GEOL 204 Dinosaurs, Early Humans, Ancestors & Evolution:
The Fossil Record of Vanished Worlds of the Prehistoric Past

Spring Semester 2014

Endless Forms: A (very, very simplified) Overview of the Diversity of Life

Animals & Vegetables
Traditionally the living world was divided into animals (which moved) and plants (which did not). But as natural world was studied in more detail, it was recognized that there were immobile animals; that fungi were not plants (they were decomposers, not photosynthesizes); that there will microscopic organisms that didn't fit any of these categories. So it was apparent that the diversity of life was much more expansive than just animals and plants.

So, what is Life? A hard thing to define, especially at the boundaries between life and lifelessness. So, in fact, we are not going to do that for this course! Instead, we'll look at some of the properties of life and its diversity, and some conditions under which it arose. All living things on Earth share certain things in common:

The Three Domains (Bacteria, Archaea, Eukarya) and LUCA
Genetics reveals three major branches to the Tree of Life. One (the eukaryotes) contain both unicellular and multicellular forms. The other two branches--the Bacteria (sometimes called Eubacteria) and the Archaea (sometimes called the Archaeobacteria)--are all single-celled, lack nuclei, lack mitochondria, and lack chloroplasts. This condition is called the "prokaryote grade." Many are heterotrophs or are chemoautotrophs (live off of inorganic chemicals); however, some bacteria (most significantly the cyanobacteria, also called "blue green algae") are phototrophs.

Many bacteria and archaeans live in what for eukaryotes would be considered intolerable conditions: extremely high or low temperatures; very high salinities; very acid or alkali conditions; etc. These organisms are called "extremophiles," and many of them occur near the base of their respective branches of the Tree of Life. In particular, most primitive bacteria and archaeans are anaerobic environments (ones where gaseous or dissolved oxygen is lacking.)

Biologists have used this evidence to make predictions about the common ancestor of all living things (LUCA, the last universal common ancestor). This ancestor would have been single-celled and have the prokaryote grade: the eukaryote condition and (later still) multicellularity would have occurred later up the branching Tree of Life. Additionally, LUCA would likely have lived in an environment that eukaryotes would consider "extreme"; it would have definitely have been an anaerobic environment, since before the evolution of phototrophs there would have been very little free oxygen in the atmosphere. In fact, LUCA would not have been very different from some types of bacteria and archaeans today.

Bacteria (also called Eubacteria)

The other major prokaryote grade of organisms are the Archaea. These are prokaryotes; many are extremophiles; most are anaerobic. Based on features of their DNA synthesis (among others), archaeans are probably more closely related to Eukaryota than to Bacteria. It is possible that Archaea is paraphyletic with respect to Eukaryota, and that the prokaryotic ancestors of the main cell (but not the mitochondria or plastids) of eukaryotes.

The remaining clade of organisms are the eukaryotes (Eukarya or Eukaryota). These are larger cells, with a nucleus surrounding the genetic material and many additional small structures (organelles) with specialized functions. All eukaryotes are aerobic. Many are heterotrophs, but some are autotrophic photosynthesizers (algae and plants).

Eukaryotes have mitochondria: another type of organelle that are the powerplants of the cells. Mitochondria take in food (basically the sugar glucose) and oxygen and convert it into chemical energy (the molecule adenosine triphosphate, or ATP): ATP is then used by other parts of cells as fuel. Mitochondria turn out to be descendants of alphaproteobacteria, and carry their own genome separate from the nuclear DNA of the main cell.

Additionally, plants (and a few other groups of eukaryotes) have specialized organelles called chloroplasts: these are little solar energy collectors. Chloroplasts contain the substance chlorophyll: plants use their chloroplasts to combine sunlight, water, and carbon dioxide into chemical energy and waste oxygen gas (the latter they release into the atmosphere). Similar to mitochondria, chloroplasts are descendants of once-free-living prokaryotes (in this case, cyanobacteria) incorporated into the eukaryotic cell, and retain their own DNA.

