occasional swim, this animal was adapted to an underwater life, and probably fed on marine vegetation.  A number of species of Thalassocnus existed throughout time, and Amson et al studied slices of their bones to track the evolution of pachyostosis and osteoschlerosis.  They found it could be tracked through the successive species as they increasingly adapted to aquatic life, offering a rare high-resolution view of the evolution of a trait, as well as further proof that sloths are awesome.

sloth bones

The ribs of Thalassocnus plotted onto a phylogeny, becoming denser as the genera adapted to marine life. The fact I put up this and resisted the urge to post one of the internet’s many sloth pictures is a testament to my dedication to palaeobiology and to you, dear reader. From Amson et al.

Pygmy Tyrannosaur

Another thing that’s awesome is, of course, dinosaurs, and we have two dinosaurs for your delectation and delight this week.  The first of these is a tyrannosaur, Nanuqsaurus hoglundi, the generic name (Nanuqsaurus) of which means ‘polar bear lizard’, surely one of the cooler (pun obvs intended) dinosaurian names.  This tyrannosaur was described last week from material found in Alaska, and defies the popular perception of tyrannosaurs as enormous carnivores by being relatively small.   As well as adding to our understanding of tyrannosaur diversity, this is particularly interesting as Alaskan members of another dinosaur genus, Troodon were found to be about 50% larger than their more southerly counterparts.  This was originally argued to be due to Troodon’s characteristically large eyes giving it a competitive advantage over other theropods in the low light conditions of the Arctic, selecting for larger body size.  The fact that Alaskan tyrannosaurs are, conversely, smaller is argued by the authors to add weight to this idea, with the claim that the opposite situation applied due to their diminished ability to see: a lowered competitive advantage resulted in selection for reduced size.

Pygmy

Size of Nanuqsaurus (A) compared to other tyrannosaurs (B-E, T. rex is B).  ‘Normal’ Troodon is F, with Alaskan Troodon as G.  From Fiorillo and Tykoski.

The ‘chicken from hell’

You’ve probably already heard of the second dinosaur that we encounter this week, the ‘chicken from hell’, Anzu wyliei.  Given that the generic name of the taxon is taken from an ancient Mesopotamian feathered demon (tbh, probably trumping polar bear lizard) one might question why it needs to be described as a ‘chicken from hell’, but hey ho.  Anzu is a new, and the most complete, member of the Caenagnathidae, members of the oviraptorsaurs like the more familiar Oviraptor. The Caenagnathidae contains taxa such as the massive Gigantoraptor, but so far is poorly known; Anzu goes some way to solving its relationships.   Like many other oviraptorsaurs it had a large, cassowary-like crest and no teeth; much debate has been had over exactly what dietary niche this group inhabited, and no firm answer has yet been reached.  Whatever its exact niche, Anzu is about 67million years old, and so it dates from immediately prior to the K/T mass extinction, and was found in the famous Hell’s Creek formation, showing that this locality still has information, and dinosaurs, to offer.

Screen Shot 2014-03-22 at 17.06.24

Anzu’s skeleton reconstructed. From Lamanna et al.

References

  • Racicot et al (2014) Unique feeding morphology in a new prognathous extinct porpoise from the Pliocene of California, Current Biology
  • Geisler et al (2014) A new fossil species supports an early origin for toothed whale echolocation, Nature
  • Amson et al (2014) Gradual adaptation of bone structure to aquatic lifestyle in extinct sloths from Peru, Proc. Roy. Soc. B
  • Fiorillo and Tykoski (2014) A diminutive new tyrannosaur from the top of the world, PLOS One
  • Lamanna et al (2014) A new large-bodied oviraptorsaurian theropod dinosaur from the latest Cretaceous of Western North America, PLOS One
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