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CRANIATA: Animals with Skulls
Craniates are all united by the presence of at least a small skull, or cranium, encasing the brain. And we all arose from a set of interesting ancestral fishlike creatures.
The earliest ancestors of craniates were plate-skinned fishes known as
ostracoderms (like the little yellow fish in this DRAMATIC video). The name comes from the Greek ostraco meaning "shell" and
derm meaning "skin".
One offshoot of this lineage, the osteostracans, evolved paired fins just behind the head. (What's the advantage of paired fins?)
- were a diverse array of armored fishes
- lacked a mandible (hinged jaw)
- lacked paired fins
The Devonian period saw the rise of the first jawed fishes, the placoderms, all of which are now extinct. Another
group, the acanthodians which lived at the same time as the placoderms, may have included the ancestors of all modern bony fishes.
One placoderm, Dunkleosteus, bears the distinction of having had the strongest bite of any known animal. Watch the cheesy, fakey video!
Craniates can be divided into:
Jawed vertebrates all show
similar ontogeny with respect to jaw formation, and so are believed to be monophyletic. The jaw bones develop
from portions of the cartilaginous arches that once formed gill supports in the earliest chordates.
The condition of being "jawless" is primitive, and is not a good diagnostic character to use for constructing phylogenies.
Let's Meet the Vertebrates
I. Hyperotreti, the Hagfishes
II. Vertebrata, the Vertebrates
Hyperoartia - The Lampreys
Predatory, with vicious circular mouthparts that attach to prey fish much larger than the lamprey, and allow the lamprey to essentially slowly eviscerate its prey. Lampreys have been introduced accidentally into freshwater lakes (they are normally marine), where they wreak havoc on the native wild fish such as salmon and trout.
B. Gnathostomata - The Jawed Vertebrates
- Vertebrate jaws evolved from the skeletal supports of the anterior pharyngeal gill
- The posterior slits were retained as respiratory structures
- Vertebrates have a complex camera eye that forms highly resolved images
- rod photoreceptors
- high sensitivity to light (night vision)
- low image resolution ("grainy" picture)
- no color information
- cone photoreceptors
- relatively low sensitivity to light (day vision)
- high image resolution (clear picture)
- different classes of cones each have a different pigment, conferring color vision
Gnathostomes - The Jawed Vertebrates
I. Chondrichthyes (sharks, skates and rays)
II. Actinopterigii (ray-finned fishes)
III. Sarcopterigii (lobe-finned fishes and tetrapods)
Sharks, skates and rays
Chondrichthyan Features and Facts all carnivorous/predatory
skeleton composed entirely of cartilage (bone has been secondarily lost)
many pelagic (free-swimming) sharks constantly swim to keep water moving over the gills, lest they suffocate
placoid dermal scales cover the skin and form replaceable
Sharks hunt via keen sense of smell, and localize prey
with the lateral line system, which detects movement in the water.
Chondrichthyans may be
- oviparous - laying eggs that hatch outside the
- ovoviviparous - brooding eggs that hatch within the
mother's body, and then releasing the young
- viviparous - young develop
within a uterus inside the mother's body, and are nourished prior to birth
via a connection with the mother's bloodstream (placenta).
The most diverse (~30,000 species) and numerous of vertebrates
Vast diversity in form, function and natural history
Fun Fish Facts
Skeleton composed primarily of bone
Skin covered with dermal scales that protrude through the epidermis and shed readily.
Skin copiously supplied with mucous glands (science terminology note: mucous is an adjective. mucus is a noun.)
Fins may be either medial and unpaired, or lateral and paired
- poikilotherm - body temperature controlled by the environment
- homeotherm - body temperature metabolically controlled
- ectotherm - body heat obtained from the environment
- endotherm - body heat produced metabolically
Fin rays are composed of cartilage or bone
Toothy, terminal (i.e., at the end of the body) mouth
Gill arches covered by a bony operculum
Swim bladder, derived from the digestive tract, may or may not be open
to the pharynx, and provides buoyancy.
Heart is two chambered (one ventricle (caudal) and one atrium (cranial)
Brain differentiated, with small olfactory lobes and cerebrum; large
optic lobes and cerebellum (what does this tell you about what fish do
best, and where they might not excel?)
