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Age of Animals

Triassic Period, 251.9 - 201.3 MYA

Triassic Period
  • The continents reached their maximum phase of fusion (the Pangaea) with much of the land lay in the equatorial belt. The climates were hot and monsoonal. Following the massive extinction at the end of the Permian, the survivors underwent adaptive radiations as they diversified and began to reoccupy many of the now-vacated environmental niches.
  • FA of dinosaurs - Out of the thecodonts in the Triassic came a most fantastic array of reptiles, the dinosaurs (terrible lizard). They were the stock from which the crocodiles and birds, as well as the dinosaurs and winged reptiles, developed. They evolved into two main groups in this period. Those shown on the right of Figure 07a are the meat-eating coelophysis, which walked

Figure 07a Triassic Period
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 and ran on their hind legs, captured prey with fore- limbs and jaws, and balanced their swaying bodies with stiffly extended tails.
The other animals in Figure 07a are the dicynodonts (which had all the incisors and lower canines replaced by a horny beak) in the middle, the first generation crocodile (phytosaur) in the water, and a diadectes (which was either an advanced amphibian or a
Triassic Confrontation primitive reptile) on the left. Figure 07b shows a confrontation between the phytosaur and a top predator. The plateosaurus is a primitive member of a group called sauropods (lizard-footed), which walked on four feet, developed massive legs both fore and aft, and had teeth that were suited only to a diet of soft, juicy plants. Plateosaurus has peg-like teeth and the hands had huge thumb claws, used perhaps to gather in plant material

Figure 07b Triassic Confrontation
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 from tall trees. The other subdivision is called theropods (beast-footed). With a few exceptions, the theropods were bipeds that walked on three birdlike toes, had short forelegs, and were carnivorous.

Dinosaurs are divided into two major groups based on the structure of the hip; they are known as the bird-hipped (ornithischian) and lizard -hipped (saurischians) dinosaurs. The names are rather misleading as it is from the saurichians that the birds actually evolved; the names were given before this was fully understood. The pelvic girdle consists of three bones as
Pelvic Structures shown in Figure 07c. The difference in the orientation of the pubis is related to the feeding habits and stance when walking. All ornithschians were herbivores and many were bipedal. As vegetarians they would have a large gut to allow the food to pass through sufficiently slowly to allow it to be digested, a process involving symbiotic bacteria. Thus, an erect ornithischian would have a "beer Belly", which has to hang between the legs with the pubis pointing backward. The bipedal saurischians were all carnivores so their guts would have been much smaller as meat is quickly digested.

Figure 07c Pelvic Structures
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Sauropodomorpha
  • Sauropodomorpha became the biggest land animals - Sauropodo-morpha means "lizard feet - form" in Greek, even though their feet look much more like a tree trunk. They are mostly herbivores although early forms may be omnivores. The primitive forms are facultative bipeds; later forms were so large they were obligate quadrupeds with long forelimb and/or long neck for reaching high into trees (for feeding) without having to rear up. By the end of the late Triassic they had surpassed all previous land living animals in size. The main sauropods of the late Jurassic and the Cretaceous belong to the clad Neosauropoda characterized by dorsally placed big nares and very large size. This clade includes the Argentinosaurus - the largest known dinosaur about 35 meters long and weighing in at 80,000 kg (80 tonnes) lived in Patagonia 100 million years ago (late Cretaceous). In comparison, African elephant weighs about 6 tonnes. Table 01 below explains the adoaptations that allow the existence of sauropods' unprecedented bulk.

    • Figure 07d Sauropodomorpha
    [view large image]

    		 The long neck is an adoaptation to the foliage over head. The structure satisfies certain requirements as shown in Figure 07e. The only animal that possesses higher neck to body ratio is the Albertonectes with the help of buoyancy in water.

