Natural History of Vertebrates
Lecture Notes
Chapter 3 - Jawless Vertebrates and the Origin of Jawed Vertebrates

These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; they are a supplement to the discussion in class and the textbook. If you were to study only these notes, you would not learn enough to do well in the course.

List of Terms

The earliest "definite" vertebrates were the ostracoderms. The ostracoderms are a paraphyletic assemblage of organisms as some of the descendants of this group are placed with early jawed vertebrates. The earliest vertebrate evidence comes from the Early Cambrian. Myllokumingia, a fossil from this time that had a dorsal fin, which is a derived character and makes Myllokumingia more derived, in some respects, than a hagfish

Some of the earliest vertebrates (Late Cambrian) were a group of little understood organisms called conodonts (figure 3-2), which are primarily known from an abundance of small tooth-like fossils from the Late Cambrian to the Late Triassic. The structure of the tooth-like fossils and the impressions of conodont fossils indicates that these organisms were early vertebrates.

There are partial reconstructions of fossils from the Early Ordovician, though no complete fossils are known until the end of the Late Ordovician. These Ordovician forms were small (12 - 35 cm) and completely encased in small plates that abut in the head region but overlap from midbody back. There is evidence of sensory canals, protection around the eye, and gill openings along the sides (figure 3-2) Jawed forms have already appeared by the Middle Ordovician, and thus numerous jawed and jawless forms were extant at the same time.

Vertebrates represent a new more active life style relative to the other chordates (table 2.1 and figure 2-4). These earliest ostracoderms possessed a muscular pharyngeal pump for forcing water over the gills. Amphioxus (a non-vertebrate chordate) uses ciliary motion to move water, this system can not be used for a larger, more active organism, like the early ostracoderms. This innovation of using a muscular pump to move water allowed these animals to maintain a much more active life style and to eventually achieve a much larger size. These animals were also primarily filter feeders but some may have been predaceous on smaller, slow-moving invertebrates.

Appearance of Bone
Primary component of vertebrate bone is calcium phosphate. Calcium carbonate is the most common mineral in invertebrates but some do have calcium phosphate. Extensive deposits of bone first appeared in a group of animals called ostracoderms, jawless and without movable paired fins, the head region was encased in a bony dermal armor. However, conodonts had bony elements (the tooth-like structures) and early vertebrates possessed odontodes (tooth-like structures composed of dentine and embedded in the skin). The bony armor of the ostracoderm may have been composed of many odontodes that fused into a solid piece of bone. Wether the very earliest vertebrates had bone or if it arose later in the evolution of vertebrates can not be determined at this time.

Bone's protective function is probably the most obvious but in addition, it serves as an important means for the storage of calcium and phosphate and may have served as an insulating mechanism for electroreception.

Marine origin
All non-vertebrate and deuterostome taxa are marine. The earliest vertebrate fossils are found in marine deposits. The most primitive extant vertebrates (hagfishes) are exclusively marine. The function of the vertebrate kidney in these early forms probably was to regulate calcium and magnesium, which are needed for muscular contractions. Later, this kidney was then useful for regulation of fluid volume as the organisms invaded freshwater.

Jawless Vertebrates
There are no fossils attributable to hagfishes until the Carboniferous, however they possess many characteristics that indicate that they must be more primitive than even the ostracoderms (table 3-1). This means that hagfishes must have diverged from the vertebrate lineage before the ostracoderms did (figure 3-3). We will revisit the hagfishes later.

Extant Jawless Vertebrates

two groups, hagfishes and lampreys
The fossil record is sparse as both groups first appear in the fossil record from the Carboniferous, which is at least 100 million years after they diverged from the other vertebrate lineages.

groups of ostracoderms (extint jawless forms) Rise in jawed vertebrates

The jawed vertebrates not only differ from jawless forms based on the presence of jaws but also differ in many other ways. The differences are found in the body musculature, the structure or even presence of fins, the gills, and the sensory apparatus are just a few of the areas (study figure 3-9 and table 3.2).

Origin of jaws
Vertebrate jaws are made from neural crest derived cartilage just at the branchial arches are. Thus jawed vertebrates converted gill arches into jaws (figures 2-9 and 3-9, pages 30-31). What was the selection pressure that led to the transition of a gill arch into jaws? How could the transitional states (the proto-jaw) have a selective advantage as well?

It could be that enlargement of the mandibular arch (the first gill arch) was to increase ventilation of the gills. This would be advantageous for a more active organism and would allow a stronger pumping action for the suction of water into the mouth. The increased suction would lead to increased prey capture ability. Further modification of the mandibular arch would increase the ability to restrain a struggling prey item. Thus the transition from ventilatory structures to feeding structures (figure 3-12).

Jaws brought several new abilities The jaws allowed the jawed vertebrates to reach much larger sizes and to quickly replace many of the jawless forms in the fossil record of the Devonian period.

Origin of fins

In a fluid environment (water or air) there are three directions of control (figure 3-11):
  • pitch is head up or down
  • yaw is head right or left
  • roll is rotation around the longitudinal axis.
    Fins provide a means of control. By pushing against the water with its fins, a fish can control its motion in these three direction. Fins actually allow very precise control and this gives the fish the ability to orient its jaws relative to a food source or other object to be manipulated by the jaws. This can really be brought home by watching fish feed in the often turbulent and seemingly chaotic flow of water over a coral reef. Fins allowed fish to greatly increase speed and agility when moving about.

    The dorsal and anal fins control yaw. The pelvic and pectoral fins control pitch. All of the fins control roll. In addition, fins generate thrust for forward motion.

    There is no real fossil evidence that sheds any light on the origin of the paired fins (pectoral and pelvic fins). Most evidence comes from embryonic data and innervation patterns. In fact, paired fins probably evolved several different times as there are only a few positions for the fins to function properly. Early jawed forms may not have had paired fins that were homologous with the paired fins of other taxa (that is, they had different evolutionary origins). However, all modern jawed vertebrates have paired appendages with at least some homologous elements. Several jawless forms possessed a caudal fin of some type (either heterocercal or hypocercal), however, jawed forms support the caudal fin with collagenous fin rays, which greatly strengthens the fin and allows for much greater thrust.

    Earliest jawed vertebrates
    Last updated on 29 January 2008
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