chpt 3
Chapter 3: Vertebrate Evolution — Key Concepts and Details from Lecture
Chapter 1–2 recap (context):
Chapter 1 focused on biogenetic trees (phylogeny).
Chapter 2 introduced chordates, their defining features, basic anatomy and physiology, and the move toward vertebrata and craniata.
Chapter 3 moves up the phylogenetic tree to jawless vertebrates (cyclostomes) and the origin/early evolution of jaws (gnathostomes).
The discussion also highlighted the geologic time assignment, touching upon questions of life's origin in the ocean and subsequent colonization of land.
From Cyclostomes to Gnathostomes: Key Features and Synapomorphies
Cyclostomes (jawless vertebrates) focus: Hagfish and lamprey.
Core synapomorphies of cyclostomes:
Lacked jaws, a prehistoric context crucial for understanding jaw evolution.
Lacked paired fins, which are important for advanced locomotion.
Lacked mineralized tissues (no bone in modern cyclostomes).
A single nostril located in the midline.
Gills supported by brachial baskets; the mouth is completely covered.
Keratinous teeth on a tongue-like structure; these are not homologous to true vertebrate teeth, emphasizing distinct evolutionary origins.
Notochord present; cranium present but no true vertebrae.
Early jawless vertebrates (“black jaws” in lecture):
Still possessed a notochord.
No vertebrae.
No paired fins.
Cranium present. These taxa provide a foundational scaffold for understanding the evolutionary steps toward vertebrates with true vertebrae.
Bony vs. mineralized tissues: Cyclostomes lack mineralized tissues; some mineralization appears later in a few lineages, specifically seen in conodonts and ostracoderms, marking early stages of hard tissue development.
Mineralization: Origins, Patterns, and Significance
Mineralization appears multiple times throughout the fossil record and may have begun in different tissues, suggesting convergent evolution in the development of hard structures.
Early evidence:
In invertebrates, shells show mineralization.
In some early vertebrates, surface mineralization is seen in the skin (dermal tissues) before true bones form internally.
Conodonts show mineralized elements (tooth-like structures) in the pharynx; these are often cited as early mineralized structures but are not considered true vertebrate bones.
The lecture emphasizes convergent evolution: the mineralized structures in conodonts likely evolved independently from the vertebrate mineralization pathway that later leads to true bones and teeth.
The term Naphthostoma (a jaw-related term discussed in lecture) refers to jaws; the root word 'naphtho-' relates to jaws, and 'stones' is a playful reference to mineralized structures associated with jaws.
Possible selective advantages of mineralization:
Protection of essential tissues (head, brain, and sensory organs) in aquatic environments.
Mechanical protection and shielding from physical damage.
Electrical insulation considerations in nervous tissue (context for future discussion of electricity in Chapter 4).
Storage/regulation of phosphorus (P) and calcium (Ca): Mineralized tissues can act as reservoirs when dietary Ca and P are limiting. The body may mobilize Ca and P from bones when needed for vital processes like muscle contraction and ATP synthesis.
The fossil record includes early armored forms (ostracoderms) with elaborate head shields that clearly illustrate the protective roles of early mineralization.
The debate on which tissues mineralized first and why remains active; many scientists argue that mineralization had multiple functional drivers and evolved in a mosaic fashion across different tissues and lineages.
Conodonts and Early Mineralized Structures
Conodonts are an extinct group noted for their mineralized elements in the pharynx, which resemble tooth-like structures.
Key takeaway: Conodont mineralization is an important clue to early vertebrate evolution but is now viewed as a clear case of convergent evolution rather than a direct precursor to true vertebrate bone/teeth.
Conodonts did not possess the full complement of bony structures seen in later vertebrates, distinguishing their hard tissues from the more complex skeletal elements of gnathostomes.
This example illustrates how mineralization can arise in separate lineages for functional reasons (e.g., feeding) without implying a direct, linear evolutionary path to bone formation in all lineages.
Dermal Denticles and Placoid Scales: Skin-Based Mineralization to Teeth
Early mineralization often begins in the skin as dermal denticles, acting as a form of dental armor.
Placoid scales (found in extant sharks) are composed of an outer enamel-like layer with underlying dentine; these structures are considered a design precedent for later tooth evolution in more advanced vertebrates.
In the lecture, dermal denticles were described as tiny teeth embedded in the skin, providing both protection and possibly hydrodynamic benefits by reducing drag.
Implication: The mineralization process initially found in the skin could migrate and specialize into oral structures, significantly contributing to the evolution of true teeth and jaws in gnathostomes.
Ostracoderms: Armor, Protection, and Early Skeletal Innovations
Ostracoderms are an extinct group of armored fishes characterized by elaborate head shields and external mineralized tissue.
They illustrate the crucial protective function of mineralization and provide valuable context for the evolutionary transition from skin-based armor to more developed internal skeletons.
