THE ORIGIN OF VERTEBRATES AND THEIR RELATIONSHIPS

All life is considered monophyletic.

     All of life is connected by a number of shared derived characters (synapomorphies), such as: RNA, DNA, cell membranes with distinctive chemical structures, a variety of amino acids and proteins, similar metabolic processes to deliver energy for the functions of life, and the ability to reproduce (Fastovsky and Weishampel, 1996).
 

The Evolution of Life on Earth

     The earliest atmosphere on Earth was without oxygen.  The primeval atmosphere evolved from the gases expelled during volcanism, a process called volcanic outgassing.

     The earliest organisms were one celled organisms that were anaerobic (did not metabolize with oxygen) and were most likely heterotrophs.  The first cells evolved prior to 3.5 billion years ago.  However, the remains of the oldest one celled organisms that have been found in the rock record are 3.5 billion year old autotrophs (organisms that make their own food).  About 3.5 billion years ago, anaerobic autotrophic organisms evolved.  These one celled organisms are called cyanobacteria (Figure 1).  Autotrophs make their own food by photosynthesis:

Figure 1. Filamentous  procaryotic microfossils from 3.5 Billion year old black cherts of the Archean Warrawoona Group, Pilbara shield of western Australia (Originally courtesy of J. W. Schopf and B. M. Packer to: Levin, Harold L.; 1991; The Earth Through Time (4th ed.); p. 265, Figure 7-18; Saunders College Publishing; 651 p.
 

Prokaryotic and Eukaryotic Cells

     The oldest one celled organisms were PROKARYOTIC cells.  Prokaryotic cells are small, have no nucleus or cell partitions.  Bacteria are examples of organisms that possess prokaryotic cells.

     About 1.5 billion years ago more advanced cells evolved.  These EUKARYOTIC cells are larger and more complex.  Eukaryotic cells have an nucleus and organelles that perform certain cell functions.
 

a)         Prokaryotic Cell                                    Eukaryotic Cell

 Figure 2. a) Comparison of prokaryotic and eukaryotic cells. From:  Introduction to Microbiology
 (http://www.eng.rpi.edu/dept/chem-eng/Biotech-Environ/FUNDAMNT/streem/ese.htm)
b)  Schematic diagrams of a prokaryotic cell (below left) and a eukaryotic cell (below right).  From: Sepkoski, John, Jr., 2001, Foundations  Life in the Oceans, in Gould, Stephen J.  (ed.) , 2001, The Book of Life: W. W. Nortan & Company, New York and London, 256 p.

     The evolution of the eukaryotic cell is probably the most important event in the history of life.

     ENDOSYMBIOTIC THEORY for the origin of eukaryotic cells.  Certain prokaryotes ingested other prokaryotes.  The ingested prokaryotes remained alive inside their host and adopted special functions as organelles for a more complex cell.  Both the host cell and the ingested cells found a mutual benefit from the association.

     There is much supporting evidence for the endosymbiotic theory for the origin of eukaryotic cells.  The DNA and RNA of certain organelles is like that of prokaryotic cells and different from the nucleus of eukaryotic cells.  Certain organelles have separate cell membranes and organelles have separate reproductive mechanisms.

Figure 3.  One theory for the evolution of eukaryotes from prokaryotes, the Endosymbiotic Theory.
(From:  Levin, Harold L.; 1991; The Earth Through Time (4th ed.); p. 264, Commentary; Saunders College Publishing; 651 p.
 

     Two other evolutionary steps had to occur before life as we know it could have evolved.  1) The origin of sexual reproduction and 2) the evolution of multicelled organisms.  Sexual reproduction allows for the reshuffling of genetic material to get different combinations of traits (than the parents had).  Whereas, asexual reproduction allows for only very slow variation due to minute mutations that occur over long periods of time.  Sexual reproduction allows recombination of DNA in the offspring, thus tremendous variety can occur in a short period of time (geologically speaking).
 

