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Age of Animals

Jurassic Period, 201.3 - 145.0 MYA

Jurassic Period
  • Dinosaurs became dominant, reaching their largest size. The brontosaurus (thunder lizard) was a huge sauropod with length up to 80 feet and a total weight of 30 to 35 tons (see the gigantic beast in Figure 08a). The large size probably helped them to escape predation by carnivorous dinosaurs. In the same picture, the stegosaur protected itself by the elaborate armour. Its small brain was compensated by large ganglia (a mass of never cells) between the shoulders and another one above the hips; those are sometimes referred to as the second brain.
  • The dinosaurs also diversified into water and air - Kuehneosaurus were the gliding reptile, pterosaurs were the flying one, while nothosaurs and ichthyosaurs returned to sea.
  • FA of birds - Archaeopteryx (ancient wing) is the oldest known creature that had feathers. Except for the feathers and the braincase, this crow-sized extinct animal is much more like a small running dinosaur.

Figure 08a Jurassic Period

Feather It was believed that feathers evolved from scale for flight. New evidence from fossils and recent idea in developmental processes indicate that they evolved for some other purpose and were then exploited for a different use. Numerous functions of feathers are plausible, including insulation, water repellency, court-ship, camouflage and defense. The development of such feature can be traced back to the theropods in the Triassic Period. In essence, all feathers start from a tube produced by proliferating epidermis with the nourishing dermal pulp in the center. The evolution involved many

Figure 08b Feathers
[view large image]

stages from an unbranched, hollow cylinder (like the pinfeather) to the asymmetrical flight feather (see Figure 08b).

Avian Evolution Archaeopteryx Characteristics The consequence of recent fossil finds has prompted reconsideration of the biology and life history of the theropod dinosaurs. Birds - modern birds and the group that includes all species descnded from the most recent common ancestor of Archaeopteryx - used to be recongnized as the flying, feathered vertebrates. Now we have to consider them as a group of the feathered theropod dinosaurs that evolved the capacity of powered flight (Figure 08c). Other dinosaurs are very likely to have had feathered skin but were not birds.

Figure 08c Avian Evolution [view large image]

Figure 08d Archaeopteryx Traits [view large image]

For many years the earliest bird fossil has been the Archaeopteryx lithographica, which lived in the Late Jurassic period about 148 MYA. Figure 08d shows the characteristics of the Archaeopteryx. It indicates that Archaeopteryx is at the transitional stage between reptile and bird. The size
Archaeopteryx Huxleyi of Archaeopteryx is about 45 cm, and it fed on insects. It has a long bony tail, three-fingered hands with claws, and jaws with teeth. The claws on its feet and hands suggest that Archaeopteryx could climb trees, and the wings are clearly those of an active flying animal. This bird could fly as well as most modern birds, and flying allowed it to catch prey that were not available to land-living relatives. In effect, it had explored a niche in the air. Figure 08e shows the first Archaeopteryx fossil from Bavaria, southern Germany, and an artist's renderings of the very first birds.

Figure 08e Archaeopteryx and Fossil [view large image]

Figure 08f Oldest Feathered Dinosaur [view large image]

It was only in the 1990's, when more evidences turned up in fossil-rich quarries in northern China. Various dinosaur fossils clearly show fully modern feathers and a variety of primitive feather structures. The dromaeosaurs discovered at Liaoning seems to represent the theropods that are hypothesized to be most closely related to birds but that clearly are not birds. It may be the missing link depicted in Figure 08c. Then a four-winged dinosaur fossil (Figure 08f) was discovered in 2009, the Anchiornis Huxleyi is dated to 151-161 million years ago making it the oldest feathered dinosaur. It has the size of a chicken (less than 50 cm) with long feathers covering the arms and tail, but also the feet. Figure 08g shows some locations of dinosaur fossils in China.

Figure 08h1 shows the 2014 status of ancient birds. The archaeopteryx is no longer the oldest one, the title has gone to the Aurornis Xui which existed about 10 million years earlier. The bird brains are not so small in comparison to their size, and the feathers are used to keep warm initially rather than for flying.

Dinosaurs in China Old Birds Evolution of Birds Figure 08h2 is a 2017 update from Scientific American. It shows the family tree starting from the archosauria about 250 MYA at the beginning of Triassic. Most of the animals in the top food chain perished at the end of Cretaceous period when an asteroid strike causing catastrophic extinction of dinosaurs leaving a few surviving groups including birds and small mammals (see "K-T Extinction")

Figure 08g Dinosaurs in China [view large image]

Figure 08h1 Old Birds [view large image]

Figure 08h2 Evolution of Birds [view large image]

Here is the references from Wikipedia for members in the clade of Archosauria (see Figure 08h2) : Dinosauria, Saurischia, Theropoda, Maniraptora, Paraves, Avialae (birds), Pygostylia, Ornithothoraces, Ornithurae. These references contain lot of information on the dinosaurs and their bird-like relatives although the groupings and namings become confusing at times.

