Interrelationships of vertebrates

What is a dinosaur and where does it fit in among other vertebrates? The answer to this question uncovers remarkable things not only about dinosaurs but also about many of the vertebrates living around us. Here, we will address questions such as: "How many times has warm-bloodedness evolved in the vertebrates?" (answer: at least two, possibly three times); "How many times has powered flight been invented by vertebrates?" (answer: three independent times), "Is a cow a fish?" (answer: in an evolutionary sense, clearly!), "Did all the dinosaurs become extinct?" (answer: definitely not), and "Which has a closer relationship to a crocodile - a lizard or a bird?" (answer: a bird).

Our approach will be to sequentially construct a series of clado-grams, beginning with the most inclusive (largest) group - Chordata. The story will unfold as we systematically encounter each bifurcation in the road, retracing the path of evolution until we reach Dinosauria.

In the beginning Life is generally understood to be monophyletic. This intuitively comforting conclusion should not be taken for granted, for who can say how many early forms of molecular "life" arose, proliferated, and died out in the primordial oceans of billions years ago? Regardless, all modern life (except for some viruses) is united by the possession of RNA, DNA, cell membranes with distinctive chemical structure, a variety of amino acids (proteins), the metabolic pathways (i.e., chemical reaction steps) for their processing, and the ability to replicate itself (not simply grow). These are all shared derived characters of life.

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Figure 4.1. Pikaia gradliens, a presumed chordate from the Middle Cambrian Burgess Shale.

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Figure 4.1. Pikaia gradliens, a presumed chordate from the Middle Cambrian Burgess Shale.

Jumping to chordates

It is certainly possible to construct a cladogram for all life, but this would require us to blithely encapsulate (given the most recent estimates) about 3.8 billion years of organic evolution. Instead, we'll zip forward to Middle Cambrian time, about 520 Ma, where we first find the diminuitive Pikaia gracilens (Figure 4.1), a creature that seems to give tantalizing insights into the ancestry of vertebrates. Pikaia harkens from the Burgess Shale, a forbidding windswept outcrop in the Canadian Rockies that was once located at the edge of a tropical, equatorial reef teeming with life, 520 million years ago. Rubble and mud periodically fell from this reef, burying thousands of small invertebrate creatures. These ancient animals are today beautifully preserved (if a bit squashed) and indicate that the Middle Cambrian was a time of remarkable diversity.

Pikaia is about 5 cm in length and, in its flattened condition, looks a bit like a miniature anchovy fillet. It was initially described in 1911 as a "worm," but, on closer examination, Pikaia seems to reveal characters that are diagnostic of the chordate body plan (see Box 4.1): a nerve cord running down the length of its back, a stiffening rod (the notochord) that gives the nerve cord support, and V-shaped muscles (composed of an upper and a lower part) with repeated segments, a character that is familiar to most of us because it is present in modern fish. We - and the dinosaurs - would appear to have chordate relatives as far back as the Cambrian.1

Although Pikaia provides an inkling about our distant relatives, what we know about the early evolution of vertebrates and their forebears among Chordata comes principally from living organisms, with sometimes a goodly mixture of information from other relevant fossils.

The chordates consist of Pikaia from the Burgess Shale, urochor-dates, cephalochordates and, most importantly for our story, vertebrates, all of which can be united on the basis of (1) features of the throat (pharyngeal gill slits); (2) the presence of a notochord at some stage in their life histories; and (3) the presence of a dorsal, hollow nerve cord. Above the notochord in chordates is the nerve cord, encompassed within a distinct tail region behind the gut. This distinctive suite of characters -pharyngeal gills, notochord, and nerve cord - appears to have evolved only once, thus uniting these animals as a monophyletic group (Figure 4.2).

Urochordates, commonly called "sea squirts," have a sessile, shapeless adult form, but evidence of their chordate ancestry is found in their free-swimming larvae (in which the notochord is evident). The larvae eventually give up their roving ways, park themselves on their noses, and develop a filter to trap food particles from water that they pump through their bodies (Figure 4.3).

The claim that Pikaia is some kind of chordate has been contested by paleontologist N. Butterfield, Butterfield believes that chordate tissues cannot be preserved in the way those of Pikaia are. Therefore, he is unwilling to place Pikaia in any modern group.

Body plans

All organisms are subject to design constraints. Organisms live in fluid media (air or water), they are acted upon by gravity, and their ancestry limits the structures that they can evolve. For example, you'll never find a propeller on the nose of a bird (even if that were the most efficient way to propel the animal): the evolutionary process works by descent with modification (of existing structures), not the wholesale invention of new ones. In the Linnaean biological classification (see Box 4.2), the term "phylum" is a grouping of organisms whose make-up is supposed to connote a basic level of organization that is shared by all of its members.The idea is that the members of a phylum may modify aspects of their morphology via evolution, but the fundamental organization - or body plan - of the members of the phylum remains constant. For example, a whale is a rather different creature from a salamander; but few would deny the basic shared similarities of their body plans.

There are many types of body plan out there, But, because organisms are subject to design constraints, many similarities are shared by different body plans.These structural repetitions do not occur as a result of a single evolutionary event. Rather; design constraints are such that different lineages of organisms reinvent each structure. When the reinvention of a structure takes place separately in two lineages, the evolution is said to be convergent (e.g., the characters that have evolved separately converge on each other in form).This means that the structures are analogous rather than homologous (see Chapter 3).

High levels of activity dictate a number of convergent, analogous structures that are repeated in one form or another throughout a variety of different organisms. Because muscles can only contract, opposing muscle masses -termed antagonistic - are the means by which most animals accomplish most movement. In vertebrates, antagonistic muscles are distributed around the rigid support provided by a jointed, internal skeleton.This is quite the opposite case in arthropods (the group that includes spiders, crabs, and insects) in which antagonistic muscle masses are enclosed within a jointed, external skeleton (see Figure 3.6).

Complex movements generally require complex musculature, and complex musculature requires sophisticated coordination.This is usually accomplished by a centralized cluster of nerves and neural material, called a brain. In highly active creatures, regardless of origin, the brain is usually located at the front of the animal in a head that is distinct from the rest of the body.The condition of an anterior head region with brain is called encephalization.With motility and encephalization comes bilateral symmetry, in which the right and left halves of the body are mirror images of each other.

Segmentation is another example of convergence in body plans. Segmentation simply involves the division of the body into repeating units, which in turn permits the isolation of parts of the body. Sequential, coordinated motion becomes possible, because one part of the body can respond independently of another Moreover; the segments themselves can be modified through evolution into specialized organs for a variety of functions, most commonly for locomotion and/or for obtaining food. We all know that arthropods are segmented animals; less well known is that vertebrates are segmented animals, too.The segmentation of the vertebrate body plan is still seen in the repeated structure of the vertebrae, in the ribs, and in muscles.

All of these features - antagonistic muscle masses working on a skeleton, encephalization, bilateral symmetry, and segmentation - are shared by arthropods and vertebrates, but of course this does not mean that arthropods and vertebrates are phylogenetically close. All evidence suggests that the development of these features was convergent in the two groups.




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  • Where does pikaia fit in a cladogram?
    8 years ago

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