This process--the permanent incorporation of once-free-living cells into the cells of eukaryotes--has been termed endosymbiosis.

Most of the main organisms we will be concerned with in this course are multicellular organisms, but there are some major fossil-forming eukaryotic unicells:

Several groups of eukaryotes evolved the ability to form complex bodies made of multiple differentiated cells all with the same genetic information. There are even more groups that form simple colonies, but these all have cells that are essentially identical. True multicellular bodies have different cell types, each with their own function.

Here are some of the benefits to multicellularity:

Among the terrestrial realm the major groups of multicellular organisms are animals, plants, and fungi. Each is more closely related to unicellular forms than to any other multicellular group. Brown algae like kelp represent another independent origin of multicellularity.

The Green Kingdom: A Brief Overview of Plants
Plants in the broad phylogenetic sense (Plantae or Primiplantae) represent a major clade of eukaryotes. "Algae" are simply all members of Plantae that do not live on land. Basal members of the various plant groups are often unicellular, but the red algae and some green algae (including land plants) are multicellular. All are phototrophs, and have chloroplast. (In fact, all non-plant phototrophic eukaryotes actually have symbiotic plants rather than just symbiotic cyanobacteria.) Rhodophytes ("red algae") have a fossil record back to Ectasian; green algae not before the Cambrian.

Land plants (technically Embryophyta) show up in the Ordovician (with possible spore evidence in the Cambrian). They are descendants of freshwater green algae. The fossil record contains many stages of plant evolution. All land plants are phototrophs and multicellular.

Basal land plants ("bryophytes": a paraphyletic grouping) (Ordovician-Quaternary): live on land, but have no vascular tissue (and thus no good stalks, stems, roots, etc.). Liverworts, hornworts, true mosses, etc. are living representatives. Reproduce by means of spores.

Vascular plants (technically Tracheophyta) (Silurian-Quaternary): vascular tissue, stems, roots, bark (for larger plants). Two major branches are the club mosses and the eupyllophytes.

Clubmosses (technically Lycophyta) (Silurian-Quaternary): reproduce by spores; covered with small leaflets rather than true leaves. An important fossilizing group is the scale trees (technically Lepidondronales) (Carboniferous): enormous, tree-sized wetlands trees of the coal swamps of the Carboniferous. Much of the coal from the coal swamps is from scale trees.

Euphyllophyta (Devonian-Quaternary): plants with true leaves. Two major divisions within this group are:

Seed plants (technically Spermatophyta) reproduce by pollen (containing the sperm cells) fertilizing specialized female organs containing the egg cells: the embryo produced is protected by a seed. The oldest seed plants are Devonian. Current evidence points to the living non-flowering seed plants (conifers, cycads, ginkgos, gnetophytes) forming a monophyletic clade Acrogymnospermae instead of the traditional interpretation of these "gymnosperms" are a paraphyletic grouping with respect to the flowering plants. There definitely do appear to be fossil gymnosperms (that is, non-flowering seed plants) which are more closely related to flowering plants than to any living group of gymnosperm regardless of the monophyly or not of acrogymnosperms. The major groups of fossilizing seed plants are:

Animals (formally the Metazoa) are multicellular heterotrophs. Their closest relatives are choanoflagellates (a group of zooplankton).

Key attributes of metazoans:

The sponges ("poriferans") (Ediacaran-Quaternary) are the most primitive types of animals. They are suspension feeders in which water comes in through pores on the inside to be picked up by flagellated cells on the inside; essentially living sacs. Shape of sac is held together by spicules that might be siliceous, calcareous, or made of the protein sponging. Recent work has shown that sponges are not a monophyletic group: some are more closely related to eumetazoans (advanced animals) than they are to the other sponge groups. Major fossilizing sponges include:

  • Glass sponges (technically "Hexactinellida") (Ediacaran-Quaternary): siliceous spicules with three rays (like jacks). Oldest known animals; far more common in Ediacaran-Devonian than later in Earth History.
  • Archaeocyathids (Cambrian; nearly all from Early Cambrian): calcareous skeleton, shaped like one small ice cream cone inside a slightly larger cone, held together by septa (partitions). Major reef formers of the Early Cambrian.
  • Stomatoporoids (Cambrian-Devonian; most common in Silurian-Devonian): calcareous skeletons, forming mound-like masses. Major reef formers of the middle Paleozoic (along with tabulate corals). Post-Devonian "stromatoporoids" turn out to be from different groups of calcareous sponges.

