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The Petromyzontiformes, or lampreys, were once grouped in a taxon called Agnatha (jawless fishes), which also contained the other extant group of jawless fishes, the hagfishes (Hyperotreti). The Agnatha traditionally formed one of the two major sister lineages of vertebrates, the other being the Gnathostomata (jawed vertebrates). Below is a cladogram showing the old view of basal vertebrate phylogeny:

However, it has since been discovered that lampreys are more closely related to jawed vertebrates than are hagfishes, and thus Agnatha is a paraphyletic group.


why are hagfishes no longer considered vertebrates

they do not possess vertebral elements surrounding the dorsal nerve cord - evolutionary precursors to the vertebral column seen in well-known vertebrates. Instead, they are classified as an outgroup to the Vertebrata, collectively forming the monophyletic Craniata.


how are craniates defined

defined by the presence of a well-defined head region, with a cranium that encases a brain and paired sensory organs, and includes the last common ancestor of vertebrates and hagfishes plus all its descendent


what habitats do lampreys live in

reshwater or marine environments in all temperate regions except those of Africa. All species live in freshwater during their larval stage and also spawn and die in river habitats. Some species are anadromous, migrating to coastal seas when mature and only returning to reproduce. Lamprey larvae have a low tolerance for high water temperature, hence they are not found in tropical regions


features of lampreys

Lack a bony skeleton and jaws (bones may have been secondarily lost, as there is evidence to suggest that their ancestors possessed a bony skeleton).

No paired fins (pectoral, pelvic), but do have unpaired fins: dorsal and caudal (tail) fin.

Presence of horny teeth, which they use as a "rasping tongue" to suck on to prey, and pierce the flesh to draw blood.

Rows of paired gill openings; water flows in and out of the gills in a tidal flow - while most fish breathe by drawing water into the mouth, past the gills, and out through the gill slits in a one-way flow of water, lampreys cannot use their mouths for ventilation as they are attached to prey!



cartilaginous fishes from Gnathostomata;
skates, sharks, rays, and chimaeras


hypnosqualean hypothesis,

batoids (rays and skates) are in fact derived sharks, grouped with the Pristiophoriformes and Squatinomorphii in the clade "Hypnosqualea


distribution and habitat

Most chondrichthyans are marine species. Only 5% (approximately 45 species) are restricted to freshwater, such as the giant freshwater stingray, Himantura chaophraya (Fowler et al., 2005; Helfman et al., 2009). Others enter estuaries and freshwater sporadically, often to breed.

Members of the Chondrichthyes can be found in nearly all aquatic ecosystems and depths, except the most extreme conditions. But most species are restricted to and specialised for a particular oceanic zone.


features of chondrichthyans

Skeletons formed of calcified cartilage - no bone.

Covered in placoid scales - a structure like teeth, with a dentine crown coated in an enamel-like material, a vascularised pulp cavity, and a bony base. They are sometimes referred to as denticles.

True teeth, which are shed and replaced regularly in modern species.

The males of all but the oldest fossil species have a pelvic clasper, used in courtship and mating. It is formed of the pelvic meta-pterygium - the basal cartilage of the pelvic fin.


Actinopterygii -

Vertebrata; Gnathostomata; Osteichthyes; Actinopterygii

ray-finned fishes

actinopterygians can be divided into two distinct groups: basal actinopterygians and neopterygians
one of the two major clades of bony fish (Osteichthyes)

12,000 extant species, which dominate the vast majority of the world's open ocean and shallow marine environments. Within this clade is the Atherinomorpha (guppies, killifishes and relatives), and the Percomorpha - by far the most diverse group of fish, containing over one-third of all ray-finned fish species, and exhibiting a fascinating array of body forms - including perches, seahorses, flatfishes, pufferfishes, and tunas.

e.g. Salmoniformes (slamons, trouts, and relatives) - Temperate Northern and Southern hemisphere; freshwater.



features of an actinoptergii

The skeleton of the paired fins is formed from many small bones, called fin rays, in a fan-like arrangement, which are supported at the bases of the fins by parallel rows of bones called radials. All living actinopterygians except bichirs and reedfishes (Order Polypteryformes) also have branching rays in unpaired fins.

