Discovery sheds light on how vertebrates see

New research led by the University of Leicester has overturned a long-standing theory on how vertebrates evolved their eyes by identifying remarkable details of the retina in the eyes of 300 million year-old lamprey and hagfish fossils.

Professor Sarah Gabbott on fieldwork in Illinois digging for new fossil specimens. In her hand is 
a hard nodule formed 300 million years ago on a warm, shallow sea floor 
[Credit: Thomas Clements]

The study, published in the journal Proceedings of the Royal Society B, led by Professor Sarah Gabbott from the University of Leicester Department of Geology, shows that fossil hagfish eyes were well-developed, indicating that the ancient animal could see, whereas their living counterparts are completely blind after millions of years of eye degeneration - a kind of reverse evolution.

The researchers examined the eye tissue in two fossil jawless fish species - Mayomyzon (a lamprey) and Myxinikela (a hagfish) found in the Carboniferous age Mazon Creek fossil bed, Illinois.

Using a high-powered scanning electron microscope to magnify the eye 5,000 times they could see that the fossil retina is composed of minute structures called melanosomes - the same structures that occur in human eyes and prevent stray light bouncing around in the eye allowing us to form a clear visual image.

This is the first time that such details in fossil vertebrate eyes have been brought to bear on the tricky problem of how their eyes evolved.

The eye is a complex structure and must have evolved through small step-by-step changes but these are not recorded in living animals and until now it was thought that these anatomical details could not be preserved in fossils.

The fossil of a 300-million-year-old primitive jawless fish - a lamprey - about 6 cm in length. The eyes are the two dark 
circles on the right-hand side and are so well-preserved the retina and lens can be seen in high magnification 
[Credit: Rober Sansom]

Professor Gabbott explains: "To date models of vertebrate eye evolution focus only on living animals and the blind and 'rudimentary' hagfish eye was held-up as critical evidence of an intermediate stage in eye evolution. Living hagfish eyes appeared to sit between the simple light sensitive eye 'spots' of non-vertebrates and the sophisticated camera-style eyes of lampreys and most other vertebrates."

The details of the retina in the fossil hagfish indicates that it had a functional visual system, meaning that living hagfish eyes have been lost through millions of years of evolution, and these animals are not as primitively simple as we originally believed. As a result they are not the most appropriate model for understanding eye evolution.

Professor Gabbott added: "Sight is perhaps our most cherished sense but its evolution in vertebrates is enigmatic and a cause celebre for creationists. We bring new fossil evidence to bear on an iconic evolutionary problem: the early evolution of the vertebrate eye. We will now scrutinize the eyes of other ancient vertebrate fossils to see if we can finally build a picture of the sequence of events that took place in early vertebrate eye evolution."

The team also found the earliest evidence of skin pigment patterning in a fossil.

She added: "This heralds the realistic possibility of inferring details of the ecology and behavior of our ancient ancestors. Animals today have stripes for many reasons from camouflage to sexual display- we now have the potential to understand behavior in long extinct vertebrates."

Source: University of Leicester [August 02, 2016]

Discovery sheds light on how vertebrates see

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The first crocodile ancestors

Did you know that birds and crocodiles are practically cousins? Around 230 million years ago, you wouldn't have been able to tell the difference between the two different lineages. This is because birds and crocodilians (which includes alligators, caiman, and gharials) are part of a much larger group called Archosauria, or ruling lizards, which means they share a common ancestor far back in time. When they split from each other, they formed two major evolutionary pathways: the bird-line archosaurs, which also includes all dinosaurs, and the crocodile-line archosaurs, which includes crocodilians and their ancestors, the crocodylomorphs.

Skeletal reconstruction of Carnufex 
[Credit: PLOS]

Back at around the time of this split, during a time known as the Late Triassic, the world was much more different than it was today. Small crocodylomorphs prowled the land, along with the earliest dinosaurs. There were a host of other bizarre reptiles, such as the predatory rauisuchids, which might be closely related to the first crocodylomorphs, and small, fox-like sphenosuchians.

One of these was an animal known as Carnufex. Not only is that an awesome name, but it was also an impressive beasty to withhold, coming in at around 3 metres in length. It had serrated teeth for tearing apart its prey, and a long, slender body for rapid movement. Importantly, it is from around the time when this dinosaur-crocodile split occurred, and therefore should hold important clues to the evolutionary history of these groups.

Susan Drymala and Lindsay Zanno from North Carolina recognised the importance of Carnufex in helping to solve the dinosaur-crocodile divergence issue, and set out to conduct an impressive anatomical assessment of the well-preserved fossils. The fossils belonging to Carnufex also come from North Carolina, and were first discovered in 2003. They consist mostly of skull material, which is important for determining diagnostic relationships in archosaurs, and several bits of the spine and limbs.

The teeth of Carnufex, perfect for piercing and slicing flesh 
[Credit: PLOS]

By analysing the anatomy of Carnufex along with a large range of other similar animals, they were able to work out its evolutionary relationships (published in PLoS ONE). What they found is that, quite like many early diverging species, Carnufex had a mosaic of features, some more crocodilian, some more dinosaurian.

What this implies is that Carnufex is actually one of the earliest diverging crocodylomorphs, and therefore was highly important in determining the early fate of this ancient group. It was closely related to another crocodylomorph called Redondavenator, which was also fairly hefty in size.

This is important for several reasons. Carnufex was no tiddly croc, but a pretty large and fearsome predator. Other crocodylomorphs around at this time were usually small, nimble hunters, quite different from Carnufex. What this means is that the very first crocodylomorphs, such as Carnufex, were much larger than we previously thought, and developed their smaller body size later on, something which we can trace based on their evolutionary relationships. This also means that evolution of a smaller body size was something that occurred subsequent to the acquisition of features defining crocodylomorphs, rather than before.

In the Late Triassic, this means that Carnufex would have been one of the top predators roaming the plains of North America. This is quite exceptional, as other crocodylomorphs at the time were by no means top tier predators, with this role usually taken on by other now extinct archosaurs. Shortly after (geologically speaking..) Carnufex, this top predator tier was taken by theropod dinosaurs, which went on to dominate for around 150 million years.

What is clear is that Carnufex was a key stage in crocodylomorph evolution, and may have been critical in helping them survive the end-Triassic mass extinction, which took out almost all other archosaur groups around at the time.

Author: Jon Tennant | Source: Public Library of Science [August 02, 2016]

The first crocodile ancestors

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