The discussion compared ostracoderm head shields to modern examples (e.g., horseshoe crabs) to help visualize the protective strategies for vital nervous tissue in these early forms.
Ostracoderms represent an important step in the evolution of skeletal tissues and highlight the selective pressures that favored the development of mineralization.
Hagfish and Lamprey: Anatomy, Life History, and Transitional Features
Cyclostomes discussed in detail: Hagfish and lamprey, representing modern jawless vertebrates.
Key anatomical features:
Cranium present, but no true vertebrae, with the notochord persisting as the primary axial support.
The mouth is not jawed; instead, it features keratinous teeth on a tongue-like structure. These teeth are not homologous to the true teeth of jawed vertebrates.
Gills are supported by brachial baskets; water flow across the gills is facilitated by a small single nostril and the production of phlegm/mucus.
The tongue bears keratinous teeth, which are utilized for jawless feeding adaptations, such as scraping food from surfaces or feeding on carcasses.
Dermal denticles are present in some jawless forms, reinforcing the skin-based mineralization pattern as a significant early evolutionary trait.
Lamprey-specific notes:
Lampreys exhibit anadromous life history, meaning adults live in the ocean but migrate to freshwater rivers/streams for reproduction, where juveniles reside.
Lampreys undergo metamorphosis, where the larvae (ammocoetes) look distinctly different from the adults.
The lecture recounted the historical context of lamprey invasions in North America, particularly their significant impact on the Great Lakes ecosystem.
In North America, the lamprey invasion of the Great Lakes became a major ecological and economic issue, severely decimating commercial fisheries.
Historical timeline: The opening of the Welland Canal and other barriers in the Great Lakes region facilitated the lamprey invasion, leading to population explosions by the mid-20th century. Various management strategies have been implemented since then.
Great Lakes lamprey control (historical case study discussed):
Barriers: Physical structures prevent lamprey from migrating upstream to spawning grounds.
Releasing sterile males: A biological control method to reduce reproduction rates.
Selective lamprey larvicides/chemicals: Target lamprey larvae; these efforts are expensive and require careful consideration of their environmental impact.
Use of small dams: Can disrupt upstream migration to breeding grounds, helping to manage populations.
By recent efforts, lamprey populations have been reduced by approximately 90\% in most areas, thanks to a combination of control methods and policy initiatives.
The Great Lakes fishery is valued at approximately 4\times 10^{9} dollars, highlighting the significant economic incentive for lamprey control.
Ethical and practical considerations of population control: Chemical controls, genetic strategies (sterile release), barrier installation, and dam management all involve trade-offs concerning ecosystem health, non-target species impacts, and long-term ecological balance.
Evolution of Jaws: From Pharyngeal Arches to Gnathostomes
The transition from jawless to jawed vertebrates involved significant modification of the pharyngeal arches (gill arches), which were initially involved in respiration.
Core idea: Jaws evolved through the folding and remodeling of the first two pharyngeal arches, which underwent structural changes to form a hinged jaw apparatus.
Early transitional forms show progressive remodeling of these arches, with initial functional changes happening gradually rather than as a single, abrupt leap.
The lecture referred to the concept that the first pharyngeal arch (mandibular arch) and the second arch (hyoid arch) provided new feeding mechanics, enabling more diverse diets and complex predatory strategies for early gnathostomes.
Spiral valve motif and gas exchange: Early jawed forms retained the spiral valve in the gut, which significantly aided digestive efficiency as body plans grew more complex.
Enabling features for jawed feeding included:
Enlargement and modification of the first and second gill arches.
The emergence of dentition (true teeth) and guarded feeding structures in the oral cavity, allowing for more effective prey capture and processing.
This transition also included changes in locomotion and body plan, accompanying the development of paired fins and vertebral segmentation, features not fully present in cyclostomes.
Anatomy and Reproduction in Early Vertebrates: Notable Details
Early jawless vertebrates exhibited distinct reproductive and anatomical features.
Internal fertilization is observed in certain fossil contexts and early gnathostomes, representing a contextual note in course materials.
Most modern fish exhibit external fertilization, so the presence of internal fertilization in early forms represents a key divergence in the lineage toward more derived vertebrates.
The role of internal fertilization in early fish is discussed in contrast to the more common external fertilization observed in many extant fishes (like salmon).
Amphioxus vs. Tunicates: Metamorphosis Debate (Contextual Reference)
The instructor referenced a prior debate concerning the metamorphosis trajectories of Amphioxus and tunicates.
Amphioxus (a cephalochordate) typically shows metamorphosis between distinct larval and adult forms, retaining many chordate features throughout its life cycle.
Tunicates (urochordates) exhibit a different pattern: their free-swimming larval stage (often tadpole-like) possesses key chordate characteristics, but the adult forms are sessile and frequently lose many of these chordate features through a process known as retro