Lecture Exam 2

What are the key characteristics and types of Class Sarcopterygii?

• Lobe-finned fishes

• Have supportive lobes at the base of paired fins (before the ray-supported portion)

• Most are extincy

• Two living groups:

  • Actinistia (Coelacanths)

  • Dipnoans (Lungfishes)
    

What are the key skeletal and dental features of Class Sarcopterygii?

• Scales and bones have an outer covering of true enamel (derived)

• In Actinopterygians, the equivalent is ganoine

• A key synapomorphy: enamel on teeth

What is the evolutionary significance of Class Sarcopterygii?

All members are more closely related to mammals and other tetrapods than to other fishes

What are the classification and extant species of Subclass Coelacanthimorpha?

• Order: Coelacanthiformes

• Family: Latimeriidae

• 2 living species (genus Latimeria):

  • Latimeria chalumnae - West Indian Ocean

  • Latimeria menadoensis - Indonesia

What is the fossil history and rediscovery significance of Subclass Coelacanthimorpha?

• Well represented in the fossil record from the Devonian to Cretaceous (~300 million years ago)

• Thought to be extinct for about 70 million years

• Rediscovered in 1938 by Majorie Courtenay-Latimer

What are the key details about the first modern discovery of coelacanths?

• First specimen caught in 1938 near the Comoros Islands (near Madagascar)

• Depth: 260-300m

• Species: Latimeria chalumnae

• Described by J.L B. Smith

• Story told in A Fish Caught in Time by Samantha Weinberg

What are the key features of coelacanth fins?

• Pelvic & pectoral fins: Strongly developed with internal skeletal elements homologous to tetrapods

• Dorsal fins: Two (most bony fishes have one); second dorsal supported by a fleshy lobe

• Anal fin: Supported by a lobe

• Caual fin: Divided into three lobed sections

What are the swimming behaviors of coelacanths?

• Capable of strong bursts using the caudal (tail) fin

• Use paired fins to scull slowly and rotate

• Can swim upside down

• Exhibit “head-stand behavior” (vertical position, head down)

What are the brain and sensory features of coelacanths?

• Extremely small brain: about 1/15,00 of body mass (smallest relative size among measured vertebrates)

• Eyes adapted to low light: mostly rods in retina

• Have a tapetum lucidum but lack melanin sacs, so they can’t adjust well to bright light

What is the rostral organ in coelacanths and its proposed functions?

• Rostral organ: Cavity in the snout filled with gelatinous substance, with three openings on each side

• Proposed functions:

  • Detect electrical signals from prey or predators

  • Sense magnetic fields for orientation at depth

• Hans Fricke showed electrical pulses triggered head-stand behavior (rostrum pointed toward source)

What are the main conservation concerns for coelacanths (Latimeria chalumnae)?

• Estimated population: ~200-500 individuals (hard to measure)

• Threats:

  • Slow growth & late maturity

  • Limited habitat and range

  • Bycatch in fisheries

• Ancient lineage (~400 million years old)

• Key question: Can it survive the next 50 years?

Extant Dipnoans - Lungfishes

• Name means “double breath”, refers to presence of functional gills + functional lungs

• Not ancestral to the first non-amniote tetrapods

• Six species:

  • Protopterus spp, - 4 species: Africa

  • Lepidosiren paradoxa - South America

  • Neoceratodus foresteri - Australia

• Extant Dipnoans have a Gondwanan distrubution

Synapomorphic Character States Extant Dipnoans - Lungfishes

• Four characters that are different than Sarcopterygians that were ancestral to non-amniote tetrapods

  • Autostylic jaw support

  • fused tooth plates - durophagy

  • Maxilla & premaxila absent

  • Endoskeleton of paired fins has a central bony axis with radials on both sides - “biradial”

Australian Lungfish - Neoceratodus

• Largest species (1.5m, 100 lbs), deep-bodied

• Two pairs of large paired fins with large lobed bases

• Scales are large & prominent

• Diphycercal caudal fin

• Dependent upon gills for gas exchange, gulps air into lungs to survive hypoxia (facultative air-breathing)

• Stomach absent - spiral valve in intestine, feeds on benthic molluscs

• Occurs only in the Burnett & Mary River drainages of Queensland, Australia

South American (Lepidosiren) & African (Protopterus) Lungfishes

• Long (1-2m) slender bodies

• Paired fins are very slender, elongate in Protopterus

• Nearly scaleless

• Skeleton is highly cartilaginous & flexible

• Gills are poorly developed, dependent upon aerial gas exchange for 98% of their O₂ acquisition (obligate air breathers)

