Ornithology Lecture Notes

Evolution & Ornithology

Intro to Phylogenies

  • Phylogenies are trees that show hypothesized evolutionary relationships.

  • They illustrate ancestor-descendant relationships at various taxonomic levels: populations, species, and larger taxa.

  • Phylogenies are constructed using molecular, morphological, and/or behavioral data.

  • Alternative names: phylogenetic trees, cladograms, or simply "trees."

Key Phylogeny Terms:

  • Root: The common ancestor (CA) at the base of the tree.

  • Nodes: Represent ancestors at speciation events in the past.

  • Tips/Terminals: Represent the most recent taxa in the present.

  • Transitions:

    • Derived Characters: Newly evolved traits.

    • Evolutionary Reversals: Loss of traits or reduction in complexity.

  • Shared Transitions & Conserved Traits: Help determine relationships between taxa.

  • Outgroup: The earliest divergent taxon in the tree, sharing a common ancestor with the ingroup.

  • Ingroup: The taxa of focus, sharing a more recent common ancestor than the outgroup-ingroup ancestor.

  • Sister Taxa: Tips/terminals sharing an immediate common ancestor not shared by other tips/terminals.

  • Polytomy: A split of three or more lineages, indicating uncertainty in relationships.

Phylogeny Representation:

  • Phylogenies can be shown in multiple formats without altering the relationships.

  • Terminals can rotate on branches without changing relationships.

  • Removing taxa changes the relationships depicted.

Assembling Phylogenies via Parsimony:

  • Parsimony assumes that the tree with the fewest transitions is the most logical and reflects reality.

  • Example: Three species A, B, and C with ancestral characters (0,0,0) and derived characters (c, h, w).

Why phylogenies are hypotheses/estimates:

  • We are never 100% sure if we have all the data, and cannot observe the past.

  • Confounding factors:

    • Missing species (extinct and/or undiscovered).

    • Lack of characters.

    • Unique characters.

    • The common ancestor is hypothetical.

    • Homoplasies (similarities not due to shared ancestry).

Homoplasies:

  • Convergence: Similarities due to similar selection pressures, not shared ancestry.

    • Example: Endothermy in birds and mammals.

  • Reversal: Loss of a derived trait, resembling ancestral taxa.

    • Example: Flightlessness in ostriches and penguins.

  • Atavism: Reappearance of an ancestral trait after loss, usually due to mutations.

    • Example: Hoatzin's two-clawed wing vs. Archaeopteryx's three-clawed wing.

Phylogeny Terms:

  • Monophyly: A group with an ancestor and all its descendants.

  • Paraphyly: A group with an ancestor and some, but not all, of its descendants.

  • Polyphyly: A group with some, but not all, of its descendants, and no common ancestor.

Birds as Reptiles

  • Birds are vertebrates.

  • Birds are amniotes.

  • Birds are avian reptiles.

Reptilian Characteristics Shared by Birds:

  • Life Cycle: Direct development (no larval stage).

  • Amniotic Eggs:

    • Amniotic membranes for gas exchange, waste storage, and feeding.

    • Larger at hatching with longer incubation due to more yolk.

    • Calcareous-shelled eggs resist desiccation.

  • Integument:

    • Epidermal scales or scutes.

    • True claws.

    • Few integumentary glands.

    • Resistant to fluid, solute, and gas exchange.

  • Ecto/Poikilothermic (many are "Heliothermic").

  • 12 pairs of cranial nerves.

  • Ribs encircle the body cavity.

  • Internal fertilization.

Reptiles vs. Other Reptiles (Birds and Mammals):

  • Reptiles are transitional and ancestral to birds and mammals.

  • Identifying unique reptilian characters can be difficult.

  • Birds: endothermic, feathered, bipedal with forelimb wings and pneumatic bones.

  • Mammals: endothermic with mammary glands and hair; more glandular skin; mostly give birth to live young.

  • Reptiliomorphs (ancestral to amniotes) diverged from amphibians ≈355 mya.

  • Sauropsid reptiles and synapsid reptiles (ancestral to mammals) diverged ≈320 mya.

  • Amniotes = Synapsids + Sauropsids

Early Evolution History of Reptiles

  • Archosauromorphs and Lepidosauromorphs derived from early reptiles and diverged ≈280 mya.

  • Extant Lepidosaurs include lizards, snakes, amphisbaenians, and tuataras.

  • Archosaurs and turtle ancestors derived from early Archosauromorphs and diverged ≈254 mya.

  • Archosaurs = The “Ruling Reptiles”

Birds as Dinosaurs

Hypotheses for Bird Divergence:

  • A: Birds diverged from Theropods ≈150 mya (late Jurassic).

  • B: Birds diverged from stem Thecodonts ≈230 mya (mid Triassic).

    • Stem Thecodonts = Ancestral Archosaurs (ancestors of dinosaurs, crocodilians, pterosaurs, etc.)

Archaeopteryx:

  • Discovered in 1861 (Jurassic period ≈150 mya).

  • First feathered dinosaur fossil discovered.

