/* */ Rosa Rubicondior: Evolution News - Explaining Mammalian Forelimbs

Tuesday, 19 March 2019

Evolution News - Explaining Mammalian Forelimbs

Photographs of the upper arm bones from seven kinds of early mammal relatives. The three bones on the left are from an early group called pelycosaurs, and the bones are all roughly the same shape. The four bones on the right are from therapsids, the group that includes today's mammals, and they show the greater variety of shapes and sizes that characterize therapsid limbs. The black scale bars represent 2cm.
Credit: (c) Jacqueline Lungmus, Field Museum
Mammals’ unique arms started evolving before the dinosaurs existed | Field Museum

Creationists and evolution deniers have been predicting the imminent demise of the Theory of Evolution almost since the day Darwin and Wallace presented their idea to the Linnean Society in 1858.

The latest claim, repeated forlornly every day by creationists like a prayer or a protective mantra, is that the notion of 'Intelligent (sic) Design' will soon replace the ToE as the accepted scientific explanation for biodiversity, so becoming the only established body of science to be replaced by magic. Imagine, a 'scientific' explanation that includes magic and an unproven notional entity that no-one can explain, doing things that no-one has ever witnessed it doing by a process that remains a mystery.

The fact that ID has been refuted comprehensively and dismissed by serious biologists, and declared in a court of law to be creationism in disguise, with the intention of deceiving people, is lost on creationists, apparently.

Curiously though (curious if the creationist Jeremiah's are to be believed) the ToE has gone from strength to strength, incorporating and strengthened by every major scientific advance from DNA and genetics, microscopy and biochemistry, to plate tectonics and cosmology. In 160 years, when almost every other body of science has seen major revision, the ToE emerged strengthened and broadened but inscathed in its underlying principle - accumulation of small changes over time, shaped and made non-random by the environment.

As this paper on the evolution of the different mammalian forelimbs shows, the ToE is the foundation of modern biology, taken for granted by biologists and showing not the least sign of being overthrown and replaced by magic. In short, the ToE remains assailed, 'overthrown' only in the minds of those non-biologists who never wanted to accept it in the first place. A crumb of comfort for those trying to live in a world from which undesirable facts have been excluded because the facts as they are don't make them feel important enough.

As the press release by The Field Museum announcing the publication of a paper in Proceedings of the National Academy of Sciences says:

Bats fly, whales swim, gibbons swing from tree to tree, horses gallop, and humans swipe on their phones—the different habitats and lifestyles of mammals rely on our unique forelimbs. No other group of vertebrate animals has evolved so many different kinds of arms: in contrast, all birds have wings, and pretty much all lizards walk on all fours. Our forelimbs are a big part of what makes mammals special, and in a new study in the Proceedings of the National Academy of Sciences, scientists have discovered that our early relatives started evolving diverse forelimbs 270 million years ago—a good 30 million years before the earliest dinosaurs existed.

“Aside from fur, diverse forelimb shape is one of the most iconic characteristics of mammals,” says the paper’s lead author Jacqueline Lungmus, a research assistant at Chicago’s Field Museum and a doctoral candidate at the University of Chicago. “We were trying to understand where that comes from, if it’s a recent trait or if this has been something special about the group of animals that we belong to from the beginning.”

To determine the origins of mammals’ arms today, Lungmus and her co-author, Field Museum curator Ken Angielczyk, examined the fossils of mammals’ ancient relatives. About 312 million years ago, land-dwelling vertebrates split into two groups—the sauropsids, which went on to include dinosaurs, birds, crocodiles, and lizards, and the synapsids, the group that mammals are part of. A key difference between sauropsids and synapsids is the pattern of openings in the skull where jaw muscles attach. While the earliest synapsids, called pelycosaurs, were more closely related to humans than to dinosaurs, they looked like hulking reptiles. Angielczyk notes, “If you saw a pelycosaur walking down the street, you wouldn’t think it looked like a mammal—you’d say, ‘That’s a weird-looking crocodile.’”