• Aestivate to survive drying up of fresh water pond habitats

Aestivation in Protopterus

• Induced by desiccation during the dry season

• First crawls from pool to pool as they dry

• Digs a 1m deep burrow with a round chamber at the bottom

• As water dries, descends into the chamber, curls up body, secretes a thick mucous which dries into a “cocoon”

• Breathes air through a small hole in the cacoon

• Metabolism is very low, lives off energy stores

• Usually aestivates for 6 mos. Have survived for 4 years in labratory studies

Evolutionary Ecology of the Water-Land Transition in Vertebrates

• Transition occured during the Devonian, 400 - 375 mya

• First vascular plants colonized terrestial habitats

• Arthropods followed

• Severe droughts occured, FW bodies dried/shrank

  • Fish density increased

  • Competition intensity increased

  • Predation pressure increased

  • Water temperature increased

  • Dissolved O₂ content of water decreased

• Shallow-water provided ecological release for fishes that meet the challenges posed by moving into these micro-habitats

Challenges Posed by Shallow Aquatic Habitats & the Adaptive Solutions (1)

• Decreased dissolved oxygen content - gas exchange using gills alone not sufficient; aerial gas exchange using physostomous G.B./ Lungs

• Shallow water - less support of body mass against gravity. Density of abrasive vegetation increasing;

  • Swimming locomotion less effective

  • Crawling/ paddling locomotion more effective

  • Strong selection for change in body shape, limb structure, & endoskeleton

Structural adaptations posed by shallow aquatic habitats

• Food organisms different, conditions for prey capture different, inertial feeding less effective, suction feeding less effective

  • Reduction of fins, changes in the mouth to take advantage of new prey (Arthropods)

• Shallow aquatic habitats are more abrasive, air exposure increased, increasing desiccation stress

  • Evolution of a more desiccation resistant integument

Eusthenopteron (Order Osteolepiformes) Symplesiomorphic traits:

• Large, free-swimming predators adapted for life in deep waters

• Cylindrical bodies, head not flattened, large (deep) dorsal, anal, & caudal fins

• Laterally oriented eyes

• Labyrinthine teeth; strong but not flexible

• Single frontal bone, snout relatively narrow

• Small, paired, crescent-shaped vertebral centra, no articulation

• Ribs short, project dorsally (more support from surrounding water)

Panderichthyes (Family Elpistostegidae)

• Sister group of the non-amniote tetrapoda

• Synapomorphic traits with the first non-amniote tetrapoda:

  • Dorsal & anal fins lost

  • Caudal fin greately reduced

  • Body & head dorsoventrally flattened

  • Snout is long, flat, & broad (2 large frontals)

  • Eyes are dorsally oriented

  • Vertebral centra are larger & articulation is increased

  • Ribs are larger & project laterally & ventrally (increased support against gravity)

Tiktaalik roseae (“fishapod”) Fossil History

• The next intermediate for between aquatic & terrestrial vertebrates

• Fossils are 375 my old (end of Devonian), from 700 miles above the current Arctic circle

• Tiktaalik fossil has high quality preservation of the head & neck region showing important details of braincase, palate,and gill arches

Tiktaalik roseae (“fishapod”) Derived Character States

• Braincase was more solid than that of fishes

• Cranio-vertebral column articulation was more flexible than in fishes

  • In ancestral fishes that live in deep waters, selection favors an inflexible cranio-vertebral column articulation for two reasons:

    • Streamlining

    • Neutrally buoyant predators can readily change direction by moving the entire body

  • Proximal portion of pectoral fins were adapted to bear the weight of the anterior body, having distinct wrist bones

  • Ribs are long & robust for support in very shallow water

Tiktaalik roseae (“fishapod”) Adaptations

• When crawling along the bottom in shallow water, cannot easily raise the body to change direction to strike prey. Increased selection pressure of indpendent head movement.