Sarcopterygii - Lobe-Finned Fishes and Tetrapods
The lobe-finned fishes include such "living fossils" as the Coelacanths and the lungfishes. (More cool lungfish information here.)
These are the closest living relatives to the four-legged vertebrates, or tetrapods.
These are the four-legged, primarily terrestrial vertebrates, derived from a specialized group of
shallow-water fishes that probably resembled extant lobe-finned fishes.
The amphibians made first landfall during the Devonian (400 mya). The
first tetrapod had come ashore, with these defining characteristics:
- Amphibia (salamanders, frogs, caecilians)
- Reptiliomorpha (reptiles, birds, dinosaurs, mammals, etc.)
There are three main Orders:
Modern Caudates (salamanders and newts) most closely resemble the
ancestral amphibians, in that they retain a tail as well as the five-digits on each limb. But all three extant
orders of amphbians are highly derived in their own ways, with each
species showing amazing specializations in both morphology and behavior.
- four limbs connected to the trunk
- each limb ending in five digits
Fun Amphibian Facts
ectothermal poikilotherms (behaviorally thermoregulate)
Primitively, larvae are aquatic, with gills and lateral lines
Skin is scaleless, and serves a major respiratory function: must remain moist
Skin is well supplied with glands, including poison glands. Some species are far more poisonous than others!
large mouth with very small teeth
Mostly insectivorous/carnivorous (they are sit-and wait/ambush predators)
Heart is three-chambered
internal fertilization in caecilians and salamanders
external fertilization in most frog species
The amniotic egg is a synapomorphy with respect to all the extant Reptiliomorpha.
Check out this link to see what's what in the amniotic egg
Three main lineages of Reptiliomorphs, classified on the basis of skull morphology:
To which group do we belong? Have a look and see if you can figure it out.
- Anapsida - No temporal opening behind the eye orbits
- Synapsida - Single temporal opening behind the eye orbits
- Diapsida - Two temporal openings behind the eye orbits
Today's living remnants of these three lineages:
- Anapsids - turtles (?)
- Synapsids - mammals
- Diapsids - tuataras, lizards, snakes, crocodilians, and birds
Anapsida - Turtles and their relatives
- external bony shell
consisting of a dorsal carapace and ventral plastron
- The shell is fused to the vertebrae and ribs, and is an integral part of the
- Turtles lack teeth, but have sharp cutting edges to the maxilla (upper
jaw) and mandible (lower jaw).
- Turtles may be marine, fresh water, or terrestrial.
- Here's a nice
gallery of a few of the many turtle species for you to enjoy at your leisure.
Dinosaurs, Birds, Crocodilians, Tuataras, Snakes, and Lizards
- scales derived from the epidermis (not dermis, as in fish)
- skin is tough, scaley and resistant to desiccation
- dermal chromatophores (pigment-bearing cells) give skin its many distinctive colors and patterns
- powerfully muscled jaws designed for gripping and crushing prey
- internal fertilization
- Kidney ureters empty into a cloaca (diapsids have no bladder)
- nitrogenous waste is excreted primarily as uric acid
- heart is three- (lizards and snakes) or four-chambered (crocodilians and birds)
heart (more efficient than three-chambered)
- higher blood pressure allows higher levels of activity.
- well-developed lungs allow complete independence from wet habitat
- skeleton and limbs designed for terrestrial locomotion
- complex, highly developed nervous system with complex sense organs.
- trichromatic color vision (possibly tetrachromatic)
- cone photoreceptors may have an oil droplet organelle that filters incoming light, further enhancing color discrimination.
- vomeronasal organ (aka Jacobson's organ) on the roof of the mouth (secondarily lost in birds)
highly derived diapsids - great diversity in both form and behavior
complex behaviors - courtship, migration, etc.
evolved from feathered carnivorous ancestors known as therapods
famous therapod: Archaeopteryx.
feathers--epidermal scale derivatives--are homologous to scales/scutes of other diapsids (check out those leg scales)
There have been rare instances of avian gynandromorphs. At this time, the genetics and development of these anomalous birds is not fully understood.
