Palaeos Vertebrates 020.000 Craniata Overview

*Palæos:*		Unit 20: Craniata
*The* *Vertebrates*		Overview

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Craniates and the Earliest Vertebrates

Here we enter a strange and poorly known area of phylospace, one in which both timing and phylogeny are poorly understood. Somewhere between 750 and 550 million years ago, a new line of chordates evolved. These were characterised by a relatively concentrated set of sensory neurons at the anterior end and a particularly pronounced notochord extending beyond the anterior and posterior ends of the torso. In other words, they had the beginnings of a head and a tail. In addition, probably as a later development, the myomeres, the charcateristic bands of muscle along the torso, were reorganized in dorsal and ventral halves, separated by a septum, and acquired a distinctive 'V'- or 'W'-shape. These folks were our ancestors among chordate-kind.

The last common ancestor of hagfish and you, with all of its descendants, is referred to as Craniata. Craniates are animals with well-developed heads and a skull of sorts. A very early craniate, Myllokunmingia from the Cambrian of China, is badly illustrated in the figure at right.

Since there is no really good reason to suppose that Haikouella belonged to this clade, we treat as a sort of "craniatomorph," but not a craniate.

From the first craniates evolved a bewildering variety of creatures. At least they have bewildered systematists for over a century. The first step seems relatively clear. The most primitive craniates we know are the living Myxinoidea (hagfishes): eel-like, rather assymmetric creatures with a strong propensity to flood their immediate neighborhood with slime at the slightest provocation. All other known craniates are Vertebrates. ATW020417.

The Vertebrates

Generally mention the word "animal" and most people think of a vertebrate animal -- and more particularly a higher vertebrate, especially a mammal. Vertebrates are large enough for people to relate to, and most like us in appearance and structure. Hence they are distinguished from all other members of the animal kingdom, which are considered "bugs", "worms", etc. Significantly, when Linneus was formulating his systematisation of the animal kingdom, four of his six classes -- mammals, birds, amphibia (including reptiles) and fish -- were vertebrates.

Vertebrates are animals with a backbone. They differ from other Chordata in that they possess at least some rudiments of a vertebral column or backbone instead of (or in addition to) a notochord. There is a well developed head, with the brain encased in a skull or cranium. Most vertebrates have an elaborate internal skeleton that acts as a support for muscles and organs. The advantage of an internal skeleton is that it allows the animal to grow much larger than is the case with the arthropod-type exoskeleton.

All vertebrates have a single heart and closed circulatory system. They also possess ductless endocrine glands that play a critical role in regulating the body metabolism. In their circulatory, excretory, and endocrine functions, vertebrates differ markedly from other animals.

The vertebrate body plan has proved extraordinarily successful. Since their first humble beginnings in Furongian seas, the Vertebrates have conquered the land, sea, and air. Thre are some 45,000 living species, and who knows how many extinct forms that have lived and died in past ages. MAK010430.

Some Early Vertebrate Systematics

The most basal group of vertebrates are the Hyperoartia (lampreys) which survive as disgusting, but rather interesting, blood-sucking parasites on fish. In addition, the vertebrates produced the Euconodonta, the Pteraspidomorphi, and a vague, perhaps polyphyletic group referred to for the time being as "Thelodonti". These will be discussed in more detail in other sections.

Work on what is now the vertebrates section of Palæos began less than five years ago. At that time, the consensus view of early vertebrate evolution was not far from the cladogram reflected when the predecessor to this site first went on line. The standard texts generally reflected craniates evolving sometime in the Ordovician, with vertebrates arising somewhat thereafter, perhaps during an apparent secondary "explosion" in the Silurian. Eventually, the Osteostraci emerged, and some Osteostracan or a closely related forms developed jaws and paired pectoral limbs -- all at about the same time -- resulting in another quick radiation of gnathostome forms during, perhaps, the Late Silurian. At that time, conodonts were not considered chordates -- much less vertebrates -- and the thelodonts were considered as little as possible, since they made no phylogenetic sense. For that matter, the thelodonts still make very little sense.

Nonetheless, things have changed a good deal, and -- although there is probably not yet a new consensus -- it has been necessary to change the very backbone of the vertebrate cladogram (to introduce a serious redundancy). After a recent series of significant revisions, the story unfolds at a much more leisurely pace.