• Head mobility was increased because bony articulation between head and gill region was broken. Involed loss of the opercular bones and extreme reduction in the hyomandibula (takes on a new function of sound transmission as an ear ossicle)

Tiktaalik roseae (“fishapod”) Ancestral Characters (symplesiomorphic)

• Functional gills present

• Dermal scales present

• Cranium & mouth large & wide

• Pelvic appendages were relatively small and fin-like

• Distal portion of pectoral appendages still membranous fin-like structures

First Tetrapoda: Classification

• Not correct systematically or ecologically to label first tetrapods “amphibians”

• They are not closely related to extant Caudata (salamanders), Anura (frogs), or Apoda (caecilians)

• More closely related to extant amniotes

• To clarify, extant “amphibia” are correctly labelled Lissamphibia = the extant tetrapods that do not lay shelled (amniote) eggs

• NAT’s to describe first tetrapods

First Tetrapoda: Structure

• The first tetrapods of the Paleozoic were much different than Lissamphibians in several ways

• Much larger (1-2m)

• Integument had dermal scales

• Did not exchange gases over integument

• Similar to extant crocodilians in both appearance & behavior, living in shallow waters aquatic microhabitats, later riparian microhabitat

Stem Tetrapods of the Devonian: Acanthostega

• Highly aquatic - evidence:

  • Forelimbs & hindlimbs were polydactylous = more than 5 digits

  • Forelimbs were very paddle-like, hindlimbs less so

  • Had a flange on the cleithrum that articulated with the operculum - indicate internal gills that functioned for gas exchange

  • “Elbow” was not able to bend in a way that would support the body weight on land

  • Articulating surfaces of the vertebrae were (less supportive)

  • Neural arches were weakly ossified

  • Fin rays of the caudal fin were long

  • Well developed ribs absent

Stem Tetrapods of the Devonian: Ichthyostega

• Appears to have been less aquatic - evidence:

  • Flange on cleithrum is absent suggesting that functional gill chamber was absent

  • Caudal fin rays were reduced

  • Had broad, overlapping ribs - support against gravity & muscular attachment, protection of viscera

  • Hindlimb polydactylous, but less paddle like (forelimb structure unknown)

  • Proportinos of the humerus & femur are similar to those of some pinnipeds (sea lions)

  • Forelimb was permantely bent at the “elbow”

  • Hindlimb was more paddle-like

  • Suggests Ichthyostega coulid better support weight on land, but still strong swimmer

Gravity and Vertebrae

• Fishes have simple lateral motion and support from water, so:

  • Arches are relatively straight and needle-like

  • So are ribs

• On land, a quadruped backbone faces same problem a bridge designer would face - sag!

  • Evolved a series of interlocking articulations on each vertebra

  • Helped overcome sag and hold backbone straight and minimal muscle effort

Evolution of the Girdle Skeletons Ancestral Condition (fishes)

Neither the pectoral or pelvic girdles articulate with the vertebral column

Evolution of the Girdle Skeletons Tetrapods

• Pectoral Girdle - throughout tetrapod evolution, the pectoral girdle never directly articulates with the vertebral column via skeletal articulation like the pelvic girdle does

• Pectoral girdle is held in position by skeletal muscles & connective tissue

Evolution of the Girdle Skeletons Pectoral Girdle

Consists of five paired elements:

• Scapula - the dorsal element

• Coracoid - one ventral element

• Clavicle - a second ventral element when present

• Clethrum & interclavicle - are smaller portions of the girdle that are remnants from ancestral fishes. Both elements reduced, and sometimes absent entirely

• The articulating surface for the humerus is the glenoid fossa

Evolution of the Girdle Skeletons Pelvic Girdle

• Beginning with the first non-amniote tetrapoda, the bones of the pelvic girdle become larger and more robust to transmit the weight of the body ventrally through the hind limbs

• The pelvic girdle consists of three paired skeletal elements that form a “triangle” when views from the side

Evolutionary Changes of Non-Amniote Tetrapods Observable in Lissamphibians

• Endoskeleton

• Integument

• Circulatory system

• Gas exchange system

• Mouth/Tongue structure & function

Tetrapod Limb Skeletal Elements

• Homologous with those in the pectoral & pelvic fin lobes of the ancestral Sarcopterygians

• Forelimb Elements

  • Humerus - proximal most element that articulates with the pectoral girdle at the glenoid fossa

  • Ulna - lateral element of the lower forelimb

  • Radius - medial element of the lower forelimb

  • Carpals - elements of the “wrist”

  • Metacarpals - elements in the hand that articulate with the carpals

  • Phalanges - elements of the digits

Evolution of the Tetrapod Jaw Articulation Ancestral Condition Sarcopterygii

• The quadrate of the upper jaw articulates with the articular of the lower jaw. The hyomandibula supported the articulation, especially the quadrate

• The angular supported the articular

• The dentary + other elements formed the anterior lower jaw

Evolution of the Tetrapod Jaw Articulation Non-Amniote Tetrapods & “Reptilian” & Avian Amniotes

