What’s news has been neglected a bit over the last few weeks, but fear not! Here are five exciting palaeo news stories from the last month or so:
For no particularly good reason, placoderms are a group close to my heart. This group of fish existed during the Devonian, and are the earliest example of vertebrate with jaws in the fossil record, As well as being generally awesome, they are also crucial to understanding how gnathostomes, the jawed vertebrates, evolved from their jawless ancestors. Dupret et al have CT imaged a primitive placoderm, Romundina, and have shown that it has a mixture of jawless fish (‘agnathan’) and gnathostome cranial characters. As with so much of evolution, the transition to attaining jaws seems to have been piecemeal, part by part, an example of mosaic evolution, with cerebral proportions remaining largely unchanged with the evolution of jaws.
Even more Burgess Shale
The Cambrian is famous for the weird and wonderful fossil taxa that existed during it, and the most famous Cambrian fossil site is the Burgess Shale, in the Canadian Rockies. Since the early 20th century the incredible preservation of this site has given us an invaluable window into life 500 million years ago. While the Burgess Shale animals were famously argued by Stephen Jay Gould to represent experimentations in body plans alien to anything alive today, more recent work has shown that actually these animals are early relatives of groups such as arthropods (ie. insects, crustaceans etc) and vertebrates (ie. you). The other week another site was described from the Burgess Shale; hopefully this will mean we’ll see lots of new Cambrian beasties over the next few years, as well as more information on known ones, continuing to fill in our picture of early animal evolution.
We already met Acanthostega, the earliest known tetrapod, a few weeks ago, when we were looking at Tiktaalik and the transition of vertebrates onto land. Acanthostega possesses a mixture of aquatic and terrestrial qualities: like a fish, it has gills and a tail fin, but like a modern tetrapod it had digits and a ‘neck’ between the head and body. This has been taken to demonstrate that tetrapods actually evolve many ‘adaptations to land’, such as digits, while the still lived in water, in contrast to the historical view that a (presumably rather optimistic) fish crawled onto land and only then adapted to it. Recent work by Neenan et al suggests that Acanthostega’s jaw was adapted to aquatic, rather than terrestrial, feeding, which they argue refutes recent suggestions that it might have fed on land, or at least above water.
Playing around with flight
Imagine, if you will, a bird. Chances are that you’re imagining something that’s probably feathered and beaked*. The fossil record, however, shows us that many traits that we associate with birds, such as feathers and beaks, evolved before birds came into existence as a group. Feathers are perhaps the best example, found in many dinosaurs not particularly closely related to birds, but it is also true of various skeletal characters. Work by Mark Puttick and others shows that this is the case with the comparatively long forelimbs of birds, which originated before the origins of the group, amongst earlier dinosaurs. Gliding dinosaurs like Microraptor suggest that many groups were independently evolutionarily ‘playing around’ with feathered gliding, it was just the birds that happened to evolve powered flight and make it through to today.
*If not, you may want to double check that you know what a bird is.
Primitive live birth in ichthyosaurs
As we’ve seen before on The Dinosirs, ichthyosaurs were a very successful group of Mesozoic reptiles, highly adapted to an aquatic life. One oft quoted adaptation viviparity, or live birth, as a fully aquatic amniote can’t get back onto land to lay eggs. This is also seen in other groups of marine reptile, such as plesiosaurs and mosasaurs. However, a recent paper by Motani and others has suggested that actually live birth was present in ichthyosaurs’ terrestrial ancestors before they became aquatic. They argue this based on a fossil of a very early ichthyosaur, Chaohusaurus, which appears to have been fossilised giving birth. While this is fairly common in ichthyosaur fossils, all previous fossils have been found giving birth to babies tail first as in modern day whales, possibly as an adaptation to prevent suffocation. This fossil demonstrates that Chaohusaurus gave birth head first, as in most terrestrial vertebrates. Motani et al argue that this demonstrates that ichthyosaurs evolved from terrestrial viviparous ‘head first’ ancestors, only later switching round as an adaptation to marine life as in whales today. This is contrary to the traditional view, but does fit with our picture of modern reptiles: many groups of lizard are viviparous.
Dupret et al (2014) A primitive placoderms sheds light on the origin of the jawed vertebrate face, Nature
Caron et al (2014) A new phyllopod bed-like assemblage from the Burgess Shale of the Canadian Rockies, Nature communications
Neenan et al (2014) Feeding biomechanics in Acanthostega and across the fish-tetrapod transition, Proc. Roy. Soc. B
Puttick et al (2014) High rates of evolution preceded the origin of birds, Evolution
Motani et al (2014) Terrestrial origin of viviparity in Mesozoic marine reptiles indicated by Early Triassic embryonic fossils, PLOS One