The Fossil Record

     Multicelled animals had evolved by 670 million years ago (Late Proterozoic Eon).  Remains of these soft bodied multicelled animals were first discovered in Australia.  They have now been discovered several places around the world.  These remains of Late Proterozoic soft bodied organisms are referred to as the Ediacaraian Fauna.
  Spriggina, a soft-bodied multicelled organism (perhaps related to arthropods) from Precambrian rocks in Australia.  Note the segmented, bilaterially symmetrical body plan.  From: http://www.ucmp.berkeley.edu/vendian/critters.html Vendian Animals
Go to this site and read about the Ediacaraian Fauna.

     By the beginning of the Cambrian Period of the Phanerozoic Eon, about 540 million years ago, many invertebrate organisms had evolved hard mineralized exoskeletons.  Also the fossils of many new forms of organisms are present in the rock record.

     Another wave of evolution occurred about 530 million years ago during medial Cambrian time.  Many new body plans for life evolved (tremendous diversity of body plans).  Fossils in Middle Cambrian rocks of the Burgess Shale in Canada show many of these strange new body plans (both soft bodied and with hard exoskeletons).  Many of these body plans did not survive to the present.

Body Plans

     All organisms are subject to design constraints.

     1) Live in a fluid medium (air or water).
     2) Acted on by gravity.
     3) Ancestry limits the structures they can evolve.

     Structures are reinvented in different lineages.  This is called convergent evolution.  Basically, this means that many structures are analogous rather than homologous.

•      Antagonistic muscle masses in motile (moving) organisms.
•      Brain (bundle of nerve cells) for coordination of muscles and other organs and structures.
•      Encephalization in active creatures (because of bundle of nerves, brain, in front portion of body).
•      Bilateral symmetry (by product of brain and encephalization).
•      Segmented Body Plans for coordinated movement.

     Burgess Shale Fauna of middle Cambrian rocks.  The Burgess Shale Fauna was discovered by Charles D. Walcott in 1910 in British Columbia, Canada.  These fossils are representative of soft bodied organisms that lived during medial Cambrian time, about 530 million years ago.

     Remarkable diversity is present within the fossils of these soft bodied organisms.  Most of the major phyla of today appear to be present in the Burgess Shale Fauna.  A link to the ancestry of vertebrates may be present within this fossil accumulation.

     Pikaia - A worm like segmented organism that appears to have a notochord, thus it would be a primitive member of the phylum Chordata (which includes all vertebrates).

Chordates

     Derived characters for chordates (for vertebrates also):

• 1) pharyngeal gill slits
• 2) presence of a notochord
• 3) dorsal hollow nerve chord

     Amphioxus is a modern form that has primitive characters of the Chordata.  Amphioxus belongs to the Cephalochordata (closest to vertebrates).

Vertebrata

     Vertebrates have a segmented vertebral column that has replaced the notochord (although the notochord present during a portion of the embryonic development).Vertebrates have calcified skeletal tissue (bone), except for the cartilaginous fish.  Many other characters are shared among the vertebrates.  Primitive vertebrates evolved by late Cambrian or early Ordovician time (example: jawless fishes called Ostracoderms).

By at least late Ordovician, certain types of fish had evolved true jaws.  Gnathostomata - had evolved true jaws.
Fish continued to evolve rapidly during the early Paleozoic, and by Devonian time were very diverse.  The Devonian is sometimes called the "age of fishes".

Tetrapoda

      By late Devonian time, certain lobe-finned freshwater fishes evolved into the first tetrapods. The appearance of limbs with digits is strongly associated with movement to a land habitat.  However, the first tetrapods may have evolved from their fishy ancestors so that they could be more mobile in shallow streams, using their limbs with digits to negotiate around and over rocks in the stream channel.

Traditional classification of tetrapods:

• 1) Amphibia
• 2) Reptila
• 3) Aves (birds)
• 4) Mammalia

The Tetrapod Skeleton

• All tetrapods have the diagnostic features of chordates and vertebrates.
• Backbone - distinctive, repeated structures.  Vertebrae,  consist of a centrum, above which lies (in a groove) the spinal chord (first developed in the gnathostomes and later modified in tetrapods).  Straddling the spinal column is the neural arch. Neural processes stick out from vertebrae (these are for muscle or ligament attachment and/or where the ribs abut against the vertebrae).  There is repetition in the vertebral column (left over relic of segmentation), which allows flexibility in the backbone.

