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Amphibians and Reptiles / Vodozemci i gmizavci

Quite unjustifiably, amphibians and reptiles are two the least favourite groups of animals on the planet. Both of these groups create the sense of odiousness and fear in the minds of most people. The basis of such prejudices lies in the lack of knowledge on their actual values and role in nature as well as in exaggerating of the threat that they can impose to us.

Amphibians are one of the most significant links in the chain of development of land vertebrates, having in mind that they are the first land vertebrates on Earth. They developed out of fish Sarcopterygians group of fish that had extinct long time ago and that were “attributed the merit” for breaking the barrier between the aquatic and air environment.

Developing throughout the period of more than 500 million years, they have preserved their primordial link with their ancestors via the unbreakable bond between the aquatic environment and their way of reproduction. Since they lay unprotected eggs in the external aquatic environment, their larvae, which breathe on gills, need a sufficient quantity of moisture for their full transformation – metamorphosis. In the larval stage, prior to metamorphosis, young amphibians depend on aquatic environment while in the later, adult stage, they frequently wander far from water. In the mating period, the “call of the water” makes them return to places they had been “born at” so that they could start a new cycle of life all over again.

On the other hand, the role of reptiles is also very important because they are the first group of vertebrates, which separated completely from the aquatic environment and “went to live on the land”. After the interior insemination, reptiles lay specially protected eggs on land and fully formed young individuals leave out of them. The occurrence of the so-called amniotic egg in which an embryo can breathe, “swim”, and feed is to be accredited merits for such fundamental break up with the aquatic environment.

Amphibians and reptiles, as ancient groups of vertebrates, were at their peak several hundreds of millions of years ago – amphibians about 300 million years ago and reptiles more than 200 million years ago.

Despite all that, amphibians with less than 6,000 and reptiles with about 6,500 recent species still make an integral part of variety richness of wildlife on Earth.

Amphibians (Amphibia) inhabit almost the whole territory of the planet today, with the exception of the coldest and driest areas and some ocean islands and, naturally, seas and oceans. The current amphibians are classified in three orders.

Caecilians (Gymnophiona) are the smallest group of amphibians according to the number of taxons. There are about 170 species in six families. Their distribution is of circum-tropical type and they can be found in South America, Africa, at Seychelles and in India. These are tropical amphibians with elongated body and reduced extremities, as well as with the tail region in some cases. Inside the skin, there are the ring-shaped folds, or annuli, often with calcite scales. Small eyes protected by skin or bones are found on a firm, blunt head. Depending on the way of life, they can be aquatic, terrestrial, and fossorius. They used to be found even at 3,000 m of altitude, in the Ands region.

Caudate or tailed reptiles have elongated, cylindrical body, most often with side slender tail. With rare exceptions, they have two pairs of short limbs set at the body side.

This group is also characterised by very interesting biological phenomenon of paedomorphosis. Paedomorph forms of tailed amphibians retain larval characteristics throughout their lives (external gills, for example), they become sexually mature and reproduce as larvae. True causes of this phenomenon have not been clarified fully yet, but it is considered that the most significant role can be attributed to some external factors that act together with changes in the internal hormonal regulation of metamorphosis.

There are 10 families with about 520 species that inhabit different aquatic and humid terrestrial habitats. They are mainly limited to the north hemisphere and they are the most numerous in the North and Central America. The most southern parts they reach are central parts of South America.

The main characteristic of tail-less amphibians (Anura) is their specific locomotion method: jumps on the land and swimming, namely diving in aquatic habitats. The adaptation to such locomotion method is reflected in the tail region reduction and elongation of hind limbs. The distribution of this exceptionally numerous group, with about 5,000 species, is almost cosmopolitan. They cannot be fauna amongst wildlife at the Arctic, Antarctic, some ocean islands, and exceptionally arid desert areas. The highest diversity of Anura species is found in tropical regions (more than 80% of the total number of species). A small number of frogs (Rana temporaria, for example) also cross into the Arctic polar circle surviving in hibernation stage even the temperatures of up to - 6^OC.

