Mammalogy, lecture 2

Evolution

Reading: Chapter 4, pp. 42-59

I. Evolution from reptiles

A. Took place during the late Paleozoic to early Mesozoic era, ~ 100 million year span

B. This lecture will cover the evolutionary morphological changes from reptilian to mammalian structural traits

1. This includes the emergence of mammals from synapsid reptiles

2. We will also cover the changes in mammals throughout the Mesozoic era

3. We will also cover the adaptive radiation (define?) of mammals beginning in the early Cenozoic

4. We will also describe the dental characteristics of early mammals and how the teeth developed into modern mammalian dentition

a. Why teeth?

i. Because they are hard and fossilize very well; they can therefore be used to reconstruct phylogenetic (define?) history

II. Origin of mammals from synapsid reptiles

A. Four early subclasses of reptiles distinguished by the number, size, and position of lateral temporal openings (fossae, sing. fossa) on the skull; see fig. 4.1, p. 43

1. Synapsida (become mammals)

2. Anapsida (become modern turtles)

3. Euryapsida

4. Diapsida (become many modern reptiles)

B. We’ll concentrate on synapsid reptiles for now

1. They appear in the fossil record ~320 mya, in North America (see Table 4.1, p. 44, for a timetable of events)

2. They were dominant land animals for about 70 million yrs, but were superseded by dinosaurs

3. However, they gave rise to two orders

a. Pelycosauria, the more primitive order

b. Therapsida, the more derived order (define primitive and derived?)

i. It is the therapsids that are known as the "mammal-like reptiles"

III. Pelycosaurs and therapsids

A. Pelycosaurs were common by end of Pennsylvanian epoch (see fig. 4.2, p. 45, for a history diagram with abundance indicated on diagram); by the end of the Pennsylvanian, they had radiated into three suborders

1. Ophiacodontia; primitive, semiaquatic, ate fish

2. Edaphosauria; herbivorous and terrestrial

3. Sphenacodontia; carnivorous and terrestrial

a. These were the dominant carnivores throughout early Permian

b. This is the suborder that gave rise to the therapsids

c. By the middle to late Permian, pelycosaurs had been replaced by the therapsids

i. The therapsids were smaller than pelycosaurs and generally more mammal-like

B. Therapsids date from the late Permian and are divided into two suborders

1. Anomodontia: One lineage of anomodonts became the dominant terrestrial herbivores for some 60 million yrs, but the last of the anomodonts were extinct by the late Triassic

2. Theriodontia: Primarily carnivorous, more diverse than anomodonts, and much more successful; there are many lineages recognized in the fossil record

a. One of these lineages, the Cynodontia, were the dominant theriodonts in the Triassic, and possessed specialized cranial and skeletal features that led to the evolution of mammals (remember, we are still discussing reptiles)

IV. Cynodontia

A. Existed for 70 million yrs, throughout Triassic and into the middle Jurassic

1. Several cynodont features developed to the point seen in mammals, including:

a. A change in dentition to tricuspid (cusps are projections on the occlusal (chewing) surface of a tooth)

b. Double-rooted cheekteeth

c. Increased dentary (lower jaw) and masseter (muscle on lower jaw) size

d. Changes in structures associated with hearing

e. Mammal-like differentiation of some vertebrae

f. Modification of pectoral/pelvic girdles

g. Modification of thoracic ribs

h. Changes in phalangeal formula (define phalanx, the formula refers to number of bones in each phalanx)

2. General changes

a. More erect posture and different type of movement (limbs rotate more under body)

b. Changes in jaw bones and musculature that reduced stress on the joint, increased bite force, and protected the brain

B. So, when does a cynodont become a mammal?

1. Reptiles have several bones in the jaw

a. The articulation of the jaw with the cranium is formed by the quadrate and articular bones (should remember this from biol 152)

b. In cynodonts, the quadrate and articular bones reduce in size, but still form this articulation

c. In mammals, the quadrate and articular have become reduced in size, and have migrated toward the ear cavity, becoming ear ossicles (the small bones in the ear)

i. Basically, the quadrate and articular bones became, over evolutionary time, part of the ear complex (the malleus and incus)

ii. Reptiles have only one ear ossicle, the stapes

2. Mammals have a single bone in the lower jaw, since the other "old" reptile bones have become incorporated into the ear

a. The dentary bone (lower jaw) articulates directly with the squamosal bone (part of the braincase)

3. Mammals also have diphyodont dentition

a. This means that they have only two sets of teeth in their lifetime; "baby" teeth and "adult" teeth

b. Other animals shed their teeth periodically and replace them throughout life

C. Cynodonts became extinct in the mid-Jurassic

1. Several well-defined groups of mammals existed by this time

2. Certainly many other structural changes must have occurred as mammals evolved from cynodont stock, but only the structure of bones is preserved in the fossil record

V. Are mammals monophyletic or polyphyletic?

A. Mammals may have evolved from a single lineage of therapsid reptiles, in which case they are monophyletic

B. Mammals may have evolved from two or more lineages of therapsids, in which case they are polyphyletic

C. There is some controversy over this point, and we simply don’t know

D. In any case, mammals first appeared in the mid-Mesozoic, ~200 mya, during the age of dinosaurs

VI. The first mammals

A. Mammals of the early Mesozoic

1. In the Mesozoic ("age of dinosaurs"), mammals were a minor component of the fauna

a. Fauna instead was dominated by reptiles

2. Mammals only became more abundant in the early Cenozoic, after extinction of most dinosaurs

3. In the Mesozoic, mammals were small (mouse-sized), uncommon, and most likely nocturnal

a. They stayed small until after dinosaur extinction

4. In Mesozoic, mammals radiated into two major groups, the prototherians and therians

a. It is not known how these two groups are related (and they may be "unnatural" groupings), but we continue to use these terms while mammalian systematists work on the problem

