Anatomy lecture 3
Amphioxus: Evolutionary History & Characteristics
Page 1 highlights
Amphioxus features at anterior end (head): sensory systems, ocellus (light sensitive), feeding mechanism, endostyle (primitive ‘thyroid’ gland), pigment spot (proto-eye?).
Pharyngeal slits function: filter-feeding organs; act like sieve to allow certain sized food particles.
Hatschek’s pit: ciliated, secretes mucus to help collect food; proto-pituitary with hypothetical endocrine function.
Velum: resembles a partial diaphragm.
Oral hood: encloses anterior entrance to pharynx.
Summary of function: slits run along the pharynx to filter feed; Hatschek’s pit and endostyle relate to feeding and endocrine roles.
Page 2 highlights
Amphioxus circulatory system as a model for chordate/vertebrate circulation; colorless blood; plasma lacks blood cells and oxygen-carrying pigments.
There is no real heart by common anatomical criteria; sinus venosus acts like a heart region but does not pulsate.
Pharyngeal arch complexes act analogously to afferent/efferent arteries of gills.
Blood flow: from body via paired cardinal veins → common cardinal veins → hepatic vein → sinus venosus; into ventral aorta; branches into ascending aortae; form dorsal paired aortae with posterior and head-directed flows.
Page 3 highlights
Question: Does amphioxus have a heart? Answer: No true heart by simple definition of a cardiac muscular pump.
Key questions about what constitutes a heart: type of muscle (cardiac vs skeletal), genotype/phenotype differentiation, homology issues.
In amphioxus, the major blood vessels likely push blood via peristaltic movement; blood carries food; gas exchange occurs through the skin and dissolved in blood.
Page 4 highlights
Basic developmental pattern: tail, anus; pharyngeal slits; sensory systems; feeding; endocrine gland; dorsal hollow nerve cord; notochord; digestive tract.
Cephalization and regionalization of the body are fundamental developmental themes that recur in vertebrates.
Summary for amphioxus and cephalochordates: head formation and dorsal-ventral organization; cecum (liver-like structure) noted on dorsal side.
Page 5-6 highlights (Urochordata overview)
Urochordata (tunicates, sea squirts) are a major chordate group.
Emphasis on larval anatomy to establish chordate relationships (notochord and nerve cord as key features).
Reproduction: most are hermaphrodites (monoecious); some species are dioecious.
Free-swimming larva (ascidian tadpole) is bilaterally symmetrical and phylogenetically informative.
Heart location near pharynx; amoebocytes in tunicate blood resemble vertebrate lymphocytes.
Page 7 highlights (Deuterostome relationships)
Figure 2.33 presents deuterostome phylogeny including Hemichordates (acorn worms) as a reference group.
Body inversion in some lineages is governed by BMP and chordin gene expression: BMP ventral, chordin dorsal; in hemichordates this arrangement is reversed for the ventral/dorsal axis.
Chordin acts antagonistically to BMP; pharyngeal slits now open ventrally.
Page 8 highlights (Vertebrate synapomorphies)
Vertebral column; olfactory epithelium in the head; pineal/parietal eye atop the skull; lateral paired eyes.
Electroreception: ability to detect electrical stimuli; largely lost in reptiles, birds, and most mammals except monotremes (e.g., platypus, echidnas).
7 visceral arches and 6 aortic arches.
Heart: typically 2 or more chambers, up to 4 chambers in some lineages.
Mineralized tissues: dentine, enamel, and bone.
Cartilaginous internal skeleton in some early jawed/agnathan lineages; becomes ossified in most vertebrates.
Endochondral bone formation (cartilaginous precursor) is mesodermal in origin.
Dermal bone forms part of the external skeleton (skull and jaw in many groups).
Page 9 highlights (Origins of vertebrate features)
Open questions: origin of bone and cartilage; origin of cardiac muscle; origin of integumentary glands; origin of sensory epithelia; origin of teeth; origin of keratin.
Genomic databases (e.g., genome data) can help understand vertebrate origins; next chapters will address these questions.
Page 10 highlights (Geological context)
Fig. 1.45: Geological ages and mass extinctions; Permian-Triassic boundary emphasized.
Siberian Traps volcanic eruptions led to high CO2, temperature rise, and marine anoxia, contributing to major biodiversity losses.
Page 11-12 highlights (Origins of vertebrate animals; Ostracoderms and early fishes)
Ostracoderms: jawless armored fish-like vertebrates; early vertebrate models.
Fig. 3.3 emphasizes amphioxus feeding as a model for the evolution of feeding apparatus; active grasping jaws emerge with active bottom-dwelling lifestyles; this precedes jawed fishes.
Page 13 highlights (Agnathans and early vertebrate evolution)
Distinguishing groups: AGNATHAN (jawless) vertebrates; jawed vertebrates.
Major groups listed: lampreys, hagfishes; extinct conodonts; ostracoderms; Cyclostomes (jawless).
