Dinosaur Family Tree: Part 2 Notes

Overview of Part 2: Dinosaur family tree

• Focus of Part 2: shift from discoveries to dinosaur groups, their evolution, key adaptations, and significance. Use synapomorphies, biogenetic trees, and trace lineage from basal archosauriforms to modern birds.
• Preceding context: recap of archosauriformes, Euphorcaria (Euparkeria) and adaptations that gave a rise to a more capable, crown-group archosauriform during the Promontriassic mass extinction.
• Early archosauriforms (late Permian) such as Marcusauropilamines (the transcript’s spelling) appear and show high survival through the Permian–Triassic extinction. They are united by a set of apomorphies (synapomorphies within the group) including:
  • Antorbital fenestra in the skull (opening in front of the orbit).
  • Mandibular fenestra in the lower jaw.
  • Serrated teeth set in deep sockets (indicating a meat-eating diet and stronger bite).
  • Hindfoot: the longest digit becomes the middle digit, yielding a foot symmetry around the third digit.
• Functional significance of synapomorphies:
  • Mandibular fenestra and deep-tooth sockets relate to jaw muscle attachment/strength for prey capture and processing meat.
  • Antorbital fenestra likely linked to air sacs and respiratory/weight considerations.
  • Symmetrical hindfoot around the third digit ties to a more upright posture and locomotion improvements.

Locomotion and posture: from water to land

• All tetrapods trace ancestry to aquatic vertebrates; early life in water required different locomotion strategies.
• In water, axial muscles along the body contract alternately, producing an S-shaped undulation for propulsion (e.g., shark-like axial propulsion).
• This axial-driven locomotion was inherited by early tetrapods and mostly retained in living amniotes, with exceptions like archosauriforms.
• Basal tetrapods and many non-archosauriforms had a sprawling posture: limbs project laterally, the fourth hind digit is typically the longest, and locomotion is tied to axial muscle work.
• The sprawling posture couples limb movement with breathing in a constraint (breathing via rib cage expansion uses axial muscles, which is problematic for simultaneous walking and breathing in sprawling forms).
• In archosauriformes, the foot becomes more symmetrical and the hip/posture allows bearing weight with limbs under the body, enabling breathing and walking simultaneously (more endurance for pursuits).
• Crocodilians (modern archosauriforms) demonstrate multiple locomotor modes:
  • High walk: limbs pulled under the body for an upright posture.
  • Belly crawl: sprawling limb posture closer to the ground.
  • Gallop: some species can achieve a true bounding/galloping gait with all four feet off the ground briefly.
• The transition to upright posture reduces reliance on axial muscles for locomotion and allows better endurance in long-tail hunting.
• Trackways support these interpretations: archosauriform trackways show forelimbs and hindlimbs more aligned and closely to midline than sprawling forms, indicating an upright posture.

Key definitions (zoological vs anthropological terms)

• Biped: walks on two legs.
• Quadruped: walks on four legs.
• Sprawling posture: limbs project to the sides; weight-bearing and locomotion rely heavily on axial muscles.
• Upright posture: limbs held underneath the body; weight is supported by the limbs with reduced axial muscle load for locomotion.
• Anthropological vs zoological terminology differences: human bipeds can be upright while some quadrupeds (e.g., dogs) can be upright; some animals can be sprawling bipeds (e.g., Basilisk lizards) or partially bipedal while sprawling.
• Jesus Christ lizards (basilisks): examples of rapid flat-running with a combination of postures; used as a contrast example for understanding locomotor versatility.

Ancestral archosauriforms: baseline body plan and size trajectory

• Ancestral archosauriforms exhibit:
  • A more powerful bite, upright posture, and the potential to move using hind limbs underneath the body.
  • Limb proportions suggesting hind limbs longer than forelimbs (potential for repeated bipedalism at times).
  • Increasing body size over time, with early forms like ourosauriforms (Parkaria in the transcript) remaining small but quickly evolving toward > $2 ext{ m}$ in length; note that a typical adult human is about $1.8 ext{ m}$ tall.
• These features form the basic body plan that crown-group archosaurs will tweak but keep core: meat-eating predators with a more efficient upright gait.
• This baseline plan is the starting point for archosaur evolution toward Archosaurs (Archosauria), which will be revisited in later sections.