Modified the ancestral fish character of a breathing lung into a swim bladder to aid and adjust buoyancy.

Single dorsal fin.

basal actinopterygians possess a primitive heterocercal tail - one in which the fin lobes are different in length (asymmetrical) - there is increasing symmetry towards the teleost group, with teleosts themselves possessing a homocercal tail,

adaptations to increase suction were crucial in the evolution of ray-finned fishes as active predators, as lunging towards prey items in water actually acts to push them away by forcing a flow of water towards them.


function of a swimbladder

ghe swim bladder is a smooth-walled (i.e., non-alveolar) modified lung that is virtually impermeable to gas. Therefore, gas can be gulped - or indeed transferred from the the bloodstream in many more derived teleosts - into the swimbladder of ray-finned fishes to reduce the overall density of the body, and afford neutral buoyancy. This allows fish to remain stationary in the water column, and thus waste less energy. As the pressure of the water column on the body changes with depth, actinopterygians must regulate the volume of air in their swim bladders to remain neutrally buoyant.

Some ray-finned fishes, such as the gars of North and Central America and Cuba, are secondary air-breathers, who have evolved an alveolar lung to survive in their low-oxygen environment.



Vertebrata; Gnathostomata; Osteichthyes; Sarcopterygii

The Sarcopterygii, or lobe-finned fishes, is a clade containing the coelacanths, lungfishes, tetrapods, and their fossil relatives, including the osteolepiformes and panderichthyids.


Sarcopterygians are characterised by?

Sarcopterygians are characterised by their fleshy pectoral and pelvic (paired) fins that articulate with the pectoral (shoulder) and pelvic (hip) girdles via a single bone. This is apparent in the coelacanths and lungfishes, which are more intuitively fish-like. These lobe-fins gave rise to the paired limbs of tetrapods, with the single bones representing the humerus (forelimb) and femur (hindlimb).


what evolved from the sacropterygii

The palaeontological record makes clear that the terrestrial verterbates evolved from lobe-finned fishes nearly 400 million years ago during the Devonian, and are therefore members of the Sarcopterygii. The only terrestrial vertebrates still living today are the tetrapods, which originated around 350 million years ago and are defined as that group which comprises the common ancestor of the living amphibians and amniotes plus all its descendants.


previously thought extinct discovery of sacropterygii

Latimeria chalumnae has been indentified since 1938 in the waters of the West Indo Pacific Ocean near the Comoros Island and eastern coast of southern Africa


features of a sacroptergii

Muscular paired fleshy fins.
Fins attached the pelvic and pectoral girdle by single basal bone.
Teeth coated with enamel.



Vertebrata; Gnathostomata; Osteichthyes;
Sarcopterygii; Tetrapoda; Lissamphibia

three living orders of amphibians - Gymnophiona (caecilians), Urodela (salamanders and newts), and Anura (frogs and toads).



The caecilians are a group of limbless, burrowing amphibians, which superficially resemble earthworms or some limbless lizards (snakes, amphisbaenians)

Together, caecilians form the order Gymnophiona - one of the three extant amphibian orders, along with Anura (frogs and toads) and Caudata (newts and salamanders).


caecillia features

No appendicular skeleton - they are completely limbless and have no shoulder girdle, but there is a kink in the spine where the pelvic girdle once was.
95-285 presacral vertebrae (those anterior to the sacral vertebrae, which once fused with the pelvic girdle).

Compound, akinetic skull formed of joined plates of bone - this is an excellent and typical adaptation for a fossorial animal (also seen in burrowing lizards, and burrowing mammals, such as the golden moles), allowing the head to be used like a spade to dig, push, and pack earth when burrowing in underground tunnels.

Reduced eyes.

200+ lymph hearts situated intersegmentally under the skin.



