Mammalogy Mammalian Origins

What is a mammal and why did they evolve? We have a number of problems: Ectothermy is very efficient. Reptiles experience an explosive adaptive radiation. How can we explain the emergence of the mammals. What is it about mammals that enabled them to persist?

Mammalian Characteristics Mammals are endothermic, at least to some extent (but then, so are some reptiles). Mammals have fur, presumably to increase the size of the boundary layer. However, the evolution of fur is problematic. Mammals nurse their young

Mammalian Characteristics Mammals posses a synapsid skull rather than a diapsid or anapsid skull. Mammals have a single bone in the lower jaw (Dentary), while reptiles have 7. Mammals have 3 inner ear bones rather than the reptilian number of 1.

Mammalian Origins What selective pressures might have produced the mammals? Mammal-like reptiles appeared before the end of the Permian, during the first amphibian adaptive radiation. This amphibian radiation was very different from the modern amphibians of today.

Mammalian Origins The vegetation, climate, and tectonic plate positions were very different as well. The mass extinction at the end of the Permian (95% of all species lost) might have resulted from the formation of Pangea, and concomitant climate/vegetation change. Some mammal-like reptiles survived the extinction event.

Mammalian Origins Following the end-Permian mass extinction, we have the emergence of the Dinosaurs. Pelycosaurs did not survive the mass extinction, but their descendants, the Therapsids persisted.

Pelycosaur (dimetrodon - Sphenacodontia) w/ reflected lamina of angular bone. and Therapsid.

Mammalian Origins Some Pelycosaurs had significant sails, and were also quite large. Implications? There is vascularization of the vertebral elements associated with the sails. Large size promotes inertial homeothermy.

Mammalian Origins 3 suborders of Pelycosaurs: Ophiacodontia - semiaquatic & piscivorous Edaphosauria - terrestrial herbivores Shenacodontia - carnivorous. Sphenacodontia was the parent group for the Therapsida

2 Therapsid suborders Anomodontia Herbivores 3 groups Dicynodontia (only group to last through Triassic) Dinocephalia Eotitanosuchia Theriodontia Carnivores 3 groups Gorgonopsia Therocephalia (secondary palate and complex cheek teeth. Cynodontia (secondary palate and complex cheek teeth).

Theriodont Cynodonts Secondary palates Complex cheek teeth AND, mammal-like origins and insertions of jaw musculature. Only Cynodonts survived into the Jurassic. This lineage gave rise to the Mammalia in the Triassic.

Mammal-like Cynodont characters. Secondary palate, Jaw musculature, complex cheek teeth. Tricuspid, double-rooted cheek teeth. Increased size of dentary and reduction of post-dentary bones. Development of glenoid fossa on the squamosal.

Early & late CynodontsPelycosaur & Therapsid

Cynodont characters Regionalization of the vertebral column. Atlas/axis complex. Modified pectoral and pelvic girdles. Restriction of ribs to thoracic vertebrae. Erect posture. Evoution/modification of masseter. Development of zygomatic arch.

Cynodont characters. Modification of neurocranium to relieve stress on jaw joint - increase bite force.

Once again, what is a mammal? A synapsid with a dentary - squamosal jaw articulation. (Reptiles have a quadrate - articular articulation). What happened to the quadrate and articular?

Mammalian Jaw Articulation Consider the tremendous implications of the new jaw articulation. What does it mean for hearing? What does it mean for bite force? We still see evidence for the role of the jaw in sound conduction in modern mammals (whales).

Again, what selective pressures led to mammals? We still do not understand the transition from heterothermy (ectothermy/heliothermy) to endothermy (heterothermy/homeothermy).

Evolution of endothermy What came first, high metabolism or insulation? If metabolism came first, then there would be significant heat loss to the environment, and endothermy would be truly expensive. If insulation came fisrt, Bartholomew has shown that they would never be able to heat up via conduction or convection.

Polyphyletic or monophyletic? There are 2 major lineages of extant mammals: Prototherians and Therians. They share some typical mammalian characters (single dentary, fur, endothermy) but differ markedly in others (teeth, girdles, urinogenital ducts, and development.