•  Figure 4. A vertebra. (From: Anatomical Dictionary http://www.dinosauria.com/dml/anatomy.htm#1a2)

• The pectoral and pelvic girdles are at anterior and posterior positions (respectively) of the backbone.  These are sheets of bone to which the limbs are attached.
• Pelvic girdle (sacrum) - fused vertebrae in the lower portion of the back.  The hind limbs attach to the sacrum.
• Sacrum of Triceratops. (From: Anatomical Dictionary http://www.dinosauria.com/dml/anatomy.htm)
• On each side of the sacrum, the ilium, pubis, and ischium bones come together to form the hip socket (acetabulum).  The nature of the pelvic girdle is very important in dinosaur classification.
• 
• The hip structure of Saurischia (left) and Ornithischia (right). (il = ilium; is = ischium; pu = pubis; ac = acetabulum; prp = prepubic process) (From: Anatomical Dictionary http://www.dinosauria.com/dml/anatomy.htm)
• Pectoral girdle - the scapulae are flat sheets of bone on each side of the body that are attached to the outside of the ribs by ligaments and muscles.  The breastbone (sternum) is a flatish bone locked into place on the chest by tips of thoracic ribs.  The coracoid bones are for rib cage supports.
• (From: Anatomical Dictionary http://www.dinosauria.com/dml/anatomy.htm)
• The limb bones -
• Forelimb -
• humerus, radius, ulna (arm bones)
• carpals (wrist bones)
• metacarpals (palm bones)
• phalanges (finger bones, singular phalanx)
• Hindlimb -
• femur, tibia, fibula (leg bones)
• tarsals (ankle bones)
• metatarsals (foot bones)
• phalanges (toe bones,ungual phalanges are bones at the tip of digits)
• The Skull and Mandible -

• 
• Figure 5. Bones of the skull and mandible. (From: http://www.dinosauria.com/dml/anatomy.htm#1a2 (Jeff Poling, Anatomical Dictionary))

Cladogram of the non-amniote chordates.  Based on Fastovsky and Weishampel, 1996 and other sources.  Drawing by Jim Florence.

     The next major evolutionary innovation of the tetrapods was the amniote egg.  This allowed tetrapods to invade dry land because they did not have to return to the water to lay their eggs and did not have to have a free swimming larval stage (as is the case with amphibians).  The amnion is a membrane that surrounds the amniotic cavity.  The amniotic cavity is filled with the amniotic fluid that baths the embryo.  In most amniotes there is a leathery or mineralized outer shell, inside of which is another membrane around the entire egg (most mammals have lost the outer shell because they evolved live birth).

     The amniote egg also had a yolk for nutrition and a special bladder for waste from the developing embryo.
 

Figure 6.  The amniote egg (From: http://www.nova.edu/ocean/biol1090/W8B-AMNIOTES.htm)
 Origin of the Dinosaurs - Amniotes

       In addition to the amniote egg, there are hundreds of other synapomorphies among the amniotes.

      The amniotes are divided into four major groups based on skull anatomy.

  Figure 7.  The four types of tetrapod skulls (from Temporal Fenestration and the Classification of Amniotes).
 

Synapsida

     The synapsids are one of two great lineages of the amniote tetrapods.  All mammals are synapsids.  Some extinct forms of synapsids are called "mammal-like reptiles"; we will try to avoid that use.  As we will see later, synapsids (even primitive ones) are not reptiles at all (according to our new definition of Reptilia).  The split between synapsids and reptiles occurred 310-320 million years ago.

     All synapsids have a distinctive skull type.  Their skull is a departure from the primitive tetrapod skull.  In the primitive condition for a tetrapod skull there is a sheet of interlocking bones covering the brain case.  Synapsids have a single opening in the skull below the postorbital and squamasal bones of the skull (behind the eye orbit).  This opening is called the Lower Temporal Fenestra (or Infratemporal Fenestra) (see Figure 7).  Jaw muscles pass through the opening to attach to the upper part of the skull roof.  The primitive synapsid Dimetrodon belongs to this group of amniotes.