North and Central America (367 species), variously inhabiting terrestrial and freshwater systems in temperate or tropical forests.


caudata features

Elongate body, usually with four short limbs and a laterally flattened tail for swimming.
Broad, flattened skull, with large orbits.
Bicuspid teeth on both the upper and lower jaw.
The rib-bearers (the elements of the vertebrae that articulate with the ribs) are bicipital (have two prongs).



frogts and toads

famous Xenopus frog!


anura features

A skeleton that is highly modified for jumping (although many forms have altered these features to specialise in other lifestyles, such as an aquatic or burrowing one):
Elongate hind limbs, including the ankle bones (tarsals) and foot bones (metatarsals and phalanges).
A urostyle: a rod-like fusion of the sacral vertebrae, running in parallel with the extended iliac blades of the pelvis, resulting in a strong, shock absorbing pelvic basket.
Short, stiff vertebral column (9 or less vertebrae proper) and no ribs. This helps to stiffen the trunk, providing a solid path for the transmission of thrust from the limbs when jumping, as well as maintaining posture.
Short and flat head.
No teeth on the dentary.
Fused radius and ulna to form a compound radio-ulna.




divided into two clades, the Cryptodira (meaning "hidden neck") and the Pleurodira (meaning "side neck"), categorised by a difference in the articulation of the cervical vertebrae.


distribution of testudines

Testudines species are terrestrial, aquatic or semi-aquatic and occupy many different habitats within these systems, from the open ocean to freshwater rivers, tropical rainforests and deserts (Pough et al., 2009).

The distribution of Pleurodires is restricted to the Southern Hemisphere (South America, Australia, New Guinea) where all species are either aquatic or semi-aquatic.

The Crpytodires, on the other hand, can be found in both the Northern and parts of the Southern hemisphere (South America and Africa) (Pough et al., 2009), and may be terrestrial (e.g., Hermann's tortoise, Testudo hermanni), freshwater (e.g., European pond turtle, Emys orbicualris), or marine (e.g., leatherback sea turtle, Dermochelys coriacea).


features of testudines

The skull has no temporal fenestrae (openings in the skull near the temples) - the anapsid condition - unlike all other extant amniotes - mammals have one (synapsid), and other reptiles have two (diapsid).

The trunk is surrounded by a two layered shell - carapace (dorsal) and plastron (ventral) - composed largely of dermal bone (fused with the ribs and vertebrae in the carapace).

The carapace is also covered with an epidermal component - a layer of broad, horny scales called scutes.

The limb girdles are uniquely incorporated into the rib cage. This is due to the fact that they are housed within the carapace, which is derived partly from the rib bones and thoracic vertebrae.

They have evolved a horny beak instead of teeth.



Squamata - lizards and snakes

over 7000 species, they are present on every continent except Antarctica

e.g. bipedal runners (e.g. Basiliscus), climbers (e.g. chameleons), and active predators (e.g. Varanus).


fun facts about squamata

In addition to the plesiomorphic reptilian condition of egg-laying (oviparity), some groups give birth to live young (viviparity). More surprisingly, at least eight groups contain a species with only female members, reproducing by a process called parthenogenesis - the development of unfertilised eggs into functional (female) offspring.


synapomorphies of squatmata

Synapomorphies of the Squamata

Cranial kinesis - a high degree of flexibility between the bones of the back of the skull, allowing relative movements between them.

Paired hemipenes (present in all lepidosaurs) are fully eversible.

Pleurodont dentition - teeth set into the side of the inner surfaces of the jaws, and periodically replaced.

Loss of gastralia (ventral belly ribs).

Double-hooked fifth metatarsal, functionally analagous to the mammalian heel.

Further complexity to the mesotarsal joint (in comparison to non-squamate lepidosaurs)



Rhynchocephalia - tuatara
Rhynchocephalia is an order of lizard-like reptiles that includes only one living species of tuatara, which in turn has two subspecies, which only inhabit parts of New Zealand

"beak headed" reptiles were once more common throughout the globe. Now though there is only one surviving family, the sphenodontidae. And of it, only one surviving genus: Sphenodon. The tuatara (spiny back).