    The remaining animals (i.e., non-sponge animals) are the Eumetazoa, or "true animals." They share the following specializations:

    Most eumetazoans are bilaterians. But the two basal branches (Ctenophora ("comb jellies") and Cnidaria) have radial symmetry: a top and a bottom, but no front/back or right/left. Both groups are very soft bodies, but at least some cnidarians have a good fossil record.

    Cnidarians (Cnidaria) (Ediacaran-Quaternary): includes jellyfish, corals, sea anemones, hydras, and their relatives. Have a gut with a single opening, fringed by stinging tentacles. Suspension feeders and/or passive predators. Some live entire life attached to surface as a polyp; others float around with tentacles underneath them as medusa; some alternate between polyp and medusa. Cndarians are soft bodied, so typically are only preserved by impressions. However, the corals produce calcareous skeletons, and so have an excellent fossil record:

    Conqueror Worm: Bilateria
    he majority of advanced animals are bilaterians ("worms" in the broadest sense): animals with a front end, a back end, a left-to-right symmetry, and a top-to-bottom symmetry. Additionally, bilaterians typically have a mouth at one end, a gut running through the body, and an anus at the other end.

    There are a great number of worm-shaped bilateral taxa (nematodes, nematomorphs, annelids, etc.) with no significant fossil record. (One group, the priapulids, show up in many Cambrian sites, and were probably a major group of early predators.) But the vast majority of bilaterians take the basic "worm" body plan and modify it into something very different. As it turns out, nearly all the good fossilizers are such non-wormy "worms."

    Two groups of bilaterians whose fossil record is far better than than present one (and for whom there are more paleontologists studying them than modern biologist):

    Shell Games: Mollusca
    Mollusks (Mollusca) (?Ediacaran-Quaternary) are "shellfish" in the strictest sense. Ancestrally have a belly-foot for "squidging" over the seafloor, a radula for rasping algae, and gills around the body to breath. Kimberella of the Ediacaran may be an early stem-mollusk whose radula was not mineralized; Cambrian forms include non-shelled mollusks with mineralized radula. Starting in the Early Cambrian are mollusks with small chitinous, and later calcareous sclerites (tiny armor plates). However, most Phanerozoic mollusks either have large calcareous shells or are the descendants of shelled mollusks. The shells (and presumably the sclerites) are formed by the mantle: a specialized tissue on the body surface surrounding the gills. Two minor groups of mollusks are: but molluscan diversity has long been dominated by the "Big Three" of gastropods, cephalopods, and bivalves:

    An Inordinate Fondness: Arthropoda
    Arthropods (Arthropoda) (Cambrian-Quaternary) are a great clade of bilaterians with an exoskeleton of chitin which is molted and eyes. Many have some form of leg, sometime modified into a mouth part. Panarthropods include a wide variety of extinct early Paleozoic forms, some surviving soft-bodied animals, and the great crown group clade of Euarthropoda.

