INTRODUCTION TO THE CRANIATA

Craniata (cran-e-A-ta) is derived from the Greek work cranio (kpavío), which literally means head or skull.  In its Latinized form, the word becomes cranium, a term that refers to the braincase.

The craniates occupy almost every environment on the surface of the earth.  They are among the most obvious animals in marine and freshwater habitats; also, they occur in a bewildering array of terrestrial habitats where they may be found on the ground, in trees, underground, in the air, etc.  They range in size from animals of just a few grams to giants like elephants and whales.  Indeed, the mass of the female blue whale is truly astounding and marks it as the largest vertebrate that has ever lived.  Furthermore, we find the vertebrates particularly intriguing because our species is found among them.

As the name Craniata implies, these animals have an anterior brain that is encased, in part, by a cranium of cartilage or bone.  The most primitive members of the phylum,  the hagfishes (Figure A), have the typical chordate features plus the cranium.  Most members of the phylum have articulating skeletal elements (the vertebral column) that develop with or supplant the notochord.  Those with a vertebral column are called Vertebrata.  The internal skeleton developed by the vertebrates has been highly successful and allowed for the development of frames that swim, walk, run, fly, etc. (Figures B-L).  They likely appeared in the early Paleozoic and became dominant aquatic organisms by the Devonian Period (sometimes called the Age of Fishes).  Tetrapods (vertebrates with four legs) appeared also in the upper Devonian.

A. Hagfish	B. Lamprey	C. Basking Shark	D. Polypterus
E. Fish from the Great Barrier Reef	F. Lungfish	G. Xenopus, the African Clawed Frog	H. Spider Monkey in Costa Rica
I. Green Sea Turtle	J. Gecko	K. Crocodilian from Costa Rica	L. Cockatiel
Images taken from: A: http://oceanexplorer.noaa.gov/explorations/lewis_clark01/logs/jul08/media/r609hagfish_532.jpg  B: http://alpha1.fmarion.edu/~bio106lab/Resources/lamprey.jpg C: http://www.ucmp.berkeley.edu/vertebrates/basalfish/chondrolh.html D:  http://www.geol.umd.edu/~jmerck/eltsite01/projects/group2/polypterus.jpg  F: http://mit.biology.au.dk/zoophysiology/education/cources/evolution/evol_of_airbreathing_lungfish2.jpg  E, G-L: The Systematic Biology Diversity Collection

SYNOPTIC DESCRIPTION OF THE PHYLUM VERTEBRATA

SYSTEMATICS OF THE CRANIATA

Although the craniates have left a rich fossil legacy, their relationships have been clouded by preconceptions of what is advanced and what is not.  That has been complicated by our own inclusion in the phylum so that concepts of advanced become more human.  The standard class structure of the craniates can be seen below:

AGNATHA -Jawless Fishes
CHONDRICHTHYES -Sharks, skates, rays and chimeras
OSTEICHTHYES -Bony Fishes
AMPHIBIA -Amphibians
REPTILIA -Reptiles
MAMMALIA -Mammals
AVES -Birds

Growing bodies of molecular (Tudge 2000) and paleontological (Benton 2005) evidence suggest a much more complex relationship between the classes.  The agnathans represent a grade in form and include at least five independent lines.  Gnathostomes represent another structural grade and require different comparative criteria for their taxonomy, which causes the number of class-level groups to increase.  Not only are there many other groups at the class level, but the distinctions between some of the groups disappear entirely.  For example, a group like the reptiles is a grade of paraphyletic taxa if the mammals and birds are not considered to be reptiles.  Also, the bony fishes merge with the amphibians in a continuum of body form from the sarcopteryigians to the earliest tetrapods and thence to all other tetrapod decendants.  The mammals merge with the basal synapsids (traditionally an extinct group of reptiles).  Birds merge with the archosaurian reptiles (a group that contains the living crocodilians).  The turtles stand alone as a separate group as do most of the modern reptiles in a collective group (lizards, amphisbaenas, snakes, and tuataras).  Despite the cladistic structure, a classification system that places mammals within the bony fish is unsatisfactory from the perspectives of organizing biodiversity and its usefulness as a retrieval system.  So, in my opinion, a strict cladistic application is inappropriate.  However, the classical approach is equally unacceptable in that reptile is a form and not a taxonomic unit.  I have taken the organization of the following system from Benton (2005), which was written in a modified cladistic format, and adapted it to conform to a standard taxonomic system.  The following system stresses both the diversity of craniates and retains the retrieval function of a classification system.  I know full well that the following system will be unacceptable in some way to almost anyone who studies vertebrates.  So, I offer it as a way to enter the conversation about craniate systematics, not to present a system in which I am fully invested.

HIERARCHICAL CLASSIFICATION OF THE CRANIATA

This page is maintained by Jack R. Holt.  Last modified 03/10/2008.