* Astronomy

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Acanthostega gunnari

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RE: Tetrapods

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Megalocephalus pachycephalus

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RE: Tetrapods

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Did bone ease acid for early land crawlers?

In a new paper in the Proceedings of the Royal Society B, scientists propose that the bony structures in the skin of many early four-legged creatures might have been there to relieve acid buildup in bodily fluids. Analysis of their anatomy suggests that as they ventured out of water, the animals would have had trouble getting rid of enough CO2 to prevent acid buildup.
The "dermal bones" within the skin, especially the bones covering the skull roof and forming part of the shoulder girdle, had a highly complex surface of ridges and furrows called "dermal sculpture." The authors suggest that these bones served as something of a reservoir of antacids - not for the tetrapods' stomachs, but for their bodily fluids including blood.
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Scientists have filmed an African lungfish using two fins to walk and "bound" along the bottom of its tank.
The lungfish appeared to use their pelvic fins as hind legs - stepping along the tank bottom.
This suggests, the researchers say, that some fundamental features of walking on land arose in similar fish before the animals made the transition to land.
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Four-legged creatures may have gained a foothold by ditching genes guiding fin development.
The loss of genes that guide the development of fins may help to explain how fish evolved into four-limbed vertebrates, according to a study.
In the late Devonian period, around 365 million years ago, fish-like creatures started venturing from shallow waters onto land with the help of eight-fingered limbs. The limbs had evolved from fins; during the transition, our back-boned ancestors lost rows of rigid fibres, called actinotrichia, that provide structural support and guide fin development. The number of digits was also later winnowed to a maximum of five on each limb.
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Title: Linked morphological changes during palate evolution in early tetrapods
Authors: Charles B. Kimmel, Brian Sidlauskas and Jennifer A. Clack

We examined the shapes and sizes of dermal bones of the palate of selected Palaeozoic tetrapods in order to identify the ancestral states of palatal bone morphologies in the earliest tetrapods, to learn how the composition of the palate varies within and among early tetrapod radiations, and to recognise evolutionary correlations among the size and shapes of skeletal elements in this important group of animals. We find that whereas the palatal bones themselves and their arrangements are usually conserved, considerable correlated evolutionary change occurs in the shapes and sizes of the bones. Some of the changes in the bones are allometrically linked to overall palate size, which varies more than 100-fold among the taxa in our sample. Often, these allometries were only hinted at in traditional independent contrasts-based regressions of log transformed data, particularly because many allometries are subtle, their slopes may vary among subclades, and the scatter around some trendlines is high. Rather, the allometries showed up in analyses of size-standardised palatal bone dimensions investigated using independent contrasts, bivariate phylomorphospace plots, and mirrored character reconstructions on the phylogenetic tree. We find negative allometry for parasphenoid lengths and widths essentially across the entire tree of Palaeozoic tetrapods, but with different trajectories characterizing the two largest clades, the temnospondyls and the lepospondyls. The lengths of several other elements may show positive allometries, either across the entire tree or in just a subclade. One possible positive allometry exists for the ectopterygoid, which appears to shorten allometrically in temnospondyls that evolve small body and palate size, and, as in Doleserpeton can be lost altogether. Both shortening and loss could be by the same developmental change, paedomorphosis, a form of heterochrony. Paedomorphosis might also account for evolution of relatively large parasphenoids in both lepospondyls and diminutive temnospondyls, but does not seem to explain evolution of ectopterygoid loss in lepospondyls. A regularity observed across nearly all taxa in our study set is an inverse correlation between the lengths of the vomer and pterygoid, bones that lie adjacent to one another along the long palatal axis. Further work is needed to learn whether such correlated evolution might be due to adaptation and/or to developmental bias, and particularly to learn how correlations and allometries themselves evolve.

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Cartoon depictions of the first animals to emerge from the ocean and walk on land often show a simple fish with feet, venturing from water to land. But according to Jennifer Clack, a palaeontologist at the University of Cambridge who has studied the fossils of these extinct creatures for more than two decades, the earliest land vertebrates - also known as tetrapods - were more diverse than we could possibly imagine.

"Some looked like crocodiles, some looked like little lizards, some like moray eels, and some were snake-like. They occupied all sorts of niches and habitats. And they varied tremendously in size - from about 10 cm long to 5 metres" - Jennifer Clack.

Long before mammals, birds, and even dinosaurs roamed the Earth, the first four-legged creatures made their first steps onto land, and quickly inhabited a wide range of terrestrial environments. These early land vertebrates varied considerably in size and shape.

Source

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The fossil record usually shows what adult animals looked like. But the appearance and lifestyle of juvenile animals often differ dramatically from those of the adults. A classic example is provided by frogs and salamanders. New discoveries from Uppsala, Cambridge and Duke Universities, published in Science, show that some of the earliest backboned land animals also underwent such changes of lifestyle as they grew up.
Professor Per Ahlberg at the Department of Physiology and Developmental Biology, Uppsala University, together with Jennifer Clack, Cambridge University, and Viviane Callier, Duke University, have studied fossil upper arm bones from the two so-called "four-legged fishes", Ichthtyostega and Acanthostega, from Greenland. These animals, which lived during the Devonian period about 365 million years ago, were among the earliest vertebrates (backboned animals) with fore- and hindlimbs rather than paired fins. They belong to the common stem group of all living amphibians, reptiles, mammals and birds.

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