    Euarthropods (eurthropoda) (Cambrian-Quaternary) are by far the most numerous group of animals in Earth history! In addition to the above traits, euarthropods have jointed limbs which are modified into many different uses (gills, legs, mouth parts, antennae, etc.). Their segmented bodies often show considerable tagmosis (tendency to form distinct specialized body units, like heads, thoraces, abdomens, etc.). Additionally, euarthropods take advantage of their molting to produce different growth stages with sometimes radically different body forms and ecologies. Major fossilizing arthropods are:

    Stars Below: Echinodermata and Cousins
    Graptolites (Graptolithina) (Cambrian-Carboniferous) are an odd, entirely extinct group. Long known from fossils as carbonized remains on rocks, they look like writing on rock (which is essentially what "graptolithina" means). They were exclusively colonial animals, living either as plankton or attached benthos. They lived by suspension feeders. Their colonies had organic-walled chambers. Because of their rapid evolutionary change, distinctive forms, and widespread dispersal makes them excellent index fossils (especially in Ordovician). Recent studies show that they were related to a group with a much more extensive fossil and living record, the echinoderms.

    Echinoderms (Echinodermata) (Cambrian-Quaternary): lose their bilateral symmetry after larvae; early forms have three-fold spiral symmetry but most advanced forms have pentameral (five-fold) symmetry. Body uses a water vascular system as a combination hydraulic power/skeleton system. Skeleton is formed of plates of calcite. Ancestrally ate by using tube-feet to suspension feed, but advanced groups show wide variety of life styles. Ancestrally benthic and relatively sessile. A wide variety of bizarre Cambrian-Ordovician groups, and many important groups from throughout the Phanerozoic:

    Show a Little Spine: Chordata
    The chordates (Chordata) (Cambrian-Quaternary) are the vertebrates and our kin. Primitive chordates are soft-bodies suspension feeders; the living swimming lancelets and the mobile larvae of the (sessile as adult) tunicates still live this way.

    The most important group of chordates, however, are the craniates (Craniata) (Cambrian-Quaternary). Craniates have heads with eyes, brains, nostrils, organs of balance. They are nektonic, and ancestrally were suspension feeders.. Primitive craniates include the living and extinct hagfish.

    The conodonts (Euconodonta) (Cambrian-Triassic) are an extinct group of craniates (possibly primitive vertebrates) known mostly from their elements (bony biting mouth parts). Major index fossils for much of Paleozoic and Triassic. They were predators on tiny animals.

    Things Seem A Little Fishy: Vertebrata
    True vertebrates (Vertebrata) (Cambrian-Quaternary) have internal skeleton of cartilage (ancestrally) or bone, divided into separate vertebrae. Primitive vertebrates lack jaws. Jawless vertebrates include:

    The remaining vertebrates are the gnathostomes (Gnathostomata), and are characterized by having jaws (and thus gills no longer had to both breath and feed). The major gnathostome groups are:

    Four On The Floor: Tetrapoda
    Tetrapods (Tetrapoda, land vertebrates) (Carboniferous-Quaternary): have necks, digits (fingers and toes), and live adult life stage on land (or descend from animals that did). Primitive tetrapods, including ancestors of today's lissamphibians, laid eggs in water and had aquatic larvae.

    The "amphibian" grade (with external gills in the larva) is a paraphyletic assemblage relative to the living amphibians and the fully terrestrial amniotes. We'll see more about Paleozoic tetrapods in a few weeks.

    However, one group of tetrapods became fully terrestrial. These are the amniotes (Amniota) (Carboniferous-Quaternary). The fully land vertebrates have amniotic (shelled) eggs and claws. The entire life cycle is on land. Amniotes fall into two main divisions: the Synapsida (mammals and our extinct kin) and Sauropsida (reptiles, including birds).

    Wild & Woolly: Mammalia and other Synapsida
    Synapsids (Synapsida) (Carboniferous-Quaternary) were the dominant group of Permian terrestrial vertebrates. Among the synapsids were the more specialized therapsids (Therapsida) (Permian-Quaternary), which may have evolved warm-bloodedness and fur even before the rise of their most diverse subdivision, the:

    "Here There Be Dragons": Sauropsida (Reptilia)
    Reptiles (Reptilia or Sauropsida) (Carboniferous-Quaternary) were the dominant group of animals during Mesozoic Era. There are many lineages. Some important fossil reptile groups include:

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    Last modified: 27 March 2014