Marine Mammal Origins

Spenadcodontian grade

• Interdeterminate growth (keep growing)

Therapside grade

• Fast and cyclical growth
• Independent evolution of sustained growth in several lineages

Eucynodont grade

• Reduced tooh replacement

Mammaliaform grade

• Diphyodonty (2 successive sets of teeth)
• Determinate growth (stops at predetermined time)
• Decrease in size

Mammalia grade - reduced developmental plasticity in growth patterns

Paleohistory and dental replacement

Documents gradual but convergent evolution of mammalian growth features

Synapsida separated from Reptilia ~320mya = mammals

Ecomorphology - study of relationship between ecological roles and morphological adaptations

Assess morphological disparity and dietry trends using analysis of lower molars

Previous hypothesis - mammalian diversity suppressed in Mesozoic and increased rapidly in K-Pg extinction event

Ecomorphological diversity of therians began 10-20my prior to K-Pg mass extinction

• Same time as radiation of multituberculate mammals and angiosperms
  • Mammals benefited from rise of angiosperms

Therian disparity decreased after K-Pg event - selective extinction againsts ecological speialists and metatherians

Taxonomic diversity increases after K-Pg = earliest extinction survivors underwent rapid taxonomic diversification but no morphological diversification

Major mammalian radiation = 66mya

Cetacea - earliest mammalian taxa return - Early Eocene

Sirenia - returned middle Eocene

Carnivora - not as well established as cetacea

• Pinnipedia - late Oligocene
• Enhydra (sea otter) - Late Piliocene
• Ursus - Pliestocene

Fossil record demonstates mammals returned to sea >7 times

Cetaceans 

• Originally grouped with Sirenians - similar morphology
• Artiodactlys (even-toed ungulates - hippos, deer, sheep, giraffes) closest living taxa
• Sperm whales and beaked whales earliest odonotocete group to branch off
• Few living mysticeti genera - form well-resolved clades

Sirenain and Desmostylia

• Sirenians part of larger mammalian group Tethytheria
  • Includes Sirenia, Proboscidea (elephants) and Desmostylia
• Proboscidea + Sirenia sister taxa - Desmostylia more distant relation
• Either common ancestor was:
  • Semiaquatic and Proboscidea secondarily fully terrestrial
  • Desmostylia and Sirenia invaded aquatic environment seperately

Marine carnivores

• Ursids and Enhydra recently derived from terrestrial clades
• Pinnipeds likely derived from basal Ursids - morphological and molecular evidence
• Some people support diphyletic origin for Pinnipeds 
  • Phocids = basal Mustelidae
  • Otarioids = basal Ursids

Reason for returning to sea:

• Eocene - high productivity in seas
• Oligocene - rapid glaciation caused concentrated areas of productivity around upwellings

Feeding ecology is key factor in evolution of marine mammals

Desmostylia - late Oligocene to late Miocene

• Fossil deposits in N. Pacific
• Bone analysis - unable to move on land
• Semi-aquatic
• Herbivores - flattened teeth
• No direct descendants of this taxon

Kolponomos - Miocene (20mya)

• Bear-like coastal carnivorans
• Lead to modern pinnipeds
• Shows multiple attempts to return to water by different taxa
• Shallow shores, similar life habit to polar bears

Thalassocnus - Miocene (20mya)

• Genus of marine sloths
• Showed bone changes seen in Sirenia = pachystotic and osteosclerotic bones
• Large and dense organism

Cope's rule - animal lineages evolve towards larger body sizes over time

Mean biovolume of marine genera increased by factor 150 since early Cambrian

Size increase reflects differential diversification across classes

• Not simple scaling-up for larger sizes within populations