Diversity of Vertebrates

This chapter will be a veritable "parade of taxa", as we start tracing the evolution of vertebrates and the derived characters that distinguish them from the chordates that we discussed in the last lecture.

General characteristics of vertebrates

Vertebrates may be characterized by 12 general derived characteristics. You should become very familiar with these traits, and identify how they are expressed in the vertebrates you will see in lab.

1. Bilateral symmetry

2. Two pairs of jointed locomotor appendages, which can include fins (pectoral and anal/dorsal fins, as well as the forelimbs and hindlimbs).

3. Outer covering of protective cellular skin, which can be modified into special structures such as scales, hair and feathers

4. Metamerism found in skeletal, muscular and nervous system. This was described in a previous lecture - structures can include ribs, vertebrae, muscles and ganglia/peripheral nerves.

5. Well-developed coelom, or body cavity completely lined with epithelium (cellular tissue), that may be divided into 2 to 4 compartments.

6. Well-developed internal skeleton of cartilage and bone, separated into axial skeleton (skull, vertebrae, ribs, sternum) and appendicular skeleton (girdles and appendages).

7. Highly developed brain enclosed by skull, and nerve cord enclosed by vertebrae. This provides advanced neural structures that are highly protected from damage.

8. Well-developed sense organs (eyes, ears, nostrils) located on the head (cephalization).

9. Respiratory system, including either gills or lungs, and located closely to the pharynx or throat.

10. Closed circulatory system with ventral heart and median dorsal artery.

11. Genital and excretory systems closely related, utilizing common ducts and pathways.

12. Digestive tracts with two major digestive glands (liver and pancreas) that secrete into it.

 Grouping the vertebrate classes

Within the vertebrate classes, there are further ways of subdividing the groups based on derived characteristics. There are eight recognized extant classes of vertebrates(Figure 3.1, p. 81 in text):

Myxini - hagfishes

Cephalaspidomorpha - lampreys

Chondrichthyes - cartilagenous fishes

Osteichthyes - bony fishes

Amphibia - frogs, toads, salamanders, and caecilians

Reptilia - turtles, snakes, lizards, crocodilians

Aves - birds

Mammalia - mammals

 These can be grouped based on their general habitat requirements: Pisces - collective term for all fishes; includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes

Tetrapoda - collective term for the terrestrial vertebrates; they have four feet unless some have been secondarily lost or converted to other uses. Includes Amphibia, Reptilia, Aves, Mammalia

 Or based on their feeding habits: Agnatha - jawless vertebrates, including Myxini and Cephalaspidomorpha

Gnathostomes - vertebrates with jaws derived from the mandibular arch, which may have (in primitive vertebrates) supported gills. Includes Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, Mammalia

 Or based on their embryonic characteristics Anamniotes - vertebrates that lack an amnion, or extraembryonic membrane that surrounds the embryo and encases it in amniotic fluid. Includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes, Amphibia.

Amniotes - vertebrates that possess an amnion. Includes Reptilia, Aves, Mammalia.

 Don't let these different terms confuse you! They are all ways of distinguishing taxa based on primitive versus derived traits. Use them to help you memorize and classify the taxa we will be talking about.

Tracing vertebrate evolution through the fossil record

Keep in mind that the evolutionary relationships among the different taxa that we are discussing have been determined from the fossil record. Vertebrates, among all species of animals in the world, have the best fossil record.

Reasons include the presence of hard parts, such as bones and scales. The exception to this are the cartilaginous fish, because cartilage does not fossilize well, and the birds, due to the fact that they have hollow bones that can be crushed and lost. In contrast, the best fossil record among the vertebrates exists among the large mammals, whose bones are preserved well as fossils.

Superclass Agnatha

The Agnatha are in some texts referred to as a class, and in others as a superclass. In general this group shares the common characteristics of:

• no jaws

• no paired appendages

• a completely cartilagenous skeleton

• a single nostril

• 6 - 14 external or concealed gill slits

• a persistent notochord

• a two-chambered heart

 Because the fossil record is very poor for these species, it is unclear whether the two Agnathan groups should be described as classes or orders.

