Vertebrate Evolution - Bird Evolution

Bird Evolution: Solnhofen Site and Beyond

Introduction

  • Lecture focuses on bird evolution and the significance of the Solnhofen site for exceptional preservation.
  • Background reading: Benton chapter 9.

General Bird Characteristics

  • Global distribution across land and sea with approximately 10,000 species.
  • Include flying and secondarily flightless forms.
  • All birds lay eggs and are covered by feathers, classifying them as tetrapods that returned to the air.

Skeletal Adaptations for Flight

  • Birds exhibit considerable skeletal modification for flight, including:
    • Pneumatic bones: Hollow, air-filled bones.
    • Pygostyle: Reduced tail.
    • Fusion and reduction of bones in the arm and leg.
    • Four toes: Typically three or two forward and one or two back.
    • Large keeled sternum: Provides attachment for flight muscles.
    • Furcula (wishbone): acts as a spring between the shoulders during flight.
    • Swiveling wrists: Enable wings to be folded against the body, demonstrated by a pigeon's morphology.

Archaeopteryx: The First Bird?

  • Ten specimens and one feather found in Solnhofen limestone, Germany, dating back to the late Jurassic period.
  • Archaeopteryx is considered a crucial transitional fossil.
  • Eight Archaeopteryx specimens, including the well-known Berlin specimen, have been discovered.

Solnhofen Lagoons: A Unique Palaeoenvironment

  • Late Jurassic lagoons in Solnhofen, Germany, offer insights into a unique palaeoenvironment.
  • Hypersaline lagoons were located on the northern side of the Tethys Ocean, protected by coral reefs, and fed by river systems draining uplands to the north.
  • The lagoon bottom water was hypersaline and anoxic, creating conditions unfavorable for indigenous fauna, therefore promoting preservation.

Exceptional Preservation in Solnhofen

  • The unique conditions in Solnhofen lagoons led to exceptional preservation of organisms.

Solnhofen Vertebrate Assemblage

  • The vertebrate fossils found in Solnhofen represent a mixture of terrestrial animals from streams to the north and marine animals from the ocean to the south:
    • Chondrichthyans (14 genera)
    • Osteichthyans: Actinopterygians (1 chondrostean, 30 holosteans, 8 teleosts); lobe-fins (1 coelacanth)
    • 4 turtles
    • 1 ichthyosaur
    • 1 plesiosaur
    • Lepidosaurs (5 lizards, 5 sphenodonts)
    • 7 crocodiles
    • 1 theropod dinosaur
    • 7 pterosaurs
    • Archaeopteryx

Archaeopteryx Morphology

  • Key morphological features of Archaeopteryx:
    • Long bony tail; S-shaped neck; short back
    • Hand with 3 clawed fingers
    • Separate tibia, fibula, metatarsals
    • Five toes
    • Downward-pointing pubis
    • Furcula
    • Pneumatic vertebrae and pubis
    • Feathers (asymmetrical)
    • Wings
    • Toothed jaws with no serrations; interdental plates present (seen in 1 mm SEM images)

Avian Evolution and Phylogeny

  • The cladogram illustrates the origin of birds within Paraves (birds, troodontids, dromaeosaurids).
  • Key groups and their relationships:
    • Saurischia: Includes Theropoda, Sauropodomorpha, and Sauropoda.
    • Theropoda: Further divided into Ceratosauria, Tetanurae, and Coelurosauria.
    • Paraves: Contains Troodontidae, Dromaeosauridae, and Aves/Avialae.

Diversity in Paraves

  • Examples of early paravians:
    • Early Cretaceous dromaeosaurid Sinornithosaurus.
    • Late Jurassic troodontid Anchiornis.
    • Mid to Late Jurassic paravian Epidexipteryx.

Evolution of Bird Flight - Feathers

  • Feathers play a crucial role in flight, insulation, and display in modern birds.
  • Variety of feather morphologies are present in saurischian and ornithischian dinosaurs.

Evolution of Bird Flight - Hypotheses

  • Two main hypotheses for the evolution of bird flight:
    • 'From the ground up' (cursorial): Suggests flight evolved from running along the ground. No clear modern analogue.
    • 'From the trees down' (arboreal): Suggests flight evolved from gliding and then flapping. Analogues include flying squirrels, lizards, frogs, and snakes.
  • Arboreal hypothesis seems more intuitive.
  • Evidence from Jehol Microraptor: a small paravian with asymmetrical feathers on both arms and legs; bird-like feet

Cursorial Evidence

  • Findings suggest flapping precedes gliding and allows more efficient climbing.

Cretaceous Avian Evolution

  • Illustrates the timeline and evolution of various bird groups during the Jurassic and Cretaceous periods.
  • Key Avialae and Pygostylia representatives, including Archaeopteryx, Jeholornis, and Sapeornis.

Cretaceous Birds - Jeholornis and Sapeornis

  • Jeholornis and Sapeornis are significant Cretaceous birds that provide insights into avian evolution.

Cretaceous Birds - Confuciusornis

  • Highlights Confuciusornis, an early bird with notable features.

Confuciusornis Characteristics

  • Key features of Confuciusornis:
    • Large clawed hands
    • Beak with no teeth
    • Pygostyle
    • Sternum with small keel
    • Sexual dimorphism

Late Cretaceous US Birds

  • Focuses on bird fossils found in the United States from the Late Cretaceous period.

Late Cretaceous US Birds - Hesperornis and Ichthyornis

  • Two taxa from shallow marine sediments of the Western Interior Basin, US
    • Hesperornis: long beak with many teeth; flightless with reduced sternum and vestigial wing bones.
    • Ichthyornis: flying toothed shorebird.

Bird Evolution in the Cretaceous

  • Overall evolutionary patterns from the Early to Late Cretaceous:
    • Claws on wings lost
    • Finger number reduction
    • Teeth lost
    • Reduction and fusion of tail bones into a pygostyle
    • Fusion of leg bones into a tibiotarsus and a tarsometatarsus
    • Evolution of alula or bastard wing for better flight control at low speeds
  • Many groups became extinct during the K-Pg event.

Modern Birds

  • Two main groups:
    • Neognaths: most modern birds.
    • Palaeognaths: tinamous and ratites (ostrich, emu, cassowary, rhea, kiwi, moas).
  • Palaeognath features:
    • Symmetrical feathers on wings
    • All found in Southern Hemisphere today
    • May have originated in Northern Hemisphere in Paleocene or Eocene

Neognath Evolution

  • Patchy fossil record, like most birds.
  • Earliest fossils from the latest Cretaceous; post K-Pg radiation.
  • Molecular evidence suggests earlier Cretaceous origin.

Eocene Birds from Messel, Germany

  • Palaeoenvironment: stagnant lake deposits surrounded by tropical forest.
  • Bird fauna includes ratites and a variety of neognaths including phorusrhacids, Gastornis, owls, swifts; no passerines.

Next Lecture

  • The evolution of mammals.