What makes an archosaur an archosaur? Crown group concepts

• Crown group definition: a monophyletic group defined by its living members and their last common ancestor.
• Crown group Archosauria consists of the last common ancestor of crocodiles and birds and all of its descendants.
• Within crown Archosauria, two major lineages emerge:
  • Crocodylomorpha (leading to crocodiles)
  • Avemetatarsalia (leading to birds) which includes Ornithodira (the sister taxon to the lineage leading to crocodilians) and splits into Pterosauria and Dinosauria (Dinosauromorpha is the pre-dinosaur sister group within Ornithodira).
• To illustrate the branching: crown Archosauria -> two main branches Crocodylomorpha and Avemetatarsalia; within Avemetatarsalia, Ornithodira splits into Pterosauria and Dinosauromorpha; Dinosauromorpha then gives rise to Dinosauria (which includes Ornithischia and Saurischia) and their descendants, including birds.
• Note on terminology in the transcript: terms such as Corrutarci and Ranitadira appear (likely typos for Crocodylomorpha and Avemetatarsalia/Ornithodira). The key point remains: Ornithodira includes Pterosauria and Dinosaurs; Dinosauria splits into Ornithischia and Saurischia.

Three key synapomorphies defining Archosauria (as discussed in the chapter)

• Four-chambered heart: complete separation of oxygenated and deoxygenated blood by a left ventricle and a right ventricle, providing more efficient systemic oxygenation. Note: most non-archosauriform archosaurs evolved a three-chambered heart with partial septation; archosaurs developed the four-chambered heart independently.

• Supracetabular crest: a bony ridge above the acetabulum (hip socket) that braces the femur as weight shifts to an upright gait. This feature supports a more erect hip posture and stable leg articulation in weight-bearing locomotion.

• Complex nesting and parental care: building nests and protecting or brooding young, a synapomorphy in Archosauria that appears in living archosaurs and their fossil relatives.

• The supracetabular crest and the shift toward upright posture are tightly linked: the crest braces the femur when the leg is pulled under the body, preventing lateral dislocation and enabling endurance locomotion without relying on axial muscles for stabilization.

Locomotion in archosaurs: evidence and implications

• Modern crocodilians demonstrate three modes of locomotion enabled by the supracetabular crest and upright posture:
  • High walk: limbs underneath the body; weight supported by a braced hip socket.
  • Belly crawl: belly close to the ground with limbs more sprawling.
  • Gallop: in some species, all four feet leave the ground in a single stride.
• Trackways: forelimb and hindlimb alignment and midline proximity in archosauriform trackways indicate an upright, semi-erect gait as opposed to fully sprawling forms (e.g., certain non-archosauriforms).

Ornithodira and its two major subgroups

• Ornithodira is the sister taxon to the first true archosaurs (the antecedents to crocodiles and birds). Within Ornithodira:
  • Pterosauria: flight-capable archosaurs that radiated in the Mesozoic.
  • Dinosauromorpha: a clade that includes the dinosaurs and their closest dinosaur-like relatives.
• The earliest ornithodirans were comparatively small and fast, with two contrasts:
  • Early archosaurs (archosaurs proper) were large and quadrupedal in many cases.
  • Early dinosauromorphs were small to medium in size and exhibit bipedal tendencies, foreshadowing true dinosaurs.
• Teleocrater: a stem ornithodiran that provides a transitional view between crocodile-like archosaurs and later dinosaurs. It shares many features with both archosaurs and early dinosaurs, illustrating gradual changes along the node toward Dinosaurs.
• The Teleocrater/Hellio-? group (Heliokreta is named in the transcript) is highlighted as a near-dinosaur fossil that helps illuminate the origin of dinosaurs around the Triassic boundary (roughly $2.45 imes 10^{8} ext{ years ago}$). The ankle and overall morphology show crocodilian-like traits in some elements, illustrating the diversity of early ornithodiran anatomy and the gradual shift toward dinosaurian conditions.
• Importance of these stem forms: they provide a real window into early archosauriform evolution and how Crown Archosauria, Ornithodira, and eventually Dinosauria came to be.

Dinosauromorpha: making the transition to true dinosaurs

• Dinosauromorpha includes taxa like Cyclosauridae and early dinosaurs; first appearance in the middle Triassic; still present today via birds.
• Synapomorphies for early dinosauromorphs include:
  • Emphasis on the first three digits (digits 1–3) in the hand, indicating a forelimb structure geared toward certain terrestrial uses.
  • Vertebrae in the neck forming an S-shaped curve, providing a distinctive neck morphology for preying and feeding strategies.
  • The construction where the posterior ventral margins of neck vertebrae are lower than the anterior margins, creating a fused, S-shaped neck with reduced muscular energy to maintain the contour.
• Example taxa for early dinosauromorph morphology include Marasuchus (also known as Leposuchus in some nomenclatural histories) and Solisaurus, which display the characteristic S-neck and three-digits emphasis.
• Psilosauridae (often discussed as Silesauridae in modern literature) has been variably placed as basal dinosaurs or basal ornithischians in different phylogenetic analyses. In this course, the more widely accepted view is that Psilosauridae are basal dinosaurs (not basal ornithischians), though recent studies occasionally revise this placement. Ghost lineages can arise when the fossil record lacks intermediate forms and creates apparent gaps in the timeline.
• The idea of widespread distribution of early dinosauromorphs across Pangaea emerges from fossil discoveries, with lineages spreading and then outcompeting one another until dinosaurs dominate terrestrial ecosystems.