Ancestral forms of this class were the Ostracoderms, which are extinct, but were heavily-armored on their heads and trunk. The ostracoderms were believed to be detritus feeders, because of their jawless mouths.

Extant Agnathans include two groups, called cyclostomes, because of their circular mouths:

Class Myxini, Order Myxiniformes - hagfishes (Figure 3-8, p. 88 in text)

• temperate, marine deep water

• feed on detritus and carrion, as well as polychaete worms

• use sensitive tentacles around their mouths in locating prey

• single nostril opens into pharynx

 Class Cephalaspidomorpha, Order Petromyzontiformes - lampreys (Figure 3-10, p. 90 in text) • temperate, anadromous (hatch/breed in fresh water, mature in marine) and freshwater

• parasitic as adults - attach to other fishes with their suction-like mouths and rasp a hole in the skin

• buccal glands secrete an anticoagulant to ensure free-flowing food source

• larvae are called ammocoetes, resembling the amphioxus - primarily detritus feeders until they metamorphose into adults, sometimes after 6 or 7 years as a larva.

 The remaining vertebrate orders are Gnathostomes (possess true jaws). Evolutionary studies have shown that in most cases the jaw is modified from one of the gill arches that were used to support gills in more primitive species.

Evolution of jaws represents an advancement in morphology, expanding the function of the mouth to a wider range of potential prey types. Thus, the jaws are an example of a derived structure that is more generalized than its ancestral form.

Class Chondrichthyes - cartilagenous fishes characterized by:

• paired nostrils

• skeleton completely cartilagenous with no endoskeletal bone

• no swim bladder

• scales dermal placoid when present

• gill arches internal to gills

• freshwater and marine species

 Contains three main groups: Subclass Elasmobranchii • 5 - 7 gill openings plus spiracle anterior to first gill

• upper jaw not attached to braincase

• teeth derived from placoid scales, deciduous and continually replaced

• claspers present in males, internal fertilization, ovoviviparous (egg contained within the uterus, where the young develop and then hatch as miniature adults) or viviparous (embryos develop internally and then emerge as a miniature adult)

• modern species present by end of Mesozoic

Order Squaliformes - true sharks

• almost purely predaceous/marine

• heterocercal tailfin - caudal fin is longer on the dorsal side than on the ventral side

 Order Rajiformes - Rays, skates, sawfishes • greatly flattened bottom dwellers

• scales not over entire body

• pectoral fins winglike

• crushing teeth - mollusk eaters

• spiracles greatly enlarged

• oviparous - produce an egg pouch covered in a very tough shell

 Subclass Holocephali - Chimaeras • upper jaw fused to braincase

• flat, bony plates instead of teeth

• operculum covering gillslits

• strictly marine feeding on mollusks

 Class Osteichthyes - bony fishes • endoskeleton made up of bone

• jaws and paired appendages

• gill arches internal to gills

• gills covered by bony operculum

• dermal scales not placoid

• many forms have swim bladder

• appeared in Devonian - dominant vertebrates since mid Devonian

• arose in freshwater, moved into saltwater

 Subclass Actinopterygii: ray-finned fish • fin rays attach directly to girdles

• internal nostrils - nares absent

• single gas bladder

• known from Devonian

 Superorder Chondrostei (sturgeons and paddlefish) • general primitive form

• typically small

• skeleton primarily cartilage

• heterocercal tail

• ganoid scales

• most died out by end of Mesozoic

 Superorder Neopterygii (most bony fishes) Division Ginglymodi - garpike • moderate ossification of skeleton

• heterocercal tail

• elongated jaws

• ganoid scales

• dominant during Mesozoic

 Division Halecostomi Subdivision Halecomorphi - bowfin

• represented by single freshwater species Amia calva

• cycloid scales

• almost homocercal tail

Subdivision Teleostei - true bony fish

• skeleton mostly bony

• tail typically homocercal

• no spiracle

• scales ctenoid or cycloid

• known from marine forms first, originating from Holosteans during Mesozoic

• major radiation associated with modifications in locomotor or feeding mechanisms and high fecundity, i.e.