Enlarged palatine tooth row - allowing the application of three-point bending to food items. This is a unique feature amongst amniotes.

Acrodont dentition - teeth fused to the crest of the jaw bone, with no sockets. These teeth are not usually replaced and tend to be added to the back of the jaw bone as it grows.

Posterior extension of the dentary.



the last common ancestor of Rhynchocephalia and Squamata, plus all its descendants. Lepidosauromorpha contains lepidosaurs and stem-lepidosaurs



crocodiles, alligators, and gharials

The 22 living members of the order Crocodylia are divided into three families:
Gavialidae - containing a single species, the gharial.
Crocodylidae - the crocodiles, comprising 14 species in 3 genera.
Alligatoridae - 7 species of alligators and caimans in 4 genera.


features of crocoylida

The presence of a secondary palate, which acts to separate the oral cavity into two defined chambers:

The nasal passage, travelling from the nostrils to the internal nares at the back of the throat, for breathing.

The mouth, from the mouth opening to the oesophagus, for the mastication of food.

This is an archosaurian derived feature (which convergently evolved in mammals) that allows crocodylians to inhale and exhale solely through their nostrils, and thus eat and breathe simultaneously - a feature usually only seen in endotherms.

Nostrils at the tip of a long snout, connected to the internal nares via the nasal passage created by the secondary palate. This feature acts like a snorkel, meaning crocodylians can lead a semi-aquatic lifestyle - the mouth can be fully submerged yet have no effect on air passing through the nasal passage.

Semi-erect gait: they can pull their bodies upwards, straightening their legs slightly, resulting in a stance halfway between that of a horse and a lizard. This allows more efficient locomotion, as each stride acts more in the direction of travel and can therefore be longer, compared with the semicircular arc of a lizard's sprawling stride. A semi-erect gait also clears the trunk further from the ground, affording increasingly efficient breathing by permitting greater expansion of the lung cavity, as well as a more even compression of the lungs during each stride.





when did birds evolve

Birds evolved from a group of small theropod dinosaurs in the Middle-Late Jurassic


features of aves



Fused clavicles, forming the furcula (wishbone).

Large keeled sternum (breastbone), for the attachment of powerful flight muscles. This keel is greatly reduced in most flightless birds, such as ratites.

No teeth; replaced by a horny beak.

Caudal vertebrae are reduced and fused to form the pygostyle, which supports the tail feathers.

Sacral vertebrae fused to form a synsacrum, which connects with a broad, elongated pelvic girdle and ossified thoracic vertebrae. These features, combined with the pygostyle, form a rigid trunk - an important adaptation for flight stability.

Fused tibia and uppermost tarsals (ankle bones) to form the tibiotarsus - what we call the drumstick. The fibula is reduced and spike-like.

Tarsometatarsus, formed by the fusion of the outermost tarsals with the metatarsals (toe bones).

Reversed hallux (big toe) on feet, specialised for perching.

In most birds, many bones, such as the sternum, pectoral girdle and humeri, are hollow and air-filled, or pneumatic.



which are specialised structures modified from reptilian scales, for insulation, display, camouflage and flight.

The insulation from these feathers allowed birds (and perhaps their dinosaurian ancestors) to become endotherms - allowing them to raise their internal body temperature by retaining the heat energy generated as a by-product of metabolism - a feature they share with mammals.



Wings, formed of the humerus, radius, ulna, wrist and three digits (of these, the first and third are greatly reduced).
The first digit forms the alula, a specialised wing slit, which acts to reduce drag when flying by keeping air close to the wing.


airspaces (sternum, pectoral girdle and humeri) functions

these air spaces have two functions:
They acts to balance out the overall body weight to keep it relatively light for flight, as the bones of the hind limbs are usually solid and heavy to position the centre of gravity on the legs for perching/standing.

They aid in respiration, by acting as a space to contain air sacs involved in the one-way (as opposed to tidal) flow of air through a bird's faveolar lungs.



