The Chordates

 

    The metazoan ("animal") group with which most people are familiar is the phylum Chordata.

 

    All chordates show five traits, at least in their larval stages:

 

- a dorsal hollow nerve chord (the nerve chord is ventral in many other metazoan phyla)

- a notochord, which is a supporting cartilage cylinder under the nerve chord

- pharyngeal slits, openings that connect the throat to the outside environment through the neck, used as gills in aquatic species

- an endostyle, a groove in the ventral wall of the pharynx which in filter-feeding species secretes mucus, stores iodine and, in higher species, becomes the thyroid gland

- a post-anal extension of the skeleto-muscular system, namely the "tail"

 

    One of the reasons why this phylum is so familiar to people is that it includes not only us, but vertebrates in general. However, vertebrates are one subphylum - Vertebrata - within this vast and diverse assemblage of animals. There are, indeed, two others: the subphyla Urochordata (tunicates and sea squirts) and Cephalochordata (lancelets), none of which will be discussed in these website. (Not while I do not have underwater photographic equipment, at any rate).

 

    The vertebrates comprise several classes, and three of them are exclusively aquatic: Agnatha (jawless "fish": lampreys and hagfish), Chondrichthyes (cartilaginous "fish": sharks, rays, and chimaeras), and Osteichthyes (bony or true fish). I will not discuss these, either.

 

    The remaining classes include the other vertebrates of the West Indies and the rest of the planet, and are collectively known as "tetrapods".

 

 

The Tetrapods

 

    Before entering the discussion of the terrestrial vertebrates that inhabit the West Indies, perhaps I should say a few words about the terms I use to refer to them.

 

    As I mentioned elsewhere in this website, the classification of organisms is often fraught with incognita, and many things are not clear. While that might in fact make the endeavors of Systematics even more fascinating, it also means that often there is no easy answer to the question: “What animal is this?”

 

    All the terrestrial chordates living on Earth are, properly speaking, “tetrapods”. The term simply means “four-legged”. Even if a good number of them do not in fact have four locomotive limbs (like caecilians, whales, snakes, and even some flightless birds), such are still considered tetrapods because they evolved from forms that did have them. (Their lack of limbs is an "apomorphy", a derived and secondary characteristic).

 

    According to their more obvious traits, such creatures have often being succinctly defined as “amphibians”, “reptiles”, “birds”, and “mammals”. However, since taxonomists have, for the last few decades, relied ever more on genetic data and phylogenetic systematics to group organisms together, the aforementioned terms - originally based exclusively on morphologies - have nowadays little value from a systematic standpoint.

 

    Although “amphibians” are rather well defined, the same cannot be said of the other three groups of tetrapods, collectively known as the “amniotes”. As one attempts to “fit” fossil members of these groups into the popular (mostly taxonomic) definition of each, the distinctions among them increasingly blur until the term “reptile”, “bird”, or “mammal” looses its objective systematic meaning.

 

    For the purposes of this website, let me simply say that the living Tetrapoda include two basic groups: the Amphibia, and the Amniota, these last being either Anapsida ("stem reptiles"), Diapsida ("modern reptiles", including birds), or Synapsida (represented nowadays by "mammals").

 

    Notice carefully that this arrangement implies that a gecko is far more closely related to an ostrich, and even to a human, than it is to a salamander. That, in spite of the fact that many people still think that a salamander and a gecko must be close kin just because they look alike. (This bias has been perpetuated through the usage of the term "Herpetology" to mean "the study of amphibians and reptiles" - meaning "non-avian reptiles", at that).

