EES 227: Paleobiology

Spring 2004

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Lecture 20:  April 20

Reading:

Web resources for this lecture:

Lecture notes:  The Origin of Vertebrates

Within the animals (Animalia), the Subphylum Vertebrates belongs in the Phylum Chordates, which is one of the Phyla in the group of the Deuterostomes , defined by embryological characteristics.

Deuterostomes contains the Phyla:

There are thus invertebrate groups (no vertebrae) within the Phylum Chordates.

All Animals which have body cavities and are bilaterally symmetrical (involved in Camrbian explosion) have 3 tissue types from their early embryonic stages on:

  1. Ectoderm: dorsal nerve chord
  2. Mesoderm: segmental muscles
  3. Endoderm: ventral gut

Most of our skeleton is endodermal, but skull bones are ectodermal.

During their evolution vertebrates have undergone multiplication of the Hox gene cluster. Amphioxus (a cephalochordate, see below) does not have the multiplication, the vertebrates do.

During the evolution of Vertebrates (and finally us) from more ancestral chordates we see characters appear one-by-one, adding up to more and more complex organisms (see  figure below). The characters appearing at each stage are given to the right of the line from lower left to upper right.

We'll start our travel from more ancestral to more complex forms in the above diagram with Amphioxus, a Cephalochordate; in the above cladogram it is represented by the name 'Branchiostoma', its official genus-name (fourth from left).

Figure of Amphioxus. Mouth at left, within the 'buccal cirri' (hairs around mouth), tentacles for food gathering. Live in mud. Note lack of brain (no swelling in nerve chord), and of head. Section with yellow wiggly lines shows cover with segmented muscles.

Amphioxus (see picture above) is a Chordate: bilaterally symmetric,  a notochord and dorsal nerve chord, segmented muscles, front-to-back polarity (clear front end: where mouth is: 'buccal cirri' means 'hairs around mouth', to the left in figure above). BUT no true head: a true head has a brain (brain=swelling of the nerve chord, associated with input from sense organs), it has very little of a heart, single cluster of Hox genes. Amphioxushas a long geological history although its fossil record is poor: Pikaia in the Burgess Shale (middle Cambrian) looks very much like it.
Next step towards more complexity: Craniates (have a head with skull=cranium). Groups of organisms all put in the group of Agnatha (jawless fish); not really a good phylogenetic grouping (see figure above), but a mixed group of organisms without jaws.

Most primitive living Craniate: the hag-fish; common deep-sea scavengers (below 1000 m depth), feeding on bottom dwelling invertebrates and scavenging fish carcasses; secrete huge amounts of slime when caught; have cartilage brain case, a heart, sensory organs (no eyes), and gills with cartilage support structures. No jaws, no paired fins! Very little is known of eggs and embryos, unfortunately. Fossil relatives since Carboniferous; are not vertebrates (see schedule above).

Next step towards more complexity: Vertebrates. Have cartilage or bone vertebrae around dorsal nerve chord. Most primitive living vertebrate: the jawless fish the lamprey. Lampreys were first classified with the hagfish (both lack jaws), but are more complex; still no paired fins. Lampreys have cartilage vertebrae around nerve chord, thus are vertebrates; sophisticated sensory organs (eyes); well-supported gills. Live parasitically; atach themselves to fish with round mouth surrounded with rasping teeth, suck living fish empty. Various fossil jawless fish appear to be more complex than the living lampreys, because they have paired fins (similar to arms and legs), but still lack jaws.

Fossil Agnathans:

Next step towards more complexity: Gnathostomes (have jaws). Major evolutionary step ahead: animals can bite!Led to major radiation of the gnathostomes. Most primitive animals-with-jaws: fossil Placodermi. Placoderms are extinct armored fish with jaws, a hinged bony apparatus attached to skull. Large jaws, but no teeth;  bony plates associated with jaws functioned as teeth. Silurian-Devonian into Carboniferous; very large (up to 18 ft). Paired pectoral fins (arms) as well as pelvic fins (legs). Very extensive exoskeleton (armor).

Origin of jaws: derived from gill arches, i.e., support for gills, made of cartilage. Several rows of these support structures; anterior (front) pair of these become upper and lower jaws, pair directly behind become 'suspensorium', means of suspending jaws from skull. Modern fish still have gill arches behind their jaws. Note: function of structure (biting) is new, structure itself (gill arches) not.

Next step: fossil spiny sharks: Acanthodians (see above cladogram); had teeth with true enamel and dentine. i.e., a truely new type of tissue. Also advanced jaw joint. Lower Siluiran-Late Permian.

Next step: cartilagenous fish (Chrondichthyes): rays and sharks. We'll not spend time on these: arose in middle Paleozoic; lost lungs.

Next step in complexity: Vertebrates with bony skeletons or Osteichthyes (note: some people place the Acanthodians in this group). Ancestral forms had lungs, retained in Sarcopterygians. Lung evolved in to swim bladder in Actinopterygians.

Sarcopterygian fish: have internal skeleton in fins, very similar to our arm and leg bones in structure. Endoskeleton (not exoskeleton as in ray-finned fish. Note: we are more closely related to a lungfish than that a lungfish is related to any ray-finned fish!

Not clear whether the ancestor of all Tetrapods was more closely related to lungfish or to coelacanths. Genetic evidence points to coelacanths, morphology of fins to lung fish.

Former mythology: in Devonian, lung-fish like animal lived in seasonally-drying pools; used lobe-fins to crawl to another pool when its pool dried up. Now we know (see reading) that legs developed while animals lived in water (had legs as well as such organs as the lateral line system in fins; tail with fins), possibly to crawl along bottom in estuaries. Many of the first amphibian-like fishes lived in coastal estuaries,  probably used their fins to wade along the bottom in dark, murky waters with coastal forests, similar to present mangrove swamps (Out of the Swamps!), maybe to stay out of way of the large Placoderms and Eurypterids hunting in open ocean.

Fin-to-limb transition occurred within this group: a lobe-finned fish like animal evolved into an amphibian-like animal because limb developed digits (fingers). All early tetrapods have more than 5 digits/leg (5-9 toes).

How did these limbs develop? Hox genes are again the genetic toolbox. Single Hox cluster multiplied into 4 in vertebrates.  Hox genes define front-and-back of limbs (where thumb, where little finger) as well as front-back in whole animal (head-tail).  Tetrapods and ray-finned fish both have Hox clusters, but they are used differently in limb generation.

First Tetrapods: Devonian (~370 Ma). Radiation of early amphibians in northen continents: Greenland, Russia. Had limbs with hand and feet, digits (fingers, toes); note that early forms had more than 5 fingers, which had been thought to be the mosrt primitive form of Tetrapod appendage. See reading for many of these early forms, which include such animals as Acanthostega (8 fingers on front leg),  Ichthyostega (7 fingers), Temnospondylus, and the strange looking Diplocaulus with its boomerang-shaped head and weak back bones. These animals must also include the ancestors of modern amphibians and of the large group called the Amniota, which includes what we in daily life call reptiles and birds and mammals. More about these in the next lecture on the origin of modern animals.