- maxilla and premaxilla independently mobile from rest of skull

- pelvic and pectoral fins adapted for speed and braking giving maneuverability

- fusiform bodies streamlined for speed

 Subclass Sarcopterygii: species previously believed to be extinct, such as the coelacanths and lungfish. • fleshy lobed fins so that fin rays do not articulate directly to girdles

• internal and external nares

• many retain the heterocercal tail

• the coelacanth is represented by a single species that lives off the Comoro Islands near Madagascar

 The rise of the Tetrapoda classes and the movement from water to land represents one of the major evolutionary events in the history of vertebrates. New structural designs were required to make the transition to land in order to cope with increased oxygen levels, decreased water supply, more fluctuating ambient temperature, and slight changes in the way sensory information is obtained.

Class Amphibia - Amphibians

• arose from Crossopterygian, Rhipidistian ancestors

• three extant orders, two extinct subclasses

• lungs and skin used as adult respiratory organs

• gills present in larvae, retained into adulthood in some neotinic forms (salamanders)

• heart with two atria and one ventricle - "three chambered"

• skin is naked or with bony dermal elements

• ectothermic - must regulate body temperature by moving to different microclimates within its environment

• group includes smallest terrestrial vertebrates up to some 5’ in length

• name implies continued tie to water - eggs must be laid in water or at least in very moist environment; young develop as gill breathing, water-dwelling tadpoles

• embryos lack an amnion, but eggs are laid in a jelly-like protective coating

 Order Urodela (Caudata) - salamanders • tail maintained throughout life

• limbs 1 -2 "normal" pairs

• elongated trunk and long tail

• can retain larval characteristics (flattened, shovel-shaped head, fleshy tail, external gills) in adult forms (paedomorphic) - the result is a sexually mature individual with many other body parts in the larval or juvenile condition (neoteny)

 Order Salientia (Anura) - frogs and toads • loose tail as adults

• caudal vertebrae fuse to form long inflexible urostyle - relates to saltatorial locomotion

• long hind limbs developed for saltatorial locomotion

• vocal cords well developed

• ear modified for reception of airborne sound waves

 Order Gymnophiona (Apoda) - caecilians • elongated, snake-like, with no limbs or girdles

• no vocal cords or airborne sound detection

• some retain scales embedded in skin

• notochord persists

• minute eyes, lack lids

• chemosensory tentacle on head

 Class Reptilia • first fully terrestrial vertebrates

• development of cleidoic (closed/self-contained) egg; embryo with extra-embryonic membrane and relatively impermeable shell

• lungs for respiration

• heart with two atria and ventricle partially or totally (Crocodilians) divided

• one occipital condyle

• skin with epidermal scales or bony plates

• ectothermic, sometimes called heliotherms because they can regulate body temperature by using solar radiation

• first appeared in late Paleozoic, so numerous by Mesozoic known as "Age of Reptiles"

 Subclass Anapsida ("no opening"); Order Testudinata (turtles) • ribs modified along with epidermal plates to form shell - carapace and plastron

• girdles inside ribs

• jaws covered with horny epidermal plates, no teeth

• little change since Triassic

 Subclass Diapsida ("two openings") Order Squamata - lizards and snakes • contains most modern reptiles

• lizards known from Cretaceous, snakes in Cenozoic

• skull has lost one or both temporal regions

• vertebrae usually procoelous

• abdominal ribs usually greatly reduced or absent

• body covered with horny epidermal scales

• quadrate bone moveable

• teeth set in sockets

 Subclass Archosauria - Ruling Reptiles • diapsid skull

• contains dinosaurs and ancestors to birds

 Order Crocodilia - crocodiles • quadrate fixed

• bony plates embedded in epidermis

• teeth set in sockets

• abdominal ribs present in Gastralia

• ventricles completely separated

• developed secondary palate

• "crop" similar to birds

 Class Aves

Subclass Neornithes - modern birds

• endothermic rather than ectothermic

• the reptile scale into a feather which is the only unique characteristic of this class