  • Shows clear link between Theropod Dinosaurs and Birds.

  • Provided major support for Darwin’s Descent with Modification.

  • Considered the first known bird, but not a “true bird” or a direct ancestor of modern birds.

  • Baminornis zhenghensis: Discovered 2023; Described 2025!

Dinosaur Phylogeny:

  • Historically, 3 major clades: Ornithischians sister to Saurischians, and Sauropods sister to Theropods.

  • Based mostly on hip structure.

  • Recent phylogenetic analysis finds all three arrangements statistically indistinguishable.

  • Despite the debate, birds are very much “settled in” Theropoda.

Derived Traits Inherited from Non-Avian Dinosaurs:

  • Lower body: Upright posture, hip structure, foot structure.

  • Reproduction: Hard-shelled amniotic eggs, egg brooding.

  • Flight Facilitation: Feathers, forelimb wing structure, furcula, hollow pneumatic bones, and possibly air sacs.

Traits Derived in Birds:

  • Axial Skeleton Modifications:

    • Toothless beaks

    • Loss of gastralia

    • Keeled sternum

    • Sternal ribs

    • Uncinate processes

    • Synsacrum

    • Reduced caudal vertebrae and pygostyle

Important Muscles of Respiration Associated with Vertebral Ribs:

  1. Intercostal Muscles

    • Inhalation: contract to expand rib cage (pull rib cage upward and outward)

    • Exhalation: Relax to contract rib cage

  2. Appendicocostal Muscles

    • Inhalation: contract to rotate vertebral ribs cranially via uncinate processes and sternum ventrally via sternal ribs Exhalation: relax

  3. External Obliques

    • Inhalation: relax Exhalation: use uncinate process as brace while contracting to help reset action of appendicocostals; moves sternum dorsally

  • Appendicular Skeleton Modifications:

    • Reduced digits 1-3

    • Alula

    • Pubis angled parallel to ischium

    • Tibiotarsus

    • Reduced fibula

    • Tarsometatarsus

Adaptive Radiation of Birds

Adaptive Radiation of Dinosaurs
Adaptive Radiation of Bird-Like Dinosaurs and Birds

Dromaeosauria:

  • Non-Avian Theropod Outgroup, commonly called "raptors" (≈168-65 mya).

"Long-Tailed” Bird-Like Theropods

  • First known "birds", but not "true birds" nor direct ancestors of modern birds.

  • Lineages may represent the first and second outgroups to “true birds."

  • Derived Traits: unserrated teeth; fully reversed hallux, reduced caudal vertebrae

  • Examples: Archaeopteryx (≈ 155-150 mya) and Jeholornis(≈ 130-120 mya)

"True Bird” Lineages

  • Baminornis (?-150-? Mya)

Confuciusornis

  • Lineage may represent first outgroup to Ornithurines (more modern birds)

  • DT: no teeth; carpometacarpus; sternal ribs; posteroventral pubis; short tail w/ pygostyle

  • However, 3 wing digits/claws and gastralia still present; no keeled sternum

Enantiornithines

  • e.g., Sinornis

  • One of two major, highly-diverse (species & ecologies) bird sister groups during Cretaceous

  • Derived Traits: weakly keeled sternum, alula, and 2 wing claws (not 3)

  • Very similar appearance to sister Ornithurines, but still had teeth, gastralia, and wing claws

    • Died out 65 mya; survived by sister Ornithurines

Hesperornithines & Icthyornithines

  • Two aquatic, cormorant-like lineages that existed during Cretaceous

  • Derived Traits: uncinate processes and no wing claws; Hesperornithines had reduced traits

  • However, still had teeth.

  • Died out 65 mya (possibly earlier); survived by Neornithines(AKA modern Avians/Birds)

Adaptive Radiation of Birds:

  • Birds may have diverged as early as ≈ 172-164 mya (mid Jurassic) and radiated throughout Jurassic.

  • Birds diversified even more during Cretaceous, way before Cretaceous extinction!

Extant Birds:

  • Two extant infraclasses possibly diverged as early as ≈ 120-110 mya (≈ early Cretaceous) or as late as ≈ 95-90 mya (≈ mid-late Cretaceous).

    • Palaeognathae (Ratites: ≈ 0.5% of extant species)

    • Neognathae (All other extant birds: ≈ 95.5% of extant species)

  • Galloanserae and Neoaves possibly diverged as early as ≈ 105-100 mya (mid-late Cretaceous) or as late as 85 mya (late Creataceous).

    • Galloanserae (All land and water fowl: ≈ 4.5% of extant species)

    • Neoaves (All other extant birds: ≈ 95% of extant species)

Key Points:

  • Modern birds were contemporaries of non-avian dinosaurs!

  • No “March of Progress”, just adaptive radiation of dinosaur groups!

  • Birds are dinosaurs!

  • Modern birds survived Cretaceous Extinction!

  • Diversified rapidly after Cretaceous Extinction and even more throughout Cenozoic!

  • Dinosaurs are not extinct; they’re still perhaps the most successful group of tetrapods ever! We just call them birds now…