Sarcopterygii; Tetrapoda; Amniota; Syanpsida

mammals and extinct relatives


synapsid lineages

Monotremata - monotremes
Marsupialia - marsupials
Eutheria - placental mammals


primary difference beween amniotes and sypasida

The primary dichotomy within the amniotes is that between the Reptilia, or Sauropsida (see amniote groups above - turtles through to birds), and the Synapsida, thought to have diverged sometime during the Carboniferous (approx. 360 - 300 million years ago).

Thus, the Synapsida is one of the two major lineages of amniotes, containing the mammals plus all extinct amniotes more closely related to mammals than to reptiles.



The order Monotremata is the only extant group within the subclass Prototheria - the oldest living taxon of the class Mammalia.

Whilst there has never been much doubt that monotremes split off from other mammal group at an early stage, due to their many ancestral reptilian characters

consist of 'egg laying' mammals such as platypus and echdinas


monotremata features

Males have a spur on their ankles, which bears poison in the platypus.
Toothless - platypuses have a leathery electrosensory bill, with crusihing horny plates to break through the tough exoskeleton of arthropods; echidnas have an elongate horny rostrum with a long sticky tongue for collecting insects.


monotremata features (mammalian)

Produce milk (lactate) from mammary glands. However, while therians have nipples, monotremes do not, and consequently the young suck milk from patches of mammary hairs - specialised areas of fur positioned around the ventral openings of the mother's mammary glands.

Epipubic bones - two thin rod-like bones extending anteriorly from the pubic bones of the pelvic girdle.
Lower jaw (mandible) made up of a single bone, the tooth-bearing dentary.

A middle ear formed of three bones: the incus, malleus, and stapes. While the stapes is present in the middle ear of all living tetrapods, the incus and malleus are modified bones from the typical amniote jaw joint.


monotremata features (reptilian)

gg-laying (oviparity); however, these soft-shelled eggs are short-lived, the young hatching after around ten days and being dependent on mother's milk for up to six months after.
Sprawling gait (although it is possible that this is a derived feature of monotremes, relating to specialisations for swimming in the platypus, and for digging in the echidnas).
A single common opening for the digestive, urinary, and reproductive tract, called the cloaca (the general amniote condition). In therian mammals (marsupials and placentals), there are two openings: one for the digestive system, and one for the urogenital tract.
Presence of the coracoid - the ventral bone of the shoulder girdle seen in all non-mammalian amniotes. This is the general amniote condition, where the humerus articulates with the shoulder girdle at the junction between the coracoid and the scapula (commonly termed the shoulder blade).
Not entirely homeothermic. While monotremes are - meaning that they regulate their body temperature using heat produced during endothermicmetabolism - they are fairly poor at maintaining a constant body temperature during extreme enviromental conditions.


how do jaws of nonmammlian amniotes and mammals differ

the jaws of non-mammalian amniotes articulate via the quadrate of the upper jaw, and the articular of the lower jaw;

in mammals, the quadrate migrated to form the incus, while the articular became the malleus, leaving a jaw joint formed of the dentary articulating with the squamosal (the angular bone of the non-mammalian amniote lower jaw is used as a bony support for the eardrum in mammals).



sister taxon to the subclass Eutheria (placental mammals), together forming the clade Theria

kangaroos, possums, marsupial moles, bandicoots, and carnivorous marsupials


marsupial feature

After a brief gestation in the womb with no placenta (except bandicoots), the females of certain species give birth to extremely immature young, who then develop to maturity through suckling in the mother's pouch.

In-turned process in the dentary.

Three premolars and four molars. (NB. This is the ancestral marsupial dentition, however, many species have actually modified this character to have a varying number of molars and premolars)

Epipubic bones - two thin rod-like bones that extend anteriorly from the pubic bones of the pelvic girdle. These bones, although often called marsupial bones, are also seen in monotremes, and are probably a ancestral mammalian character, which was lost in the Eutheria.



are placental mammals (e.g. placentals; primates)