 

 

The Amniotes

 

    The term "amniote" makes allusion to the structure of the eggs of these organisms. The amniotic egg has three distinctive membranes. The most important - and the one that gives the entire group its name - is the "amnion", which surrounds an amount of watery liquid, itself surrounding the embryo which develops in this little, self-contained "pond". (The amnion and its contents comprise the "white" in a chicken's egg). This provides the moisture that the developing embryo needs to develop. The "chorion" is a protective membrane around the whole egg, and eventually fuses with the rubbery or calcareous "shell". The "allantois" attaches to the ventral surface of the embryo and is pressed against the chorion, thus allowing the exchange of gasses between it and the atmosphere, and serves as a "trash can" that keeps metabolic waste away from the developing organism. (The allantois becomes the urinary bladder in adult amniotes that have one).

    Even amniote organisms which, like most mammals, do not reproduce by laying external eggs are still considered amniotes for having evolved from others that did lay amniotic eggs.

    While seemingly simple and so often taken for granted every time we eat an omelet, the amniotic egg was a gigantic evolutionary step: it is what allowed amniotes to fully conquer land, and the lack of which maintains amphibians as prisoners of their need of water, even to this day. This is the reason why in a desert you can find reptiles and mammals galore, yet very few amphibians.

    According to the most recent understanding (not perfect but rather, as ever, changing and deepening) of the relationships among things living, a convenient and encompassing way to classify the amniotes would rely on the structure of their bones. These are their best preserved tissues in the fossil record and, consequently, can be used with relative ease to elaborate the best possible (at present) systematic understanding of them.

 

    In several ways, the most eloquent part of chordates’ skeletons - including those of amniotes - are their skulls. And it is indeed the skulls of animals what are most often used by paleontologists and taxonomists to differentiate among amniote groups.

 

    The amniotes can be classified according to three basic types of skull:

 

 

The Anapsida

 

    The ancestral (most primitive) type of amniote skull is the “anapsid”. This kind has only two openings on each side: one for the eye, and one for the nasal passage.

 

    Although turtles have often been considered the only living Anapsida, more recent data seem to point to their proper inclusion among the Diapsida, because their skulls seemed to have derived from a diapsid condition.

 

    Anapsids gave rise to the two other groups of amniotes: diapsids and synapsids.

 

 

The Diapsida

 

    The next kind of amniote skull is the “diapsid”. This skull has on each side, additionally to the eye and nasal openings, two more located behind the eye, and these are called the “temporal fenestrae” (“upper” and “lower”; the singular in Latin is fenestra: “window”). The fenestrae seem to fulfill the function, at least on some animals that possess them, of serving as attachment points for the jaw muscles. Forms that - like modern birds - have lost one or both fenestrae are still considered diapsids for having evolved from ancestral forms with them.

 

    The Diapsida are the most varied and abundant of living and extinct tetrapods. From the phylogenetic standpoint (that is, not according to most modern vernacular languages) the diapsids are the "reptiles". As such, they include the squamates (lizards and snakes), rhyncocephalians (tuataras), chelonians (turtles), archosaurians (pterosaurs, crocodilians, and dinosaurs - these last including birds), and several totally extinct taxa. Thus, they include a bewildering array of organisms both poikilothermic and endothermic.

 

 

The Synapsida

 

    The third type of amniote skull is the “synapsid”, which has only one temporal fenestra on each side of the skull. This single opening fulfills the same purpose as the two fenestrae of diapsids. Once again, taxa which have secondarily lost the fenestra are still considered synapsids. Such is the case of the extant mammals, in which the fenestra has been modified into a simple concavity, the "temporal fossa".

 

    Indeed, mammals are the only living Synapsida, but extinct forms included the pelycosaurs, therapsids, and cynodonts, creatures increasingly less "reptilian” and more "mammalian” as one moves up the fossil record. That means that what we know as “mammals” constitutes a "taxon" but not a true natural group. Systematically, they are rather a poorly defined portion of the Synapsida.

    For more information on Systematics you can visit the Tree of Life Web Project at http://tolweb.org/tree/, and to http://www.ucmp.berkeley.edu/glossary/gloss1phylo.html.

    Now that you are wretchedly confused, please proceed to the next section…