The origin of chordates now seems to date back to Pre-Cambrian times, perhaps 600 My. Haikouella was described by from the Early Cambrian (about 530 My) of South China in 1999. It is a derived chordate, just short of being a crown group craniate. Myllokunmingia, from about the same time and location, probably is a crown group craniate. Shu et al. (1999). Crown group vertebrates, i.e., the last common ancestor of lampreys and gnathostomes, can scarcely have been much later. In fact Haikouichthys, also described in Shu's paper, may well be a very primitive hyperoartian (lamprey cousin).

By Furongian times, at 500 My, vertebrates had evolved a variety of forms, including the extraordinarily strange and successful euconodonts. Conodonts have been known for almost as long as there has been paleontology, but their peculiar dentition was all that was known of the group. Since conodont remains are very common, they were much-studied as stratigraphic markers but otherwise little understood. As Donoghue et al. (2000) state in their recent study: "Just a year before the first conodont fossil with preserved soft tissues was found, Müller (1981) compiled a list of groups to which conodonts had been attributed; his list includes at least three kingdoms and almost every major animal phylum." Id. at 192.

It now seems fairly well accepted that conodonts are chordates. The Donoghue group's own detailed cladistic study places them within the vertebrates, just above the Hyperoartia. Interestingly, the authors state that the earliest (Pre-Cambrian to Early Cambrian) conodonts, known as the Protoconodonta, are probably unrelated. They assert that the Early to Late Cambrian Paraconodontida are related, but do not include them in their analysis because there are no known soft tissue remains. Thus their analysis is based only on the relatively derived Euconodonta. If paraconodont soft tissue remains are ever found, the picture may be different.

The exact placement of this group is of some interest because the inclusion of conodonts has other strange effects on the vertebrate cladogram. Donoghue et al.'s final result looks like this:

Note that the headshield forms (osteostracans and their close relatives) are now monophyletic -- no longer gnathostome ancestors. This is odd, but appealing. The Osteostraci have never been satisfactory gnathostome ancestors because they are very highly derived forms. The general direction of osteostracan evolution seems to have been toward a loss of fins and an ever-more benthic existence. Only the most basal forms, like Ateleaspis, really suggest a gnathostome relationship. The separation of anaspids from lampreys is less congenial. Janvier (1996) shows a rather neat series between the anaspids and lampreys which can be seen below. However, this separation does seem to be required by the current data.

Somewhat similar results were obtained by Wilson & Caldwell (1998) in their study of the "fork-tailed" thelodonts, the Furcacaudiformes. The thelodonts, like conodonts, are another old group of misfits. They come in a great variety of forms: some resembling anaspids, osteostracans, and gnathostomes. Thelodonts appear to be united only by lacking large dermal plates and having very small scales somewhat similar to those of primitive sharks, and have been widely believed to be para- or polyphyletic. See, e.g. Thelodonti. Some appear to have paired fins (located over the gill slits) and other advanced features, including a well-developed anal fin and possible fin spines. Unfortunately, since thelodonts lack large dermal plates or massive cartilaginous headshields, they are rarely found as articulated specimens. Wilson & Caldwell, however, used their new data on the aberrant furcacaudiforms to try to anchor the group. Although some of their conclusions are too difficult to accept at the moment, they, like Donoghue et al., also conclude that Loganellia and other thelodonts (e.g. Turinia) are close to the gnathostome base and that that the osteostracans are several steps removed.

Given these results, it may now be possible to advance seriously the hypothesis that the thelodonts are the main stem of vertebrate evolution, and that the other groups are only branches. Consequently, we have taken the liberty of temporarily promoting the thelodonts to "Thelodonti" to include anaspids, gnathostomes, furcacaudiforms, and the headshield forms. This solution is only slightly radical and finds comfortable homes for both of the perennial orphans of early vertebrate paleontology: the conodonts and thelodonts. No doubt these are only foster homes, and some new and different arrangement will be necessary in a few more years, particularly for the conodonts. However this hypothesis surely makes more sense than relegating to footnotes the conodonts, who make up 65% of all non-gnathostome chordate species, and thelodonts, our closest known relatives among jawless fish. ATW 010126

Ancient Bloodsuckers

The vertebrate family tree has always had the lampreys splitting off early, as it still does. Until the 1980's it was believed that the lampreys were close cousins to the hagfishes (Myxinoidea), with which they were included in the order Cyclostomata ("round mouths"). It is now known that the hagfish are too primitive to be considered vertebrates, althought they qualify as Craniates, as discussed above. However, lampreys are known in the fossil record only from the Carboniferous. The teaching became that the lampreys were the sister group, perhaps even the descendants, of the anaspids. The two groups were united in the clade Petromyzonida, and indeed, they seem to belong together.