     The synapsids radiated during late Paleozoic and by medial Triassic were the dominant terrestrial vertebrates, with a world wide distribution.  They had diversified into many herbivorous and carnivorous forms.

     The synapsids suffered greatly during the late Triassic mass extinction event and by late Jurassic were reduced to a clade of tiny, scrappy, furry night dwellers called mammals (however, mammals first arose in late Triassic time).

Reptilia

     Reptilia is the other great clade of amniotes.  The clade Reptilia includes the Anapsida and Diapsida, and all forms down to their most recent common ancestor.

    Modern representatives of Reptilia are turtles, snakes, lizards, crocodiles (and alligators), the Tuatara, and the birds (notice here that if we do not include the birds we would have a paraphyletic group).

    From the past, Reptilia includes the dinosaurs, pterosaurs, plesiosaurs, and icthyosaurs.  Today there are 15,000 species of Reptilia (of course this number includes the birds).

• Within Reptilia two Clades:

Anapsida - fully roofed temporal region	Diapsida - two temporal openings in the skull roof (upper and lower temporal fenestrae)
Includes Turtles - not much change in 210 million years	Includes modern snakes and lizards, as well as crocodiles, dinosaurs, pterosaurs, birds, and some primitive archosaurs of the Triassic Period.

• Note:  Icthyosaurs and Plesiosaurs were euryapsid reptiles.

    There are two major clades of diapsid reptiles:

• 1) Lepidosauromorpha - snakes, lizards, and the Tuatara
• 2) Archosauromorpha - which includes the clade Archosauria.  The archosaurs are the crocodiles, dinosaurs, pterosaurs, and birds (and some primitive archosaurs of the Triassic Period, certain ones of which were in the ancestral line to the dinosaurs).

 The Archosauria

     The Prolacertiforms of the Triassic Period are basal archosaurs (archosauriforms) that look like reptilian pigs.  However, they possess a number of significant derived characters.  These characters also unit the birds, crocodiles, and dinosaurs (see cladogram below). The most significant shared of derived characters is the Antorbital Fenestra (an opening in the skull in front of the eye orbrit). Changes in ankle structure were also important shared derived characters that unite more advanced archosaurs, the ornithodirans (including the pterosaurs and dinosaurs). 

Changes in ankle structure among the diapsid reptiles.  Drawing by Jim Florence.

"Students of archosaur evolution are blessed with a wonderful fossil record for many groups of archosaurs, including some very bizarre extinct taxa. The first archosauromorphs (relatives of the true archosaurs) appear in the fossil record in the Early Triassic; about 245 million years ago, just after the great end-Permian extinction. They include weird hippo-size beaked herbivores called rhynchosaurs, long-necked reptiles called prolacertiforms, evil-looking terrestrial predators like the erythrosuchians and proterosuchians, and close relatives of the true archosaurs, including Euparkeria. Many of these early groups are limited to the Triassic period, not enduring the extinctions in the Late Triassic that the dinosaurs and other taxa survived." (from: http://www.ucmp.berkeley.edu/diapsids/archofr.html)

Cladogram of the amniotes.  From various sources.  Drawing by Jim Florence.

     The Ornithodira are close to the ancestry of dinosaurs.  They had an upright posture due to the following modifications of the limb bones:

• 1) head of femur distinctly offset from the shaft.
• 2) head of femur is barrel shaped and allows forward and back motion in the plane parallel to the body.
• 3) ankle bone modified - single, linear articulation (modified mesotarsal joint - allows only forward and back motion).

Dinosaurs

     Now we come to the clade Dinosauria.  The Dinosauria have a host of shared derived characters (synapomorphies).

     There are two major groups of dinosaurs, the primary criteria to separate these two clades of the Dinosauria is their hip structure (as we have talked about previously):

• Ornithischia (bird-hipped)
• Saurischia (lizard-hipped)