Animal Reproduction and Evolutionary Traits

Viviparous

Viviparous animals have embryos that develop inside the mother and receive nourishment directly from her. The offspring are born alive. Example: Humans, dogs.

Ovoviviparous

Ovoviviparous animals have embryos that develop inside eggs within the mother. The embryo is nourished by the egg yolk, not the mother, and the egg hatches inside the mother or just before birth. Example: Some sharks, rays.

Oviparous

Oviparous animals lay eggs outside the body. The embryo develops in the egg and is nourished by the yolk. Example: Birds, amphibians.

Cambrian Explosion (Cambrian Era)

Occurred about 541 million years ago. It was a period of rapid evolutionary development during which most major animal phyla appeared. This marked the first real evidence of complex, multicellular, and hard-bodied animals. It was likely triggered by increased oxygen levels, predator-prey interactions, and genetic innovations.

Chordates

Chordates have four key traits at some point in their development:

- Notochord (a flexible support rod),

- Dorsal hollow nerve cord (develops into spinal cord),

- Pharyngeal slits/clefts (become gills or jaw/ear structures),

- Post-anal tail.

Includes all vertebrates (fish, amphibians, reptiles, birds, mammals) and some invertebrates (like lancelets and tunicates).

Mammal Traits

- Hair or fur,

- Mammary glands (milk production),

- Endothermic (warm-blooded),

- 4-chambered heart,

- Differentiated teeth (molars, incisors, etc.),

- Mostly viviparous,

- Kidneys for water conservation,

- Efficient respiratory and circulatory systems,

- Large brain-to-body size ratio,

- Extensive parental care,

- Modified teeth for various functions (grinding, shearing, etc.)

Swim Bladder (Bony Fish)

A gas-filled sac that helps bony fish maintain buoyancy. It allows them to stay at a desired water depth without sinking or floating.

Fish Breathing

Fish breathe underwater using gills. Water flows over the gills, and oxygen diffuses into the blood while carbon dioxide diffuses out.

Mammal Breathing

Mammals use lungs for gas exchange. The diaphragm contracts to draw in air. Even aquatic mammals like whales and dolphins breathe air using lungs.

Sharks

Cartilaginous fish (Class Chondrichthyes).

- Have gills for respiration, no lungs.

- No swim bladder; maintain buoyancy with movement and liver oil.

- Streamlined for speed but not agility.

- Use pharyngeal muscles or swimming to keep water flowing over gills.

- Fertilize eggs internally; can be oviparous, ovoviviparous, or viviparous.

- Some bone traces exist in teeth, scale bases, and vertebral coatings.

Book Lungs (Spiders)

Arachnids such as spiders and scorpions have book lungs—thin, leaf-like structures stacked together to maximize surface area for gas exchange.

Arthropods

- Exoskeleton made of chitin,

- Segmented body (head, thorax, abdomen),

- Jointed appendages,

- Open circulatory system (pumps hemolymph).

Groups: Insects, Arachnids, Crustaceans, Myriapods. Chelicerates include sea spiders, horseshoe crabs.

Chelicerae: clawlike feeding appendages.

Arachnids have 6 pairs of appendages: chelicerae, pedipalps, and 4 pairs of walking legs.

Pedipalps are used for sensing, feeding, defense, and reproduction.

Cladograms

A cladogram is a diagram that shows evolutionary relationships among organisms based on shared traits. Each branch point (node) represents a common ancestor. Traits like a backbone, jaws, lungs, amniotic egg, and fur are usually listed on the line leading to a group.

Derived Characters of Vertebrates

- Two or more sets of Hox genes (lancelets and tunicates have only one).

- Enabled evolution of advanced nervous systems and skeletal features.

- Neural crest cells give rise to features such as: Teeth, Bones and cartilage of skull, Sensory capsules, Neurons.

Filter Feeders

Animals that capture particles suspended in water that pass through their body. Example: Sponges (Phylum Porifera) use a central cavity to filter water and an osculum to expel it.

Cnidarian Body Forms

- Diploblastic and radially symmetrical.

- Gastrovascular cavity: central digestive sac that also acts as a hydrostatic skeleton.

- Polyp: sessile, adheres to substrate by aboral end (opposite mouth).

- Medusa: free-swimming, bell-shaped, mouth on underside.

Reptiles

- Scales with keratin for protection.

- Lay shelled eggs on land. Internal fertilization.

- Mostly ectothermic (regulate body temperature via environment).

- Birds are reptiles (archosaurs) adapted extensively for flight.

Endothermic vs. Ectothermic

- Endothermic: Maintain body temperature using metabolic heat.

- Ectothermic: Rely on external sources for heat; regulate temperature behaviorally.

Mammal Overview

Amniotes with about 6400 living species.

Traits include:

- hair,

- milk production,

- endothermy,

- large brains,

- efficient systems.

Subgroups:

Marsupials: Short gestation, live birth, development in pouch. Eutherians: Complex placenta, long pregnancy, internal development completed in uterus.

Primates

- Grasping limbs, flat nails, and fingerprints.

- Large brains,

- short jaws,

- forward-facing eyes for depth perception.

- Fully opposable thumbs for grasping.

Includes lemurs, lorises, bush babies, tarsiers, monkeys, and apes.

Apes include: gibbons, orangutans (Pongo), gorillas (Gorilla), chimpanzees and bonobos (Pan), and humans (Homo).

- Adaptations for tree life: grasping limbs, depth perception, eye-hand coordination.

Paleoanthropology & Hominins

Paleoanthropology studies human origins.

Hominins: extinct species more closely related to humans than chimps. About 25 extinct hominin species discovered.

Traits:

- Reduced canines,

- Flat faces,

- Upright posture and bipedal locomotion,

- Foramen magnum under skull,

- Larger brain,

- tool use,

- symbolic thought,

- Shorter digestive tract.

Humans and chimps share ~99% DNA but differ in 19 Hox gene expressions.

Bilateria & Eumetazoa

Bilateria: Animals with bilateral symmetry and three tissue layers.

Eumetazoa: Animals with true tissues.

Invertebrates

Animals that lack a backbone. They make up about 95% of all known animal species.

Porifera (Sponges)

Among the simplest animals.

Body structure: sac with pores, central cavity, and osculum. Function: filter feeders that pull water in and filter nutrients.

Exoskeleton

A rigid external skeleton made of calcium carbonate or chitin, providing support and protection.

Lophotrochozoans

Diverse clade of 18 phyla.

Some have lophophore (ciliated feeding structure).

Others have trochophore larval stage.

Flatworms (Platyhelminthes)

Live in marine, freshwater, and damp environments.

Planarians:

- Free-living,

- freshwater flatworms with eyespots

- lateral flaps.

- Complex nervous system (compared to cnidarians).

Parasitic groups:

Trematodes: Multiple hosts, complex lifecycles.

Tapeworms: Live in vertebrate intestines, absorb nutrients directly.

Nematodes (Roundworms)

Cylindrical, unsegmented bodies, tapered at ends. Covered in cuticle.

Example: Trichinella spiralis—human parasite from undercooked meat.

Ecdysozoans

Clade of animals that shed their exoskeleton or cuticle as they grow (molting).

Includes nematodes and arthropods.

Gnathostomes (Jawed Vertebrates)

Outnumber jawless vertebrates.

Groups: Sharks, ray-finned fishes, lobe-finned fishes, amphibians, reptiles, mammals.

Jaws with teeth evolved from skeletal rods supporting pharyngeal slits.

Traits:

- Genome duplication (4 Hox gene sets),

- Enlarged forebrain,

- Enhanced smell and vision,

- Lateral line system (aquatic species).