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Age of Animals

Devonian Period, 419.2 - 358.9 MYA

First Land Animal, Fossil Eusthenopteron A 365-million-year-old arm bone fossil was found in 2004 (see Figure 04g). It came from one of the first creatures able to do push-ups, an evolutionary step that was necessary for animals to move from the sea to dry land. This four-legged creature had a humerus, or upper arm bone. Such a bone, far different from the flipper bones of fish, gave the creature an important new ability - it could raise its upper body like an athlete doing push-ups. The defining moment has been captured by the drawing in Figure 04h. These are lobe-finned fish called Eusthenopterons, which were more than a fish but less than a true amphibian. They are supposed to be the first creature that crawled onto land about 380 million years ago.

Figure 04g Fossil [view large image]

Figure 04h First Land Animal
[view large image]

Tetrapod Transformation A popular scenario suggests that fish like Eusthenopteron, stranded under arid conditions, used their muscular appendages to drag themselves to a new body of water. Over time those fish able to cover more ground - and thus reach ever more distant water sources - were selected for, eventually leading to the origin of true limbs. Recent research in 2005 on the fossil of Acanthostega indicates that although this animal had four legs, they would not have been able to support its body on land. It seems that they may have initially functioned to help the animal in lifting its head out of oxygen-poor shallow water instead of moving on land. Only later did they find use ashore. Figure 04i shows the transformation of body structure from lobe-finned fish to modern reptile.

Figure 04i Tetrapod Trans-formation [view large image]

Tetrapod Transition Tetrapod Transition, Forelimbs Discovery of the Tiktaalik fossil in 2006 has illuminated more detail on the transition between fishes and land vertebrates. As shown in Figure 04j, Tiktaalik and Panderichthys (red) represent the transitional forms between the lobe-finned fish Eusthenopteron and the primitive tetrapod Acanthostega. The skull roofs (left) show the loss of the gill cover (blue), reduction in size of the postparietal bones (green) and gradual reshaping of the skull. It also shows the pectoral, and distal fins gradually

Figure 04j Tetrapod Transition

Figure 04k Transition of Forelimbs

Arm Bones Evolution transformed into forelimbs and digits. A peculiarity of Tiktaalik is its poorly ossified vertebral column that seems to contain an unusually large number of vertebrae. The larger ribs may mean it was better able to support its body out of water. The longer snout suggests a shift from sucking food towards snapping up prey, whereas the loss of the gill cover bones probably correlates with reduced water flow through the gill chamber as the animal had become partially living on land. Figure 04k shows the transformation from fins to elbow and wrist-like structures as indicated by the parts in different colors. Figure 04l traces the evolution of arm bones from fish to humans.

Figure 04l Arm Bones Evolution

Eventually, a tetrapod evolutionary tree was drawn up by paleontologists according to the the available data as of 2009 (Figure 04m). A discovery later that year has forced the re-drawn of the tree to allow for the earlier appearance of the tetrapods (Figure 04n) such that they would coexist with the transitional forms for at least ten million years. This is the trackways discovered on the Polish
Tetrapod Evolution Tree, Old Tetrapod Evolution Tree, New Tetrapod Evolution Tree, New marine tidal flat sediments. The footprint fossils had been securely dated to 397 MYA in the early Middle Devonian period. The tracks show a very large tetrapod exceeding 2 meters in length, lived in fully marine intertidal to lagoonal environments some 18

Figure 04m Tetrapod Evolutionary Tree, Old [view large image]

Figure 04n Tetrapod Evolutionary Tree, 2010 [view large image]

Figure 04o Tetrapod Trackways
[view large image]

million years before the earliest-known tetrapod body fossils were deposited as shown in Figure 04o.

Vertebrate Evolution Figure 04p shows the evolution of the living vertebrates in terms of substitutions per site (in a 2013 study), which is a measurement of mutation rate by checking on the replacement of one nucleotide in a DNA sequence. Thus the length of the lines indicates how much the DNA of each lineage has diverged from their common ancestor. The evolutionary tree is rooted on cartilaginous fish, and shows that the lungfish is more closely related to tetrapods than the coelacanth (settling a debate on tetrapod evolution), and that the coelacanth is evolving slowly. Pink lines (tetrapods) are slightly offset from purple lines (lobe-finned fish), to indicate that these species are both tetrapods and lobe-finned fish.

Figure 04p Vertebrate Evolution, 2013

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