- homeothermic endotherms
- four-chambered heart
- upper maxilla and lower mandible fused, covered with keratinous
sheath that forms the toothless bill (or "beak")
- long, flexible neck
- saurischian pelvis, derived from the Saurischian Dinosaur Ancestors
- (Side note: the ornithischian pelvis, in which the pubis points towards the tail, is found in the Ornithischian Dinosaurs. The bird pelvis, in which the pubis bone also points towards the tail, evolved that convergence with the ornithischian pelvis from a saurischian blueprint.)
- forelimbs modified for flight
- some birds (ratites, such as otrich, emu, cassowary, etc.) have secondarily become flightless
- respiration by means of slightly elastic lungs connected to air sacs nestled among the viscera and bones
- air spaces in the hollow bones reduce weight
- oil gland at the base of the tail provides feather waterproofing via preening
- highly developed nervous system with complex sense organs
- tri- or tetra-chromatic vision plus oil droplets in the cone cells: complex color vision important for intraspecific communication
- voice box near distal end of trachea, just before the bronchi of
the lungs: vocalization is important!
- egg shell highly calcified and hard; large yolk.
- young may be either altricial or precocial, depending on species
- Sex determination is via the female parent:
- male = ZZ sex chromosomes
- female = ZW sex chromosomes
- (in mammals, male = XY and female = XX sex chromosomes)
Mammalia: Our Friends, the Mammals
- There are about 4500 extant species in more than 14 orders.
Monotremes - Egg-Laying Mammals (Platypus and Echidna)
These are the only living mammals that lay eggs, which are reptilian in
structure and development. (Their cone photoreceptors have oil droplets, which have been lost in all other mammals.)
Marsupials - Pouched Mammals
Eutherians - Placental Mammals
These are the mammals that give birth to live young that develop in the
mother's uterus, and are nourished during development via the
connection to the mother's bloodstream, the placenta. The placental
membranes are homologous to the amniotic egg membranes. YOUR
ASSIGNMENT: Find out which membrane (amnion, chorion and allantois)
has developed into WHAT in the mammalian system. (HINT: Review the
Amniotic Egg link at the UC Berkeley Museum of Paleontology)
The Synapsid vertebrates are probably the most familiar to all of us, as we happen to
be one of the many species of synapsids. What's so special about the synapsids?
They were the first vertebrates to radiate into a wide diversity of terrestrial
- mammary glands
- fleshy lips for suckling (marsupials and eutherians only)
- hair (grows between the analog of the reptilian scute/scale)
- red blood cells have lost the nucleus
- high rate of metabolism
- ancestral vertebrate jaw bones have become modified to form the bones of
the inner ear.
- jaw musculature increased in size and complexity
- bony plate (hard and soft palates) separates the mouth and nasal cavities, allowing the animal to hold food in its mouth and still breathe
- This character also allows young to breathe while suckling (mammalian inovation)
The Importance of Teeth
The earliest true mammals showed up in the late Triassic, and were
They had diphyodont dentition, meaning they
were unable to constantly replace lost teeth (as their amniote ancestors
could), but rather replaced them only once over the span of a lifetime.
- deciduous ("baby") teeth
- permanent teeth
Humans and all other mammals show this pattern today.
Another dental inovation: differentiated
teeth (we are HETERODONTS, as
opposed to HOMODONTS as all other vertebrates
The number of each of these types of teeth varies with
species, and the
"dental formula" is one diagnostic character of mammal species.
In humans, the adult dental formula is 2:1:2:3/2:1:2:3 - This is the
number of teeth in half of the upper jaw (2 incisors, one canine, 2
premolars, and 3 molars) and in half of the lower jaw (two incisors,
1 canine, 2 premolars and 3 molars).
Note that a few mammals have secondarily reverted to the homodont condition, but this is derived with respect to other mammals.
Some mammals are hypsodonts. This means that their teeth grow continually throughout their lives, and must grind against each other to remain a normal, manageable shape. Some examples: rodents, lagomorphs (hares, rabbits, and pikas), horses, elephants and many other herbivores. What do you suppose is the advantage of having continually growing teeth? What could be some disadvantages?
What makes us mammals?