Page 14-16 highlights (Hagfishes and Lampreys)
Hagfishes (Myxinodea) and Cyclostomes lack vertebral bone surrounding the notochord; they respire via primitive gills; life history includes marine to freshwater settings; lampreys and hagfishes share features but differ in many respects.
Lampreys: gill pouches in adults; larval gills function differently and larvae are suspension feeders; metamorphosis can lead to a non-feeding adult stage in some species (e.g., American Brook Lamprey).
Hagfish: deep-sea marine, burrowing; their defense includes extreme mucus production and a unique knotting behavior that helps them extract tissue; they can form slime that clogs predators’ gills.
Notation of two YouTube links as supplementary material for hagfish and lamprey behavior.
Page 17 highlights (Lamprey life cycle)
About 50% of lampreys are parasitic as adults; larval lampreys are suspension feeders.
Genetic manipulation can affect formation of the endoskeleton in some species.
American Brook Lamprey example linked for visual reference.
Page 18-19 highlights (Larval vs adult lampreys and respiration)
Larval lampreys’ gills are used for filter feeding; adults use true gills for respiration.
Adult lamprey anatomy and gill ventilation, swimming, and spawning have been documented (-links provided for visuals).
Page 20-21 highlights (Integument and keratin in lampreys)
Histology of adult lamprey skin shows two key cell types with unclear functions; may produce distasteful products to deter predators.
Keratinized epidermis in lampreys; horn-like keratinized teeth cover keratin rather than true teeth; keratin scales/plates and cell movement patterns discussed.
KRT80 keratin gene in lampreys: 8 exons; example of pleiotropy where one gene affects multiple traits; the slide emphasizes focusing on impacted organs rather than memorizing all pathways.
Concept of pleiotropy: a single gene influences multiple phenotypic traits.
Page 22 highlights (Using Genes to Explore Vertebrate History)
Genome browser: Ensembl (www.ensembl.org) provides gene and protein sequence data for many animals.
Gene trees: used to infer evolutionary relationships.
Genomic approaches are crucial for understanding origins of tissues such as bone, dentin, and enamel.
Page 23 highlights (Vasotocin and vertebrate hormones)
Vasotocin: an oligopeptide homologous to oxytocin and vasopressin; found in non-mammalian vertebrates (birds, reptiles, fishes, amphibians); probable presence in mammals during fetal development.
Arginine-vasotocin (AVT): hormone produced by neurosecretory cells in the posterior pituitary; regulates water balance/osmotic homeostasis; also involved in social and sexual behavior in non-mammalian vertebrates.
In developing mammals, vasotocin shows properties similar to oxytocin; vasotocin is considered a progenitor of vertebrate hypophysial hormones, illustrating pleiotropy and deep homology in hormone systems.
Page 24 highlights (KRT80 keratin gene and pleiotropy)
Structure of the KRT80 keratin gene in lampreys described as having 8 exons.
Example serves as an instance of pleiotropy: a single gene (KRT80 or its receptor gene) influences multiple traits.
Practical takeaway: do not memorize all downstream effects; focus on which organs and systems are impacted by these gene-level changes.
Page 25-26 highlights (Conodonts and early vertebrates)
Conodonts: small, laterally compressed soft-bodied early vertebrates; initially known only from pharyngeal elements (fossils) until 1983 clarified their nature.
Occurred from Cambrian to the end of the Triassic; their elements are mineralized dental tissues used to crush food; large eyes and “teeth” hint at predatory lifestyle.
One imaginative reconstruction (artist’s concept) shows conodonts with tooth-like elements and eyes; fossils provide dating utility for geological strata.
Key cross-cutting themes
The evolution of the vertebrate body plan involves gradual changes in feeding apparatus, circulation, skeletal tissue, and skin.
Cephalization and regionalized body plans recur across chordates and vertebrates as foundational developmental themes.
Genomic data increasingly clarifies homology and the origin of tissues once thought to be unique to vertebrates.
Practical and ethical implications: understanding evolution informs comparative anatomy, paleontology, and biomedical research; cautions about oversimplifying homologies across deep time.
Glossary of core terms (quick reference)
Cephalization: development of a head region with sensory organs.
Notochord: flexible rod-like structure providing support in the embryo.
Endostyle: primitive thyroid-like organ in invertebrate chordates.
Hatschek’s pit: ciliated mucus-secreting structure involved in feeding and possibly endocrine function.
Pharyngeal slits/arches: openings and associated structures used in filter feeding and gas exchange; highly conserved across deuterostomes.
BMP and chordin: genes that regulate dorsal-ventral patterning; antagonistic interactions influence axis formation.
Conodonts: early vertebrates with mineralized dental tissues; important for stratigraphy.
Vasotocin/AVT: ancestral neuropeptides involved in osmoregulation and social behaviors; precursors to oxytocin/vasopressin systems.
Summary takeaway
The transcript outlines a progression from simple chordate features (amphioxus) to more complex vertebrate features (jawed vertebrates), highlighting key anatomical, developmental, and genomic threads that connect body plans across deep time.