• The hyomandibula ceases to support the jaw articulation between the qudrate & articular

• The hyomandibula is reduced to a small, slender cylindrical element (stapes = columella) that transmits sound waves from the tympanic membrane to the inner ear

• Sound wave transmission is increased in air

• Jaw articulation is qudrate-articular

Evolution of the Tetrapod Jaw Articulation Synapsida

• Squamosal (upper) & dentary (lower) elements become involved in the jaw articulation

• Quadrate, articular & angular are reduced, migrate posteriorly

Evolution of the Tetrapod Jaw Articulation Mamalia

• Jaw articulation is completely between the squamosal and the dentary

• Articulation involving two large bones is stronger

• Other bones become involved in sound transmission

Evolution of the Tetrapod Integument Ancestral Condition Sarcopterygii

• Dermis is the “dominant” layer (dermal armor & scales)

• Epidermis is a relatively thin outer layer with numerous unicellular mucous glands

• In fishes, the mucous functions to reduce resistance while swimming & protect against pathogens

Evolution of the Tetrapod Integument Derived Condition Non-amniote & amniote tetrapods

• Epidermis becomes much more prominent

• New epidermal cells are produced in mitotically active cells adjacent to the dermis

• Cells migrate toward the body surface as they age

• Produce keratin & become progressively flattened

• Keratin is a tough, evaporation resistant protein

• Reduces evaporation of water from deeper cells, and protects against abrasion

Integument in Extant Lissamphibia

• Thickest in terrestial forms (e.g. Bufonidae)

• Thin in aquatic adults & larvae

• Keratin does not stretch. Therefore, eventually limits growth

• Ecdysis - to accomodate growth, periodically a new keratinous layer produced beneath the old one which is sloughed off

Integument in Extant Lissamphibia Glands & Gas Exchange

• Multicellular mucous glands - secrete mucous that protects from pathogens, & deters desiccation, so long as it is kept moist (limits to moist microhabitats)

• Specialized mucous glands - in some species, glands are specialized to produce toxins that deter predators (dendrobatids), or secretions that function in social communication

• Integumentary Gas Exchange - moist mucous layer allows O₂ to go into solution
  on the surface of the integument & diffuse into capillaries

Integument in Amniote Tetrapods “Reptilian” Amniotes

• Becomes thicker & more complete (limits O₂ exchange)

• Provides increased protection from desiccation stress, important for success in terrestial habitats

• Forms specialized hard structures: e.g. claws, spines, coverings of horns, rattles, rhamphotheca (beak), egg-teeth (sharp projection on bill when first born)

Integument in Amniote Tetrapods Avian Amniotes

• Feathers

• Beaks (covering), claws, scales

Integument in Amniote Tetrapods Mammalian Amniotes

• Fur & hair

• Claws, scales, “horns”

Evolution of the Circulatory System in Non-Amniote Tetrapods Heart

• Ancestral condition in fishes - an undivided tubular heart

• Increasing reliance on lung ventilation seelcted for a beginning separation of the tubular heart into right & left chambers

• Separation is in the atrium, resulting in right & left atria

• The ventricle remains undivided

• The heart is three chambered

Lung Ventilation in Early Non-Amniote Tetrapods & Extant Lissamphibia Ancestral Condition: Buccal Force (Positive Pressure Ventilation)

• Early non-amniote tetrapods lack/lacked a rib cage

• The body wall was/ is not muscular & not rigid

Lung Ventilation in Early Non-Amniote Tetrapods & Extant Lissamphibia Ventilatory Cycle

1. - A bolus of air is present the lungs & gas exchange is on-going
   - Valvular nares are opened, valvular glottis is closed
   - Muscular floor of the bucco-pharyngeal cavity is forcefully depressed
   - Air is sucked into the bucco-pharyngeal chamber

2. Glottis is opened, external nares opened
   - Body wall muscles contract pushing air out of the lungs over the air bolus being held in the bucco-pharyngeal chamber (exhalation)

3. External nares are closed, glottis is opened
   - Muscles of the bucco-pharyngeal cavity contract forecully pushing air bolus through the opened glottis into the lungs (inhalation)

4. Glottis is closed, gas exchange occurs
   - External nares are opened to allow air to be moved in-and-out for olfaction
   

Evolution of Negative-Pressure (suction) Lung Ventilation Derived Condition

• Development of longer ribs that projected ventrally eventually culminated in a “rib-cage” in later non-amniote tetrapods, and amniotes