• four-chambered heart

• epidermal scales on bill, legs, feet

• bill instead of teeth; teeth absent in modern forms

• modifications for flight include hollow bones, pectoral appendages modified as wings, air sacs, large eyes and large cerebellum

• modifications for vocalization

 Class Mammalia • possess hair/fur

• mammary glands to nourish young

• endothermic

• viviparous (oviparous in one order)

• two occipital condyles

• zygomatic arch and secondary palate

• single dentary bone in lower jaw

• dentary-squamosal jaw articulation

• muscular diaphragm

• arose from synapsid reptiles which branched off at base of reptilian tree

 Subclass Prototheria - egg-laying mammals • oviparous

• mammary glands without nipples

• cloaca still present

• pectoral girdle with separate precoracoid, coracoid, and interclavicle bones

 Subclass Theria

Infraclass Metatheria - marsupial mammals

• viviparous, young born extremely altricial

• abdominal skin pouch (marsupium) supported by epipubic bones

• lack typical chorioallantoic placenta, have yolk-type

• vagina doubled, no cloaca

• mammary glands located inside marsupium

• restricted to New World tropics and Australia

 Infraclass Eutheria - "true" or placental mammals • viviparous

• chorioallantoic placenta

• vagina single

• mammary glands with external nipples

• precoracoid and interclavicle gone

• arose in Cretaceous, great radiation of insectivore-like ancestors during Cenozoic

 Final note: We will be using the terminology and phylogenies developed in this lecture as we go through the different systems. Try to become comfortably familiar with this phylogeny, as I will be referring to it later. Because this is comparative anatomy, the focus of this course will be on how different systems developed in different taxa, without emphasis on any single taxonomic group (such as humans).

General Definitions:

Adaptive radiation - evolutionary process in which descendants from an ancestral species multiply and diverge to occupy many different habitats and modes of life

Agnatha - jawless vertebrates, including Myxini, Cephalaspidomorpha

Amniotes - vertebrates that possess an amnion or extraembryonic membrane that surrounds the embryo and encases it in amniotic fluid, including Reptilia, Aves, and Mammalia

Anamniotes - vertebrates that lack an amnion . Includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes, Amphibia

Cleidoic egg - self-contained egg that allows animal to bypass larval stage

Cloaca - posterior chamber of most fishes, nonmammalian tetrapods, and monotreme mammals into which the digestive tract and urogenital passages discharge).

Coelom - body cavity completely lined with epithelium (cellular tissue

Gnathostomes - vertebrates with jaws derived from the mandibular arch, which may have (in primitive vertebrates) supported gills. Includes Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, Mammalia

Heliotherm - regulates body temperature by using solar radiation .

Marsupium - skin pouch in which immature young are carried

Neoteny - paedomorphosis that results from the slowing down of somatic development relative to reproductive development.

Oviparous - eggs are laid and the young develop outside of the mother

Ovoviviparous - egg contained within the uterus, where the young develop and then hatch as miniature adults

Paedomorphosis - retention of juvenile characters into the adult stage

Pisces - collective term for all fishes; includes Myxini, Cephalaspidomorpha, Chondrichthyes, Osteichthyes

Placenta - composed of parts of the uterine lining and fetal extraembryonic membranes through which exchanges between mother and embryo occur.

Swim bladder - sac of air that acts as a hydrostatic mechanism, allowing a fish to control its vertical position in the water

Tetrapoda - collective term for the terrestrial vertebrates; they have four feet unless some have been secondarily lost or converted to other uses. Includes Amphibia, Reptilia, Aves, Mammalia

Viviparous - embryos develop internally and then emerge as a miniature adult