Hair (a derivative of the integument)
- undercoat - provides most insulation
- guard hairs - coarser and longer, this provides protection against
wear, and is usually the part of the pelage that has color patterns
- hair length is genetically determined, as we already have discussed
- hair usually sheds annually or twice a year
- specialized hairs called vibrissae ("whiskers") are used for touch
- Horns and antlers (another derivative of the integument)
- horns are sheaths of keratinized epidermis (keratin is the
protein that makes up hair) wrapped around a bone core rising from
the skull. They are permanent, and never shed. (Note: Rhino horn
isn't a true horn: it'sa swirl of hair-like filaments rising from the
dermis and firmly "cemented" together with glycoproteins to form the
structure, which is not attached to the skull.
- antlers are composed of solid bone at maturity, but are shed
- sweat glands
- eccrine - secrete water and salts, some organic compounds, trace amounts of urea and other waste products. Function: cooling
- apocrine - secrete a milky substance associated with reproduction
and emotion. The sweat from apocrine glands contains organic molecules (lipids, proteins, pheromones) that are broken down by resident bacteria, and produce a stong odor. Function: intraspecific (and possibly interspecific) communication
- scent glands - vary greatly in number, function, and location
- sebaceous (oil) glands - function to keep skin and hair soft and
pliable with its secretion, sebum.
- mammary glands - the gland that gives our class its name,
evolved for feeding the young. Our fleshy, muscular lips have evolved
for the function of suckling.
Mammals may be
insectivores - feed on insects
carnivores -feed primarily on herbivores
- herbivores (including frugivores)
Such mammals often have a complex intestinal flora of specialized
microorganisms that can digest cellose (a complex plant starch), which is
not digestible by mammals.
Many herbivores (horses, rabbits, elephants, some primates), have a
cecum, a side pocket that branches off the junction between small and
large intestine. This contains a complex ecosystem of microorganisms
that provides essential nutrients via fermentation of the herbivore's
Lagomorphs (hares and rabbits) and some rodents re-ingest their
cecotropes, strong-smelling pellets delivered from the cecum via the
anus. Your text calls this "coprophagy", which means "ingestion of
feces. This is not truly correct, as cecotropes are NOT feces. They are
an essential part of the lagomorph diet. Hence, your bunny engages, not
in coprophagy, but in cecotrophy.
Other herbivores (bovids (cow family) and ovids (sheep family)) are ruminants which have a large,
four-chambered stomach. Ruminants swallow food, which passes to the rumen,
where it is partially digested and regurgitated as cud. This is re-chewed
to break down fiber, and then sent back to the rumen for further
Let's talk about the "carnivore intelligence" myth....
omnivores - feed on a variety of plant and animal matter
Live birth of altricial young which are then raised in a pouch:
Internal brooding of young via a placenta - Eutherians (placental
Most mammals have defined mating seasons, with females accepting male
advances only during her estrous cycle. When not in "heat"
(estrus), the female is unreceptive to male sexual advances.
- One estrus per breeding season: monoestrous
- Multiple estrus per breeding season: polyestrous
- Induced ovulators: ovulate upon the act of mating
Family Ties: The Primates
The first primate was
This lineage gave rise to the
- small (about the size of a rat)
How has our evolutionary history made us who we are today? The
earliest hominids (human-like apes) were not only social, but
cooperative, and evolved into the most primitive human social group,
- prosimians - lemurs,
- simians - monkeys and apes (including us)
(And some very derived
And of course, of great interest to most of us are our own primate relationships.
The primate ancestors that gave rise to the simians were
- diurnal, with good trichromatic color vision
male - hunter of meat
female - gatherer of vegetable matter
What do these two types of foraging strategies require, and what
genetically-based behaviors might confer a selective advantage to
Our Favorite Ape: Homo sapiens
Paleoanthropology is the study of human origins and evolution. Because
humans and chimpanzees have existed as separate species for only a few
million years, this branch of science examines only a very small, recent
portion of the fossil record.
Terms to know:
Humans, apes, and monkeys all diverged from
a common anthropoid ancestor that shared all the characteristics common to all primates. Natural selection has driven the specialization of each primate species to be what it is today, and that includes our own species.