As shown sequentially from right to left in Figure 08i, early chick embryo starts with all 5 digits, then the 1st and the 5th become vestigial, eventually the 2nd, 3rd, and 4th digits emerge together to form the wing. A controversy has developed for more than a century over the
Digits of Bird and Dinosaur relationship between the wing of birds and the digits in theropod dinosaurs when palaeontologists mistakenly identified the dinosaurs' to be the 1st, 2nd, and 3rd. Until now in 2009, analysis of the digits in a Limusaurus fossil shows that those are indeed the 2nd, 3rd, and 4th digits - the same as the modern birds (Diagram d, Figure 08i). This explanation vastly simplifies the current convoluted evolutionary story which, either assumes that birds lost their 1st digit and re-grew their 4th one or that birds descend from another kind of dinosaurs.

Figure 08i Digits of Bird and Dinosaur [view large image]

Meanwhile, the lizard ancestor evolved into an entirely different form as snake (Figure 07i). Evolution of venom was thought to occur around 60 million years ago. It was assumed that venom has evolved independently in each of the three modern families -
Snakes Viperidae (vipers), Elapidae (cobras and coral snakes), and Atractaspididae (stiletto snakes). New research in 2006 suggests that venom evolved in a lizard ancestor before snakes appeared (Figure 08j). Even the supposedly harmless Colubrids such as those sold in pet stores have enough poison in their venom glands to kill a human. Fortunately for the would-be pet owners, they have no front fangs, leaving them with a rather crude venom-delivery system in the back teeth. Snakes such as boas may have lost their venom as they evolved to kill by constriction. It is also found that venom didn't evolve from ever more toxic saliva but from altering cells from other parts of the body including the brain, eye, lung, heart liver, muscle, ovary and testis. Over generations these proteins, usually involved in key biological processes such as blood clotting or regulating blood pressure, were mutated into more potent varieties and concentrated into catastrophic overdoses. The common ancestor had nine such toxins in its venom. Modern snakes have recruited 17 more.

Figure 08j Snakes [view large image]

Half Snake Report in 2007 purported to find the missing link between lizards and snakes. The 95 million years old fossil has greatly reduced forelimbs, a diminished supporting skeletal girdle and an elongated neck (see Figure 08k), as seen today in snakes including pythons and boas. But researchers still cannot conclude that snakes evolved directly

Figure 08k Half Snake [view large image]

from such lizards without other fossils to fill the evolutionary gaps.

A 2014 update on the evolution of snake indicates that losing their legs was the least of the amazing modifications. The major change is on the metabolism. After a big meal, there is a huge change in the mass and size of the internal organs as well as soaring in metabolic rate (see Figure 08l in time scale of 10 days, deeper colors indicate the changes). Actually, they lay dormant between meals (within 2 weeks) keeping the metabolic rate the lowest level known in any vertebrate. It seems that they evolved about 100 million years ago as burrowing lizards. Thus they lost their eyes and recovered part of it when moved back to the surface. They are still having problems in the eyes with blood vessels running in front of the retina and the fused (but transparent) eye lids. They may have poorer eyesight but they can "see" in the infrared (with special heat-
Snake Evolution sensing pits on their faces) and are able to trace scent of the venom left by the bitten preys. To swallow the prey much larger than their head, they developed unusually folded skin around the mouth as well as very flexible jaw muscles. Sometimes the venom serves another function of breaking down the prey's tissues before swallowing it. Study of the 2 genomes sequences belonging to the Burmese python and the king cobra (inserts in Figure 08l) shows that only a few hundred genes among the 7442 genes common to all land vertebrates had been modified. The venom genes have been assembled from 20 families of genes, which had day-to-day housekeeping functions within the cells.

Figure 08l Snake Evolution
[view large image]

The leg genes are still active in the embryos, but the cells in these areas just ignore the signal, so no legs form. They have also eliminated one lung and retained only one lobe of the liver.

Snake, ZRS Gene A research paper published in November 2016 reveals that mutation or loss of the ZRS gene is responsible for the loss of limbs in snakes. It is an enhancer for promoting the transcription of the Shh gene, which regulates limb formation (Figure 08m,b). Such effect is demonstrated by coloring the ZRS proteins blue and watching the development of blue patches in the back of their small budding limbs at day 11.5 (E11.5, Figure 08m,a,f) in embryos of various animals. It is also found that the basal snakes such as boa and python still retain pair of vestigial hindlimbs because they evolved earlier, while the corn snakes such as cobra forgo the limbs completely (Figure 08m,c).

Figure 08m Snake, ZRS Gene Mutation or Loss [view large image]

The effect is confirmed by replacing the ZRS gene in mouse embryos with the snake version to produce legless mouse (Figure 08m,d). The other way is to restore legs to snake by replacing the its ZRS gene with the non-snake version (Figure 08m,e).

The study becomes feasible only with the new gene editing technique of CRISPR, which reduces years of work to just a few months. See a news article on "What a Legless Mouse Tells Us About Snake Evolution".

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