B. Prototherians (follow along using fig. 4.8, p. 49)

1. The earliest mammals are in the family Morganucodontidae, genus Morganucodon, and they are abundant in the fossil record

a. These may have been related to modern prototherians (monotremes, Echidna and platypus), but we don’t know for sure

2. At any rate, morganucodontids have several typical mammalian characteristics

a. Dentary-squamosal articulation, but with the involvement of the quadrate and articular bones (basically, this means that the migration of these bones to the inner ear is incomplete in these fossils)

b. Cheekteeth differentiated into incisors, canines, premolars, and molars

c. Occlusal surfaces were mammalian

d. First two vertebrae were somewhat differentiated into atlas and axis, as modern mammals are

e. Thoracic and lumbar vertebrae present, pelvis not reptilian

f. Had mammalian posture (i.e., stood more upright than reptiles)

a. Like morganucodontids, triconodonts were small and carnivorous mammals

4. Another successful order was the Multituberculata

a. These survived 120 million yrs, from late Jurassic to the late Eocene

i. This was concurrent with the evolution of modern flowering plants

b. Primarily herbivorous, with two enlarged incisors similar to modern rodents

c. Order is named for the 8-cusped molars

d. These also went extinct, probably outcompeted by rodents, primates, and various eutherian herbivores

C. Early Therians (i.e., marsupials and placental mammals)

1. Early therians were concurrent with the morganucodontids; two orders are known

a. Symmetrodonta

i. These were probably carnivores and insectivores

b. Eupantotheria

i. The advanced therians (metatherians (marsupials) and eutherians (placentals)) evolved from this order

ii. They probably originated within the family Peramuridae by middle to late Cretaceous, known only from the genus Peramus

2. The most significant feature of early therians was that they had more advanced tribosphenic molars

a. These have three large cusps arranged in a triangular pattern

i. These are depicted in fig. 4.10, p. 51

ii. The main thing to remember about these is the fact that they are more efficient in shearing and crushing food

b. This basic pattern is seen today in the marsupials and insectivores, and has been modified in other modern mammals

i. In humans they are modified into 4-cusped molars, which are quadritubercular

VII. Cenozoic mammals and mammalian radiation in the early Cenozoic

A. Two major events probably caused this radiation

1. Extinction of the dinosaurs (except crocodiles and birds, the only living descendents) at end of the Cretaceous

a. This obviously removed a major competing group of animals

2. Breakup of Pangaea, which was the single large continental landmass present throughout much of the age of dinosaurs (see fig. 4.11, p. 52)

a. This always allows differentiation of many lineages on many different landmasses, since they evolve in isolation from each other

3. There has also been a general increase in faunal and floral diversity throughout evolutionary history, allowing many niches to be filled

4. For these reasons, mammals have been dominant for 65 million yrs

a. Most extant orders known from the early Eocene (65 mya)

b. Most extant families from before the Miocene, ~ 37 mya

B. Fossils and increased metabolism (endothermy)

1. Important evolutionary changes occurred in association with metabolism, physiology, and reproduction

a. These are all associated with the evolution of endothermy

2. Obviously, soft features do not fossilize

a. However, the bones do, and these changes can be inferred from the bones

3. Endothermy requires at least 10 times the amount of food and oxygen that is required for an ectotherm of a similar size

a. Therefore, there are changes in structure including bones

b. Most of these changes from reptilian to mammalian characteristics are related to efficient homeostasis

VIII. Summation of anatomical trends from mammal-like reptiles to mammals (these are not necessarily observed in some orderly fashion, but are general trends)

A. Temporal opening of therapsids enlarged

1. Associated with the evolution of stronger jaw musculature

B. Evolution of larger, heavier dentary is associated with the stronger jaw musculature

1. Remember that this went along with development of the dentary-squamosal articulation and movement of articular and quadrate bones to inner ear

C. Maxillary and palatine bones extend posteriorly and medially, forming a secondary palate (see fig. 4.14, p. 55)

1. This forms a secondary palate, through which air flows (if breathing through nose) to lungs

a. Creates much more efficient air flow to lungs than in mammals

D. Dentition changes from homodont (teeth all same) to heterodont (teeth differentiated in form and function)

1. Increases efficiency of obtaining and processing food

E. Evolution of two occipital condyles

1. This allows finer control of head movements

F. Limbs rotate from the splayed reptile stance to limbs directly beneath the body

1. Mammals move with less energy expenditure than reptiles

G. Changes in vertebral column associated with flexibility, again allowing increased movement with less energy expenditure

H. Number of carpal and tarsal bones reduced, again increasing efficiency in movement by reducing the "swing" in movement

IX. What defines a mammal

A. Presence of the dentary-squamosal articulation

1. This was first seen in the fossil record 200 mya

X. Characteristics of modern mammals

A. Presence of hair or fur on some or all of the body

B. Mammals have a 4-chambered heart with a left (not right) aortic arch

C. Presence of erythrocytes that are enucleate

D. Female mammals have milk-producing mammary glands

E. Mammals have a muscular diaphragm that separates the thoracic and abdominal cavities (explain in more detail)