• In addition, amniotes developed a muscular partition separating the thoracic & abdominal cavities (= diaphragm)

• During the transition from non-amniote to amniote tetrapods, increased rigidity of the body wall of the trunk (ribs + skeletal muscle), promoted ventilation of the lungs by negative pressure (suction)

• Air is sucked into lungs with negative pressure

WLT and the Evolution of Mouths in Tetrapods

• In water, prey are brought into the mouth using suction + forward intertia. Aquatic prey do not require moistening for swallong, already wet

• WLT placed different selection pressures on tetrapod mouths that influenced the tongue, teeth, & oral glands

WLT and the Evolution of Mouths in Tetrapods Ancestral Condition (fishes & aquatic NAT’s)

• Primary tongue = a fleshy lobe lacking intrinsic musculature, skeletal support, & is not highly glandular

• Few oral glands elsewhere in the mouth

• Teeth are homodont, and can be widely distrubuted in the mouth (jaws, palate, tongue), function for grasping prey

WLT and the Evolution of Mouths in Tetrapods Derived Condition

• Evolution of the definitive tongue:

  • Intrinsic musculature present

  • Skeletal support (hyoid)

  • Makes mobile; protrusible

  • Glandular

  • Tongue functions in prey capture & manipulation of prey inside the mouth

  • Number of oral glands increases, and they become concentrated in amniotes (= salivary glands)

  • The total number of teeth decreases & they become confined to the jaws

  • Teeth are homodont = similar size & shape, continue to function for grasping, crushing, some shearing of prey (no true mastication until mammals)

WLT and the Evolution of Mouths in Tetrapods Derived Condition Differences between Lissamphibia, Amniote, and Turtles

• Lissamphibia teeth are pedicellate = have a base (pedicel) that supports a crown. There is a zone of weakness between the crown & pedicel that allows teeth to bend some without breaking

• Amniote teeth are not pedicellate. Show three types of attachment to the jaw bones:

  • Acrodont - attached to the top of the jaw

  • Pleurodont - attached to the side of the jaw

  • Thecodont - anchored into sockets in the jawbones

• Turtles - teeth are absent, jawbones covered by a thick layer of keratin (derived from epidermis)

Subclass Lissamphibia

• “Modern amphibians”

• Lissos (greek) → “smooth”, moist and scaleless skin

• Thought to be a monophyletic group composed of three distinct lineages

• Non-amniote tetrapods, lay eggs in water

Subclass Lissamphibia Shared Derived Characters

• Moist, permeable skin with substantial cutaneous gas exchange

• Pedicellate, bicuspid teeth

• Operculum-columella complex (two bones that transmit sounds to the inner ear)

• Green rods ( in addition to red rods common elsewhere) max sensitivity to blue light in low-light conditions

• Levitator bulbi muscle - thin sheet in floor of eye socket causes eyes to bulge outward, enlaring buccal cavity

• Relatively immobile tongue, not picky carnivores

Distinguishing the Three Lissamphibian Clades

• Caudata - salamanders, newts, efts (~ 350 species)

  • Have well developed tails

  • Most have 4 limbs

  • Terrestial locomotion by crawling

  • Swim with lateral undulation (tail) when in water

• Anura - frogs & toads ( at least 22 families, 3,000+ species)

  • tailless

  • All have 4 limbs

  • Terrestial locomotion by hopping (ability variable)

  • Swimming using webbed hind feet when in water

• Apoda - caecilians (many fewer species)

  • Appendages lost as an adaptation for fossorial (i.e., burrowing) life

  • Terrestial locmotion by burrowing

  • Aquatic locomotion by swimming using lateral undulations

Order Caudata A Caudata Curiosity

• Most vertebrate genomes are 1-7 picograms, but the mean for salamanders is 35pg (up to 120!)

• Goes hand-in-hand with larger cell size than most other vertebrates

• Requires morphological and developmental adjustments

Caudata Life History Overview

• Locomotion

• Paedomorphism

• Loss of lungs

• Complex life-cycles

• Internal fertilizatoin by spermatophore

• Complex courtship & mating behavior

Crawling Locomotion in the Caudata

• Least specialized skeleton & musculature

• Least speciliazed locomotion

• In many salamande species, alternate legs on opposite sides of the body move at the same time

Paedomorphism in Caudata

• Retention of the aquatic larval lifestyle in adults & character states that are adaptations for aquatic life:

  • external aquatic gills

  • Fin-like tail

  • Reduced appendages

  • Lack moveable eyelids

  • Thin skin covering

  • Lateral line system