    Prerequisites for Long Neck

    Figure 07e Prerequisites for Long Neck [view large image]

    Function Requirement Adoptions Disadvantage
    Protection and Competition Large size Eat more, breed slow First to die when food is scarce
    Reproduction Enough # of young Laid eggs in clutches more often Provide no care for offsprings
    Growth High growth rate
    (2 tonnes/year)    		 Fast metabolism Need lots of food (1 tonnes/day)
    Support and movement A body plan to move massive body Long neck, small head, barrel-like body, thick sturdy legs, and walked on all four. Movement becomes cumbersome
    Breathing Lots of oxygen Bird-like lungs, and air sacs inside the body No disadvantage; but also help to reduce weight and stop overheating
    Digestion Eating huge amount of poor nutritional vegetation Long neck and peg-like teeth to pluck leaves and branches; long retention time in the digestive tract Lengthy microbial fermentation inside producing lot of gas

    Table 01 Sauropods' Special Characteristics

    Early Mammal Diversification Early Mammal Middle Ear and Molars The classic scenarios of mammalian evolution posits an orderly acquisition of key evolutionary innovations leading to adaptive diversification (first column in Figure 07f). But newly discovered fossils in the 2000's show that evolution of such key characters as the middle ear and the tribosphenic teeth (cutting and grinding molars) is far more labile among Mesozoic (250 - 65 MYA) mammals (Figure 07g). Many of such mammal groups led to dead-

    Figure 07f Diversification
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    Figure 07g Middle Ear and Molars [view large image]

    		 end lineages. But some iteratively developments eventually succeeded into modern mammals (Figure 07f).

    Early Mammal Evolution Early Mammals As more mammalian fossils have been unearthed in the past few years, a very different picture of early mammals has emerged to replace the shrew-like description. Dinosaurs may have been the dominant creatures, but mammals were very much a part of their world. They invaded many more ecological niches and developed many more lifestyles than was previously thought possible before the extinction of the dinosaurs. Figure 07h presents a brief guide to early mammal evolution. According to this diagram, mammals evolved from a group of "mammals-like" reptiles called cynodonts that

    Figure 07h Early Mammal Evolution [view large image]

    Figure 07i Early Mammals [view large image]

    		 prospered during the Triassic period. The larger members of this group went extinct at the end of the Triassic and were displaced by dinosaurs, but some
    smaller ones survived into the Jurassic. The line that gave rise to mammals is called Mammaliformes. In the early to middle Jurassic these evolved into the first true mammals. The monotremes - a group containing just two living species, the platypus and echidnas - split from the main lineage first, further splitting occurred later as shown in Figure 07h. Figure 07i shows some of these early mammals with a brief description for each in the followings: The Multituberculates in Figure 07h get their name from their teeth, which have many cusps, or tubercles arranged in rows. These rodent-like mammals were distributed throughout the world, but seem to have eventually been outcompeted by the true rodents, and became completely extinct by early Tertiary leaving no living descendants.

    Toward the end of the Triassic period, another wave of extinction wiped out some 30% of the genera (Figure 07j). The suspected causes include severe volcanism, asteroid impact (in Wale according to a 2011 exploration), and global warming. But whatever
    Extinctions Triassic Extinction happened, it didn't affect everything. Plenty of groups, including small predatory dinosaurs, the early mammals, and some crocodile relatives survived into the Jurassic. Yet large groups of archosaurs mysteriously vanished at the end of the Triassic (Figure 07k). It really isn't obvious why the non-dinosaurs get hammered the most. Anyway, the end-Triassic extinction pruned a number of dinosaurs, but the group as a whole marched on, and prospered in the Jurassic period.

    Figure 07j Extinctions
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    Figure 07k Triassic Extinction [view large image]
    Half Turtle For a long time palaeontologists have the idea that turtles evolved in a terrestrial environment and the shell was formed by osteoderms (bony deposits) fusing together. The 2008 discovery of a turtle fossil Odontochelys semitestacea in China dated back to 220 million years with the top part (carapace) missing implies that the top and bottom parts of the shell evolved separately. This fossil could be seen as the missing link of turtle evolution. But some palaeontologists argue that the old theory is still correct, the carapace has been reduced as the result of adoption to living in a marine environment similar to today's sea turtles. Proponents of the new theory point out that the aquatic turtles alive today (that have reduced carapace) do not match the patterns seen in the fossil. The new specimen looks like the embryonic pattern. The drawing at the top of Figure 07l is an artist's impression of the Odontochelys semitestacea, the image at the bottom is the real fossil.

    Figure 07l Half Turtle

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