BSCI392
9-14-07
The Principles of Animal Design

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Animals aren't "designed" per se. Indeed the variety of ways in which they have addressed biomechanical problems - each in some way suboptimal but all good enough - testifies to that fact. E.G.: rapid swimming in vertebrates and cephalopods. Rapid undulations of the body is more energetically efficient than cephalopod "jet-propulsion," in which they must expend energy hauling their reaction mass (water) around inside their mantles before expelling it; yet there squids are. A benevolent designer, creating them from scratch would not saddle them with so inefficient a method of propulsion. That they have it is a function of their molluskan heritage.

Because there may be no "intelligent designer" doesn't mean that organisms are random or haphazard. Two general patterns clearly show that the mechanistic processes of evolution can be highly non-random:

  • Evolution hates waste. Organisms typically exert the minimum of metabolic energy necessary growing tissues. E.G.: The antorbital fenestra (an opening in the skull on the side of the snout) is a common feature of archosaurs, whose function has long been debated. In theropod dinosaurs, it can be very large and accompanied by numerous ancillary excavations, apparently all in order to lighten the skull and avoid wasting metabolic energy growing and maintaining bone where it isn't really needed. Thus, ammonoids of the early Triassic who were not significantly threatened by vertebrate predators, were neither heavily armored nor strongly streamlined. By the late Cretaceous, when oceans were full of ammonoid eating vertebrates, they tended to be heavily armored or streamlined.

    Similarly, organisms don't waste energy on extraneous behaviors. In a common example, a land animal changing gaits as it picks up speed is seeking an optimal output per unit of locomotor energy. (This is why race-walking is limited to athletes in special events.) Apparently wasteful activity usually has some secondary benefit. E.G. Male vertebrates may show-off to secure mates.

  • Organisms are grouped in ecological guilds. Organisms with similar lifestyles tend to evolve similar phenotypes, even if they are descended from very dissimilar ancestors. A familiar example is fast-swimming long distance pursuit predators tending to evolve tall, half-moon shaped tail fins and very narrow tails:
  • Tuna (derived from less specialized actinopterygians)
  • Mako shark (derived from less specialized chondrichthyans)
  • Bottlenosed dolphin (derived from terrestrial mammals)
  • Ophthalmosaurus Jurassic ichthyosaur (derived from terrestrial reptiles)

    NOTE: We can't observe Ophthalmosaurus directly, but because of its extreme similarity to living members of the "marine pursuit predator" guild, it is reasonable to infer a similar lifestyle. - Comparative method.

  • A more obscure example: Sessile (i.e. attached to substrate) Benthic (i.e. bottom dwelling) suspension feeders.

    These tend to grow upright conical skeletons, often with a lid on top. This also seems to represent some sort of biomechanical optimum. It is tempting to speculate that, in the case of the coral, a polyp that secretes a calcareous cup for a skeletan and that moves upward in that cup as it grows would coincidentally end up with a conical skeleton, just because it a roughly cylindrical organism that grows. With the clam and brachiopod, however, this makes no sense, as they are derived from ancestors with non-conical shapes. Thus, we conclude that conical attached bottom dwelling suspension feeders are also an ecological guild. Of course, guilds can be identified with repsect to different parts of the organism. The spotted hyaenas and wolves belong to the same locomotor guild - pack-hunting pursuit predators, but to different dental guilds - one is a generalized carnivore, the other a bone-crusher.

    The topic of ecological guilds is one that we will revisit frequently.

    Make no mistake: a guild is NOT necessarily made up of creatures that share a recent common ancestry!

    Some essential ecological terms

    A quick review of some basic terminology. Animals are either:

    If they are marine, we can break them down by life habit depending on:

    Thus, a sea urchin is motile, benthic, and epifaunal. A blue marlin is nektonic. A razor clam is benthic and infaunal. A coral is benthic, epifaunal, and sessile.

    Basic body plans

    On the largest scale, animal bodies fall into two vast patterns based on their symmetry. These patterns reflect both ecology and phylogenetic pattern:

    Radial symmetry is phylogenetically ancestral for metazoans, so in considering its adaptive value, we have to be careful. Some creatures my have it only because having inherited form their ancestors, they are stuck with it. Nevertheless, it is highly persistent in forms like sessile benthic suspension feeders. Indeed, some suspension feeding bilaterally symmetrical groups have secondarily evolved a form of radial symmetry. Thus ancestrally radial animals like cnidarians have plausibly preserved this form of symmetry for a reason. Generally, radially symmetrical animals are:
    • Sessile or slow moving
    • Have a 360 degree sensory field
    • Have small amounts of muscle, either because they lack mesoderm altogether or have secondarily reduced it.
    • Have low metabolic rates.
    Cnidarians are a special case, being morphologically very simple (E.G.: Sack-shaped gut with a single opening). Indeed, morphological complexity in cnidarians is typically achieved by coloniality. In some, individuals in a colony have specialized shapes and functions.

    Note: In radial animals, terms like front, back, right, and left have no meaning. The only real polarity is toward or away from the mouth (or is it the anus?), thus: oral or aboral.

    Bilateral symmetry is that in which the body is divided in half and is symmetrical around a sagittal plane and is polarized with a distinct front, rear, top, and bottom. WATCH OUT! Such symmetry is a synapomorphy of Bilateria, the clade containing most metazoans, however ctenophores (comb-jellies) also display it. Be careful not to confuse the ecological guild of bilaterally symmetrical animals with the clade Bilateria. Notably, both groups consist mostly of motile active creatures. (OK, ctenophores are limited in being propelled by cilia, but they, at least, are nektonic.) Bilateral symmetry seems to facilitate higher degrees of motility. Bilaterians, have exploited the possibilities by evolving:
    • Mesoderm - giving rise to large muscle volume.
    • A flow-through gut, allowing more efficient processing of nutrients
    • A coelom or body cavity against which muscles can work.

    Many groups have independently evolved:

    • complex gas-eschange and circulatory organs
    • complex osmoregulatory systems
    • Cephalization - distinct special senses and a central nervous system to integrate their input and coordinate activity.
    The combination of improved food processing, gas exchange, and coordination makes most bilaterians significantly more metabolically active than non-bilaterians. This has provided an impetus for the evolution of feeding and locomotor strategies that use this energy effectively. Among bilaterians we see features like:
    • Streamlined bodies
    • Fluid-filled Coelom - body cavities functioning as hydrostatic skeletons against which muscles can act.
    • Segmentation, enabling distinct modules of the body to do different things in an individual's body
    • Tagmosis (sometimes "tagmatization") where distinct body segments (tagma) evolve into functionally distinct units. (E.G.: arthropods)

    Now for something completely different - the interpretation of an enigmatic fossil

    Tullimonstrum gregarium AKA the "Tully Monster" - a common soft-bodied organism from the Pennsylvanian age Mazon Creek fauna of Illinois. Also the Illinois state fossil.

    Tullimonstrum gregarium (cont.) Characteristics:
    • Ovoid body with faint transverse markings
    • Anterior proboscis with teeth
    • Triangular posterior fin
    • Bar organ with expanded tips & dark pigment
    • Paired swellings near base of bar organ
    • Linear, complete gut

    This creature does not especially resemble ANY living creature, and it is a true enigma to taxonomists even though we have interpreted its anatomy in detail. As for its life style?????

    And yet there is one possible relative or analog. Living specimen.

    So how do we like the Illinois state museum's reconstruction?