- anthropoid - of or pertaining to monkeys and apes
- hominoid - of or pertaining to the great apes (including humans)
- hominid - member of family Hominidae
The earliest fossil members of our genus (Homo) range in age from
about 2.5 to 1.6 million years, and are currently classified as Homo
Homo erectus shared common ancestry with H. habilis, and
shows up in the fossil record from about 1.8 million to 0.5 million years
Homo sapiens neanderthalensis , named for the Neander Valley in
Germany where its fossils were first found, may have arisen from an H.
erectus-like ancestor, as H. erectus is known to have migrated into both Europe
and Asia (timing uncertain).
There's a good deal of argument about how anatomically modern Homo sapiens
sapiens came onto the scene, but it's generally agreed that we
originated in Africa, where all the oldest hominid fossils are found.
Here's a nice
overview of our hominid relatives, for those who would like to delve a
- Multiregional Hypothesis:
Homo sapiens evolved in each of the
regions where its fossils are now found from ancestral Homo
erectus that migrated out of Africa about 1.5 million years ago.
Advocates of this hypothesis consider H. erectus to be an early
version of H. sapiens, and not a different species.
Constant interbreeding between neighboring populations of this "archaic"
may have prevented reproductive isolation, resulting in our
present-day races of Homo
rather than multiple species of Homo.
- "Out of Africa" Hypothesis:
All H. sapiens now living evolved from a second major migration out of
Africa that occurred about 100,000 years ago, and not from wandering Homo
erectus. These later migrants replaced the descendants of the
earlier H. erectus migrants.
So far, DNA analyses have supported the "Out of Africa" ("Replacement")
hypothesis. But as any hypothesis, this one is subject to further
testing as new analytic methods become available.
What makes humans different from the other great apes?
- Brain size
Hominoids of 6 million years ago had brains of about 400 -
450cm2. This is about the same as modern chimpanzees. Modern
humans' average brain volume is 1300cm2. This threefold
increase in volume is associated with cultural trends such as development
of complex language
- Jaw shape
Anthropoids and ancient hominoids have prognathic jaws: the
upper and lower jaws protrude beyond the nose. Recall that the human
face is paedomorphic with respect to that of other modern apes. The
face is flatter, the prognathous jaws lost. Changes in dentition
accompanied this change in jaw shape.
- Bipedal posture
Ancestral anthropoids and some of the earliest hominoids walked on all
fours, though--like modern apes--they could probably walk on their hind
legs with some degree of balance and skill. Humans are different from
all other apes in that the body posture is fully upright and
locomotion is entirely bipedal. A number of skeletal and muscular
modifications make this possible, but there is still a great deal of
academic argument about why humans became bipedal.
- Reduced sexually dimorphic size differences
In orangutans and gorillas, the male weighs about twice as much as the
female. In chimpanzees and Bonobos, males weigh about 1.4 times as much
as females. In humans, the difference is still less, with males
averaging 1.2 the body weight of females. (Why do you suppose this is the
case? HINT: social structure is a major selective factor here.)
- Key changes in family and other social structures
- Gibbons are social, with a dominant male defending a group of
females from other male rivals.
- Orangutans are solitary and do not form permanent social groups
- Gorillas are social, with a single "silverback" dominant male
getting all the mating opportunities with the females in his band.
Immature and subordinate males are allowed to stay in the group, but
do not get many mating opportunities.
- Chimpanzees are social and promiscuous. When a female comes into
estrus, all males will attempt to mate with her, and she will mate
with multiple males.
- Bonobos will have sex with anyone who holds still long enough.
- Hunter-gatherer human societies may be polygamous or
polyandrous. But in most human societies, monogamy is the norm.
(But is it biologically programmed?)
- Young are even more altricial than other great apes' young.
Newborn humans are exceptionally dependent on their mothers, and parental
care lasts longer after birth than in other ape species. This extended
period, coupled with the enlarged brain, enhances learning and is one
factor that contributes to the behavioral complexity of the human
EXTRA CREDIT OPPORTUNITY! View "Walking with Beasts" video, and be prepared to answer Bonus Questions on the video on the final exam. Here are the links. Each episode is a little under 30 minutes long.
Episode One: New Dawn
Episode Two: Whale Killer
Episode Three: Land of Giants
Episode Four: Next of Kin
Episode Five: Sabre Tooth