Natural History of Vertebrates
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.
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.
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.
Myxinoidea (hagfishes, figures 3-5 and table 3.1)
- must have diverged in the Ordovician but maybe the Cambrian
- the sister taxon to all the rest of the Vertebrata
- do not possess vertebrae
- all modern forms are strictly marine
- body fluid is isotonic with seawater
- about 40 species known worldwide
- burrow in mud
- some forms are colonial
- elongated, scaleless
- single terminal nasal open
- terminal mouth surrounded by six tentacles
- two horny plates in the mouth are used to grasp prey or help in tearing off pieces of flesh from
other vertebrates (dead or dying)
- one semicircular canal on each side of the head
- They secrete large amounts of mucus which is a deterrent to predators. After the danger has passed it ties itself in a knot and passes its body through the knot to scrape off the mucus; it also "sneezes" to clean out its nasal opening.
- food is digested in a mucoid bag
- Our knowledge of reproduction and mating behavior is very limited.
lay eggs, no larval stage, some may be hermaphroditic
Petromyzontoidea (lampreys, figure 3-6)
- similar in size and shape to hagfishes, but are very different
- possess vertebrae (though very small)
- about 40 species worldwide in two genera
- nearly all are anadromous (that is, ascend rivers from the ocean to reproduce)
- All are parasitic in that they attach to side of a fish, rasp a hole in the skin and suck out bodily
fluids. They do not usually kill their host but detach to leave it greatly weakened.
- digestive tract is greatly reduced
- large well-developed eyes
- two semicircular canals on each side of the head
- heart is innervated by the parasympathetic nervous system
- well-developed kidneys that allow lampreys to be euryhaline
- lay eggs
- fertilization is external
- adult lampreys die after breeding once
- Young hatch into a larval stage called an ammocoete, which moves downstream to burrow into the mud in a quiet part of the stream. The larva spend several years burrowed in the mud and obtain food by filter feeding. Metamorphosis to the adult form produces a juvenile that then may
spend a couple of years as a parasitic adult in the ocean.
groups of ostracoderms (extint jawless forms)
Heterostraci (also called the Pteraspida or Diplorhina, figure 3-7
- Early Silurian to Late Devonian
- size from 10 cm to 2 m.
- anterior part of body encased in bony plates, with the head shield having several protruding ornaments, paired lateral ornaments, some dorsal and some ventral
- openings for lateral paired eyes, and median eye or pineal organ.
- opening for a terminal mouth
- a moveable hypocercal tail extended from the bony plates with the notochord extending into the
lower lobe of the tail
- body cross section was generally round and they probably resembled a tadpole when moving.
The Heterostraci diverged from the vertebrate lineage after the hagfishes (figure 6-1).
Anaspida (figure 3-7)
- Early Silurian through the Late Devonian
- 15 cm in length
- single median nasal opening
- narrow scale rows
- head naked or covered with small scales (no bony head shield as in the Osteostraci)
- hypocercal tail, probably bottom feeder
- dorsal, anal and lateral projections or folds used for stability
Thelodonti (also called the Coelolepida, figure 3-7)
- fossils from the Late Silurian and Early Devonian
- possessed small scales each with a pulp cavity like a tooth
- size 10 to 20 cm
- dorsoventrally flattened anteriorly and laterally compressed posteriorly
- some had laterally projecting flanges where paired appendages would later appear
- lateral placed eyes, pineal opening
- jawless mouth (some may have crushed prey with internal denticles similar to gill rakers)
- some forms with a well-developed stomach
- seem to be more numerous in estuarine environments
- The relationship of the Thelodonti to the other vertebrates is very uncertain (figure 6-1)
cephalaspida (also called the monorhina)
- This is a paraphyletic assemblage of ostrcoderms that are generally more derived that the Heterostraci.
- Represents several distinct groups that all shared the derived feature of a single, large nasal opening in the center of the head and anterior to the eyes
Galeaspida (figure 3-7)
- Early Silurian to Late Devonian
- similar to the Osteostraci
- no paired fins
- large opening on the dorsal surface of the head shield (the opening was connected to the pharynx
and may have been an inhalant canal)
Osteostraci (figure 3-7)
- from the Middle Silurian to Late Devonian
- solid head shield that lacked sutures and did not grow after it was formed
possible larval form that metamorphosed into an adult with a head shield
over time, the trend is towards a reduced head shield
- heterocercal (epicercal) tail, which tends to raise the head; leads to increased mobility
- paired fin-like structures
- fed by sucking items from the bottom by expanding the pharynx
- internal anatomy of the brain and nervous system is similar to that of modern lampreys
Pituriaspida (figure 3-7)
Rise in jawed vertebrates
- similar to the Osteostraci
- paired pectoral appendage that contain muscles
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.
- ability to grasp prey
- With the development of teeth they gained the ability to chew up prey into smaller pieces, thus were able to take
larger prey items.
- ability to pick up objects for nest building
- ability to grasp a mate during courtship
- ability to grasp young during care of the young.
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
- appear in the Early Silurian and extinct by end of the Devonian, though numerous throughout the
- up to 10 m in length (figure 3-16)
- the sister taxon to all other jawed vertebrates (Eugnathostomata) (figure 3-14, 3-15)
- unique jaw musculature with the muscles median to the palatoquadrates (the upper jaw
elements). This is opposite of all other jawed vertebrates where the muscles are external to the
palatoquadrates. This also indicates that jaws may have evolved twice in the evolution of the
- upper jaws were attached tightly to the cranium or head shield, which limited the mobility of the
- no teeth that resemble those of other jawed vertebrates
- only some individuals within a species had pelvic appendages, which indicate they were male and that the species had internal fertilization
- hyoid arch is different from other gnathostomes
- The head region was covered with thick dermal bone. Some forms had a moveable joint between the
plates where the neck would be, which allowed the jaws to be opened very wide.
- The sister taxon to the Osteichthyes (figure 3-15, 3-17)
- Earliest jawed fishes in the fossil record dating from the Early Silurian, but disappeared by the
- Had stout spines anterior to the dorsal, anal, and many paired fins (figure 3-16)
- 20 cm in length
- teeth lacked enamel
- few enlarged scales, though some lacked scales
- three semicircular canals
- presence of a tooth whorl as seen in sharks
- cranium composed of cartilage
- neural and haemal arches but no vertebral centra are known
- shared several characteristics that aligns this taxon with the Osteichthyes and places them in the
group the Teleostomi (presence of an ossified dermal operculum, mechanism of opening the
mouth via the hyoid apparatus transmitting motion to the lower jaw, presence of an interhyal
bone, branchiostegal rays).
Last updated on 29 January 2008
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