About 270 million years ago, though, a more diverse (and sometimes furry) line of our family tree emerged: the therapsids. “Modern mammals are the only surviving therapsids—this is the group that we’re part of today,” explains Lungmus. Therapsids were the first members of our family to really branch out—instead of just croc-like pelycosaurs, the therapsids included lithe carnivores, burly-armed burrowers, and tree-dwelling plant-eaters.


So much of what we do every day is related to the way our forelimbs evolved—even simple things like holding a phone.

Ken Angielczyk
Field Museum curator
The root of the present diversity seen in mammalian forelimbs was established 270 million years ago, in a group of stem therapsids from which all existing mammals have evolved. This was some 30 million years before another group of terrestrial tetrapods began to diversify into the dinosaurs!

The team examined the upper arm bones of hundreds of fossil specimens representing 73 kinds of pelycosaurs and therapsids, taking measurements from where the bone was jointed to the shoulder and the elbow. They then analysed the results using a technique called geometric morphometrics. This revealed much more variation in the therapsids than the pelycosaurs. Variation was especially marked in the shoulder joint of the therapsids giving them a much wider range of movement.

Significance
Mammals and their closest fossil relatives use their shoulders and forelimbs for many functions, which is reflected by the great range of mammalian forelimb shapes. We found that forelimb shape diversity in the early mammalian lineage (Synapsida) began to increase about 270 million years ago, with the emergence of a group called Therapsida, and is accompanied by new forelimb functions. The functional diversification of therapsid forelimbs was curtailed by the Permo-Triassic mass extinction, but eventually continued as more mammal-like therapsids evolved new ecologies. Our analyses characterize the deep time origin of a quintessential part of the mammalian body plan: evolutionarily labile forelimbs that can be deployed in a wide range of functional and ecological roles.

Abstract
Mammals and their closest fossil relatives are unique among tetrapods in expressing a high degree of pectoral girdle and forelimb functional diversity associated with fully pelagic, cursorial, subterranean, volant, and other lifestyles. However, the earliest members of the mammalian stem lineage, the “pelycosaur”-grade synapsids, present a far more limited range of morphologies and inferred functions. The more crownward nonmammaliaform therapsids display novel forelimb morphologies that have been linked to expanded functional diversity, suggesting that the roots of this quintessentially mammalian phenotype can be traced to the pelycosaur–therapsid transition in the Permian period. We quantified morphological disparity of the humerus in pelycosaur-grade synapsids and therapsids using geometric morphometrics. We found that disparity begins to increase concurrently with the emergence of Therapsida, and that it continues to rise until the Permo-Triassic mass extinction. Further, therapsid exploration of new regions of morphospace is correlated with the evolution of novel ecomorphologies, some of which are characterized by changes to overall limb morphology. This evolutionary pattern confirms that nonmammaliaform therapsid forelimbs underwent ecomorphological diversification throughout the Permian, with functional elaboration initially being more strongly expressed in the proximal end of the humerus than the distal end. The role of the forelimbs in the functional diversification of therapsids foreshadows the deployment of forelimb morphofunctional diversity in the evolutionary radiation of mammals.


This is something that’s so cool about our evolutionary lineage. These animals are in the same group as us—part of what makes this research compelling is that these are our relatives.

Jacqueline Lungmus, lead author.
Research assistant, Field Museum
So we see a coherent and rational explanation for the present range of forelimbs in mammals - far more than in any other group of tetrapods - provided by the ToE. As we would expect, the foundation of this diversity has its origins in the earliest stem ancestors of the entire order and is the result of evolution into the different niches made available by this early variability. Structures as diverse, yet fundamentally similar, as the bat's wing, the human arm, the whale's flipper and the mole's digging hands can all be explained as the result of evolutionary processes with a common origin.

Does this look to any sane person like a theory in crisis and a situation that is best explained by magic, because the scientific one isn't up to the job?







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