The invasion of the conodonts has altered our view of the vertebrate tree and separated the lampreys and anaspids; but the position of the conodonts remains quite controversial. The lamprey-anaspid divorce is by no means finalized. Both authors of this site -- who disagree on many matters of systematics -- have strong suspicions that the two will reconcile and again take up residence in the Petromyzonid homestead. Thus, we examine in detail the composition of the taxon Petromyzonida, even though we currently present the Anaspida separately, in a later section.

In this view of these taxa, the Petromyzonida are the only surviving lineage of the ancient agnaths (jawless vertebrates), represented only by the parasitical lampreys, "living fossils" which first appeared (in only slightly different form) during the Carboniferous period. The figure below, from Janvier (1996), illustrates the anaspid-like fossil craniates and their possible relationships in the Petromyzonida.

Stratigraphic range and cladistic relationships of the Paleozoic Anaspisda and related forms: L = Lower (Early), M = Middle, U = Upper (Late), Q - Jamaytius; R - Legendrelepis; S - Endelolepis; T - Pharyngolepis; U - Pterygolepis; V - Birkenia; W - Lasanius; X - Phlebolepis; Y - Loganellia. Diagram from Janvier & Blieck (1993)

The Paleozoic (Silurian and Devonian) anaspids lacked the heavy head shields of the conventional cephalaspidomorphs. Covered in thin scales, their bodies were slender and flexible, with stabililising fins. They are known from the Late Silurian of Europe and North America, but it is almost certain that these active swimmers had a much wider distribution. During the Devonian they gave up a marine existence for life in rivers and lakes. By the Carboniferous they had already evolved forms very like modern lampreys.

Tarlo (1967) gives the following Linnean arrangement, which can be usefully contrasted with Janvier's cladistic tree.

In this view, the Anaspida (in the broad sense) can be included under the Petromyzonida (or vice versa!). The Birkeniiformes correspond to the Anaspida in the narrow or strict sense. Despite their early age they are among the most specialised of the Anaspida-Petromyzonida, and also the most armoured, with forms like Birkenia having a thick coating of scales. The Lasaniiformes (includes only the species Lasanius problematicus) and can be included under the Birkeniiformes (Anaspida proper). MAK000113.

Craniate Resources

BIOS 447 Comparative Vertebrate Anatomy - Rory Dickinson - a detailed (but also quite technical) account of the stages of vertebrate evolution, in the form of an annotated cladistic diagram. Quite an interesting approach.

Life Before Man by Zdenek V. Spinar, illustr. by Zdenek Burian - basic text, superb illustrations. Some of the illustrations are rather dates - e.g. the sauropod dinosaurs. Nevertheless, this is still the best paleo picture book around. Despite the title, there are also some neat paintings of early man

Vertebrate Life by F. Harvey Pough, Christine M. Janis, John B. Heiser - basic level textbook, a very good and readable overview of vertebrate physiology, evolution, etc. Technical concepts all well explained

Vertebrate Palaeontology by Michael J. Benton, illustr. by John Sibbick - more readable than Carroll's masterwork, a good basic-level textbook

Vertebrate Paleontology and Evolution - by Robert L. Carroll. This is the classic work on the subject! I cannot recommend it too highly. All about everything from jawless fish to dinosaurs to mammals. Quite technical, but worth persevering if this is your interest. An invaluable reference work. Carroll must be one of the last non-cladists on the planet. Good on him!

Early Vertebrates, Philippe Janvier, 1996, (Clarendon Press, Oxford) absolute masterwork on early vertebrates!


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Craniates and the Earliest Vertebrates

Abbreviated Cladogram

Contents

Craniata

The Vertebrates

Some Early Vertebrate Systematics

Ancient Bloodsuckers

Craniate Resources

General

Vertebrate History

Vertebrate Phylogeny

Books and CD Roms