Age of Animals

Devonian Period, 419.2 - 358.9 MYA

• FA of insects - The primitive (wingless) types still survive in springtails. Insects are the largest and most successful class of arthropods; in fact, they are the most successful group in the entire animal kingdom. The insect body is sharply divided into head, thorax, and abdomen. Six legs are attached to the thorax. Young, or larvae, may be aquatic and breathe with gills, but adults take air into tubes. Most adult insects also have hard, chitinous exoskeletons.

  Figure 04a shows an oldest fossilized proto-spider and its silk dated back to 386 MYA. Scientists have found that the creature did not have a spinneret (the dexterous appendage that spiders use to shape silk into webs and other structures) without which it could not have precisely controlled the emerging silk. Thus, the proto-spiders lacked the equipment to weave webs, they may have used the sheets of silk to line burrows, wrap eggs or even to have sex.

  Figure 04a Proto-spider

• FA of bony fishes - There are two groups, those with ray-like fins, ancestors of most fishes today (more than 90% of species) from carp to salmon, and sea-horse to tuna, and the lobefins. The ray-fins of the Devonian include cheirolepis, a fast-swimming predator with advanced jaw apparatus to swallow prey up to two-thirds of its own size. Lobe-finned fish is characterized by fins with fleshy bases, or lobes, containing a jointed series of bones. It is from fish in this assemblage that the first tetrapods, the amphibians,

  		developed. Holoptychius is one example, dipterus (a primitive lungfish) is another one that can breathe air. Figure 04b shows some of the fossilized Devonian fish from Yunnan, China. The one on upper left is the Youngolepis - a specimen in between the lobe-finned fish and the lungfish. Figure 04c summarizes the evolution of fish in a sequence from the jawless fish in the Cambrian, to the primitive jawed fish with amour in the Silurian, and finally advanced to bony fish in the Devonian.
  Figure 04b Devonian Fish [view large image]	Figure 04c Evolution of Fish [view large image]

  
  Figure 04d shows the evolution of jawed vertebrates with a phylogenetic tree. The node represents the split of species within a group. A crown group (clade or monophylum) consists both the extant and extinct species, while the stem group contains primitive relatives no longer in existence. The tree emphasizes the curious placement of the ostracoderms (a jawless fish) in company with

  		the jawed fish instead of with the modern jawless species such as the hagfishes and lampreys. It turns out that the ostracoderms fossils share many features similar to jawed vertebrates. They bear well-developed pectoral fins with associated girdles, a epicercal tail, and perichondral and cellular bone (see Figure 04e for the morphological specializations of Eusthenopteron - a jawed vertebrate in transition from aquatic to land dwelling).
  Figure 04d Jawed Vertebrates Phylogeny [view large image]	Figure 04e Jawed Vertebrates Features [view large image]

  
  

  FA of sharks - Their skulls and skeletons were made up of cartilage (strong elastic tissue) and not bone. They have a series of separate gill slits that are not covered (see Figure 04f). FA of amphibians - Ichthyostega appeared on the scene about 362 MYA. They had four legs, but still retained fins on its tail. Members of this group spend part of their life in water and part on land. They often lay their eggs in water. When the tadpoles hatch out, they breathe with gills and are totally aquatic. Later, they undergo metamorphosis, gain lungs and legs, and are able to live on land.

  Figure 04f Devonian Shark [view large image]

		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]

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]

		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

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

			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 million years before the earliest-known tetrapod body fossils were deposited as shown in Figure 04o.
Figure 04m Tetrapod Evolutionary Tree, Old	Figure 04n Tetrapod Evolutionary Tree, 2010	Figure 04o Tetrapod Trackways

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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