Dinosauria: origin, synapomorphies, and earliest members

• Dinosauria first appears in the middle Triassic and persists to the present via birds.
• Two subgroups within Dinosauria: Ornithischia and Saurischia.
• Key synapomorphies of Dinosauria (as described in the transcript):
  • An opposable thumb (a corrected, specialized grasping ability).
  • The hand shows the first digit (the thumb) offset, enabling grasping actions. An example is shown using the hand of Dilophosaurus, illustrating the offset knuckle and the orientation of the first phalanx.
  • The opposable thumb movement in dinosaurs is distinct from human opposable thumbs (in humans, the movement is between the metacarpal and the carpal; in dinosaurs, it occurs between the metacarpal and the first phalanx).
  • The perception that dinosaurs could grasp and potentially manipulate objects (e.g., vegetation or prey) with their hands.
• Additional notable dinosaurian synapomorphies discussed:
  • Perforated acetabulum: the acetabulum is an opening rather than a cup-shaped socket, allowing a different range of hip motion.
  • The femur head in dinosaurs is sharply inflected and fits into an open socket (the acetabulum) with a supracetabular crest providing superior stabilization.
  • The dinosaur femur is oriented such that the shaft runs vertically downward when the animal is in a typical posture, with a right-angle turn of the femoral head, enabling a fixed upright posture.
  • The combination of a perforate acetabulum, a sharply inflected femoral head, and a supracetabular crest leads to a fixed upright gait and a movement system that is less reliant on axial muscles.
• The evolution of the pelvis and locomotion:
  • An ancestral saurian pelvis had a cup-shaped acetabulum.
  • The archosaurs introduced the supracetabular crest, which braces the femur as weight moves to a more vertical gait.
  • The shift from a cup to an open socket acetabulum (peg-and-socket articulation) underpins a fixed upright posture in dinosaurs and reduces the need for axial muscle control to move the hind limb.
  • This morphological shift also changes muscular arrangements: limb-propulsive muscles move onto the hip and tail base, decoupled from rib cage expansion for breathing.
• The three-stage locomotive transition visualized (anterior view of the axial skeleton) shows:
  • An ancestral saurian with sprawling limbs and cup acetabulum.
  • An ancestral archosaur intermediate form with both sprawling and upright capabilities, supported by a supracetabular crest.
  • A true dinosaur with a fixed upright posture, open acetabulum, and braced hip joints.
• Basal dinosaurs and early body plans:
  • Herrerasauridae (Herrerasaurus, Stircosaurus) are early dinosaurs with carnivorous tendencies; placement has varied (stem Saurischia vs basal dinosaurin). They are not very large (rough sizes around a few feet in length).
  • Eoraptor: early predatory dinosaur from the Triassic era.
  • Early dinosaurs were small, primarily bipedal, with long tails, grasping hands, and an upright posture optimized for efficient locomotion and predation.
• Feeds and potential omnivory in early dinosaurs:
  • Although early dinosaurs are often described as predators, there is emerging discussion about possible omnivory or broader dietary strategies among basal dinosaurs and the dinosaur morphs near the ancestry of non-dinosaurian archosaurs. This remains a topic for ongoing research.

Feathers and the origin of feathers in dinosaurs

• Evidence for feathers in non-avian dinosaurs comes from several sources including:
  • Amber fossils showing hatchling Sillosauridae-like dinosaurs with preserved feathers on the tail.
  • Sinosauropteryx: a small predatory non-avian dinosaur covered with small, striped, quill-like feathers; dark and light banding visible in carbonized fossils.
  • Microraptor and other dromeosaurs: fossil evidence of feathers on the wings; Microraptor shows full wings on the forelimbs and tail feathers; Velociraptor likely had feathers though not directly preserved.
  • Hypo-typical taxon Hypteryx (an Oviraptorosaur) with indications of feather-like structures on the arms and tail fragments; supports widespread feathering among theropods.
  • Cytachosaurus (a ceratopsian) with tail quill-like displays; a ridge of feathers on tail tip as a potential display structure.
  • Tianyuan (a heterodontosaurid) with a comb-like ridge of quill-like feathers along the back to tail; Calindromius (a basal ornithopod) with plate-like scales and an extensive feather-like covering (hoods and baler-like feathers) covering much of the body.
• The Feather origin question and parsimony:
  • Principle of Parsimony: in phylogenetics, the simplest explanation with the fewest character-state changes is preferred, because complex evolutionary changes are expensive and less likely to arise repeatedly by chance.
  • By applying parsimony to feathers, the simplest explanation is that feathers originated once at the base of Dinosauria and were inherited by all descendants, rather than evolving independently in multiple branches.
  • Consequently, feathers can be considered a fourth synapomorphy of Dinosauria (in addition to the three earlier archosaur synapomorphies). The reasoning: the basal presence of feathers in multiple lineages within Dinosauria (including some ornithischians and many theropods) supports a single origin at the base of Dinosauria, followed by diversification and retention.
• Diagrammatic reconstruction notes used in class (summary):
  • Feathers appear in modern birds, in some dromaeosaurs (e.g., Microraptor and Velociraptor), and in several non-avian theropods; partial or full feathering is found across various lineages.
  • No clear evidence of feathers in all saurischian lineages (e.g., some large sauropodomorphs or some early saurischians); however, feather presence shows a broad distribution among many theropods and some ornithischians (as per the discussed examples).