Polyphyletic or monophyletic? If mammals are polyphyletic, then the defining characters for mammals evolved twice independently. If mammals are monophyletic, then prototheres are clearly very unique. Unfortunately, the fossil evidence is very fragmentary, and monotremes have no teeth as adults.

Early Mesozoic Mammals So, modern mammals have a dentary/squamosal jaw articulation. They are diphyodont, rather than the reptilian condition. Mesozoic mammals were mouse-size, they are relatively rare in the fossil record, and they were probably nocturnal. WHY?

Mesozoic mammals Prototheria Triconodonta Morganucodontidae Amphilestidae Docodonta Docodontidae Allotheria Multituberculata

Mesozoic Mammals Morganucodonts (Triconodonta) Dentary / squamosal w/ some involvement of the quadrate and articular. Heterodont w/ premolars and molars. Large cochlear region 2 cervical vertebrae 2 occipital condyles. Mammalian vertebrae, girdles, and posture

Mesozoic Mammals Triconodonts were carnivorous. Molars had 3 cusps arranged in a single row.

Mesozoic Mammals Triconodont Amphilestids Teeth very much like morganucodonts Deciduous molars.

Mesozoic Mammals Docodonta may be derivatives of Triconodonts. Known only from teeth and jaws. Retained reptilian jaw articulation. Complex molars - omnivores.

Mesozoic Mammals Multituberculata Radiation coincides w/ angiosperms. Herbivores w/ single pair of procumbent lower incisors. Molariform teeth w/ as many as 8 conical cusps.

Mesozoic Mammals Multituberculata For anterior molars, cusps arranged in triangular patterns. Posterior molars had cusps arranged in rows. Lower posterior molars were large & used for shearing (as in some Burramyids).

Early Therians These are the ‘Pantotheres’. There are 2 orders: Symmetrodonta Kuehnoetheriidae Eupantotheria Dryolestidae Peramuridae

Early Therians Symmetrodont Kuehnoetheriids were small carnivores and insectivores in the Triassic, but radiated during the Cretaceous. Molars has 3 main cusps in a triangular arrangement. This is the precursor of the tribosphenic tooth of modern mammals.

Tribosphenic Teeth It is triangular. This upper molar is the ‘Trigon’ and shows the Protocone to be medial, and the Paracone to be lingual and anterior. The ‘conules’ are smaller cones.

Tribosphenic Teeth The lower molar is the ‘Trigonid’ - the id suffix denotes the dentary. Here, the Protoconid is lateral, not labial. The tooth also has a ‘Talonid’ or heel.

Tribosphenic Teeth When upper and lower molars occlude, the Protocone meets the Talonid of the lower molar. Thus, food is both crushed and sheared by the cones and conids.

Tribosphenic Teeth The tritubercular teeth are the basis for all modern mammalian teeth, and can still be seen in insectivores and marsupials. Humans (and many other mammals) have quadritubercular teeth through the addition of a hypocone posterior to the protocone.

Tribosphenic Teeth Things get really weird when we look at the teeth of rodents and a nubmer of other groups. However, these patterns are highly derived, and homologies can be established.

Cenozoic Radiation of Mammals Major radiation of mammals occurs after the end-Cretaceous mass extinction. Did an asteroid wipe out the Dinosaurs? Did continental drift and climate change wipe out the Dinosaurs?

Cenozoic Radiation Most modern mammal orders are extant by the Eocene, and most modern families are there by the Miocene. Clearly, the mammalian adaptive radiation has just gotten underway.

Didelphidae Chironectes minimus

Didelphidae Chironectes minimus: HF

Didelphidae Didelphis aurita

Didelphidae Lutreolina crassicaudata

Diprotodontia: Petrogale brachyotis

Acrobates pygmaeus

Petrogale brachyotis

Fisher: Martes pennanti

Hyaena paraoccipitals

Hyaena upper tooth row

Hyaenidae: ardwolf

Hyaenidae: ardwolf ltr

Hyaenidae: ardwolf skull

Hyaena: lack of allisphenoid

Otariidae: Stellar sea lion

Otariidae: Zalophis californicus

Otariidae Zalophis californicus allisphenoid

Zalophis californicus UTR