The dinosaur timeline and consequences for learning

• The two major divisions of Dinosauria: Ornithischia and Saurischia.
• Ornithischia (the “bird-hipped” dinosaurs) and Saurischia (the “lizard-hipped” dinosaurs) develop after the establishment of the dinosaurian body plan with upright posture and specialized hands and feet.
• The earliest dinosaurs were small, dog-sized, bipedal, with long tails, grasping hands, and an upright posture that allowed breathing and walking concurrently.
• Early diets for dinosaurs: primarily carnivorous in the earliest examples (e.g., Eoraptor, Herrerasaurus); a question remains about whether some basal dinosaurs were omnivorous or herbivorous early on.
• Early feathering evidence challenges the simplistic view of dinosaurs as exclusively scaly predators and suggests more complex integumentary structures in a broader range of taxa.
• The lecture ends with an explicit plan: Ornithischia will be explored first in the upcoming lectures, culminating in an exam on Ornithischia; the next major section (after Ornithischia) will cover Saurischia (dinosaurs proper and birds) with a Pterosaur lecture saved for later in the semester.

Quick reference to key terms and ideas mentioned

• Archosauriformes: early archosauriform lineage leading to archosaurs.
• Eupharcaria (Euparkeria) and related early archosauriforms: examples of adaptations that provided advantages during tumultuous early Triassic periods.
• Antorbital fenestra: opening in the skull in front of the orbit.
• Mandibular fenestra: opening in the lower jaw.
• Supracetabular crest: bony crest above the acetabulum stabilizing the femur during upright locomotion.
• Perforated acetabulum: open socket hip joint in which the femur head fits; differs from cup-shaped acetabulum.
• Dinosauromorpha: clade including dinosaurs and their close dinosaur-like relatives.
• Ornithodira: sister taxon to crocod magnets; includes Pterosauria and Dinosauria.
• Teleocrater and Heliokreta (as described in the transcript): stem ornithodirans that illuminate early anatomy just before true dinosaurs.
• Psilosauridae/Silesauridae: debated placement as basal dinosaurs or basal ornithischians; current course stance favors basal dinosaurs with ghost lineages possible.
• Dinosauria synapomorphies: opposable thumb; three prominent digits of the hand; perforated acetabulum; inflected femoral head; upright, parasagittal limb posture; four-chambered heart; nest-building and parental care.
• Feathers in non-avian dinosaurs: evidence across multiple taxa, indicating a broader distribution of feather-like structures and possibly full feathers in some lineages.
• Ghost lineage: a gap in the fossil record where phylogeny predicts a lineage existed but fossils have not yet been found.

Possible exam-style takeaways

• Understand the synapomorphies that define Archosauria and how they relate to locomotion, respiration, and reproduction.
• Explain how the supracetabular crest supports upright posture and why this matters for locomotion efficiency and breathing.
• Describe the differences between sprawling and upright postures and how these affect the ability to breathe while moving.
• Summarize the key transitions from archosauriforms to Ornithodira and then to Dinosauromorpha and Dinosauria, including the significance of stem forms like Teleocrater and Heliokreta.
• Define the two main dinosaurian lineages (Ornithischia and Saurischia) and list the primary early features shared by Dinosauria (e.g., opposable thumb, open acetabulum, inflected femoral head).
• Discuss the evidence for feathers in non-avian dinosaurs and how parsimony supports the idea of a single origin of feathers at the base of Dinosauria.
• Understand concepts like ghost lineages and how they help explain gaps in the fossil record, as illustrated by discoveries like Nisasaurus in Tanzania.
• Be prepared to contrast the various locomotor modes shown by crocodilians (high walk, belly crawl, gallop) with the transition to an upright dinosaurian gait.

Closing reminder

• The course will continue with Ornithischia first, then proceed to Saurischia in subsequent lectures. A dedicated Pterosaur lecture is planned for later in the semester.