They can be found in highly diverse habitats. They live in both land and aquatic habitats and there are also species that spend most of their lives on trees or buried under the ground. Aquatic larvae (tadpoles, occasionally known as polliwogs) are mainly of round and short body, with internal gills. Most larvae are herbivorous. In some groups, tadpoles eat dead animals, while there are also known cases of carnivorous feeding or plankton use (filtration feeding).

Adult frogs are most often carnivorous and they feed on invertebrates, mostly insects, worms, snails, spiders, centipedes, and even other vertebrates. Cannibalism is not rare in this group either. Their prey can be any animal they can subdue and swallow. It is interesting that there are completely herbivorous species, such as one specie from Hyliidae, Hyla (xenohyla) truncata family that feeds on fruit, and Euphlyctis hexadactylus frog the diet of which is made up of aquatic vegetation.

Regretfully, amphibians are the only basal group of vertebrates that is considered to be endangered at the global level nowadays. It is considered that more than 30% of the total number (meaning, more than 1,850) species of amphibians are endangered all over the planet, and certain number of them has been irreversibly lost. Although concrete figures on the extinct species are not known, it is estimated that the current rate of extinction is higher than ever within the last 100,000 years. It is estimated that more than 500 species of frogs and tailed amphibians were reduced in number until the end of the last century or they were classified in the category with special conservation status.

Reptiles (Reptilia) are similar to amphibians in as much they represent a specific group of vertebrates that marks the overall development of wildlife on the plant in a unique way. Out of 12 groups of reptiles in total, the largest number of which ruled the planet for many years (almost 160 million years) only four orders have remained until today.

The first order – turtles (Testudines) are characterised by a thickset body that is shielded by a special bony or cartilaginous shell that is open at both ends that parts of the skeleton are attached to (spine, ribs etc.) Turtles do not have any teeth and their jaws are covered with horny ridges in the form of a beak. Land turtles feed mainly on juicy parts of plants while salt and fresh water turtles are carnivorous. Today, we know about 240 species of turtles classified in 12 families. Better known families include terrestrial turtles Testudinidae that live mainly in tropical and sub-tropical regions, but they can also be found at all the continents except for Australia and Antarctic. Snake-necked freshwater and land Chelidae inhabit Australia, New Guinea and South America. Freshwater Emydidae live in streams, rivers, lakes, and ponds of all the continents except for Australia and sub-Saharan part of Africa. Families of Dermochelyidae and Cheloniidae are marine groups of turtles. The leatherback turtle (Dermochelys coriacea) is the only specie from Dermochelyidae family and it is the largest marine turtle. It can reach the length of 2.5 meters and weight of more than 900 kilograms. This is the widest spread specie of reptiles in general. It mainly inhabits tropical and sub-tropical seas, reaching the polar circle in the north and New Zealand in the south.

The second order - Tuataras or Sphenodontias, represents a small group of primitive reptiles. Current representatives are the remaining successors of a group of reptiles that used to be vastly spread in the past (about 200 million years ago). They can be found only at New Zealand and some nearby islands.

The third order – Squamata includes lizards, snakes, and Amphisbaenia With about 6,400 species they make far most numerous and widest spread group of reptiles.

Amphisbaenia are a small group of completely fossorious reptiles that inhabit parts of the North, Central and South America, Africa, southern Europe as well as India and south-west Asia. There are about 160 known species today. It is interesting that they often crawl backwards as well and hence they were named after that phenomenon (”both ways”). Due to this quality, they take a significant place in mythology of ancient and medieval authors among imagined monsters, such as amphisbaena snake with a head at each end of its body.

The number of lizard species is estimated to about 4,700 today. They are widely spread and they inhabit very different habitats. Many of them are terrestrial, arboreal, fossorius as well as aquatic. A large majority of them are carnivorous although some are also herbivorous. They can be numerous even in the driest parts of the planet and climb up to 5,000 m of altitude.  Many lizards have the ability to reject a part of their tail when they are attacked or endangered. The rejected part of the tail continues moving for a while and draws the attention of the attacker. After rejection, the tail is regenerated as cartilaginous extension.

Iguanas with about 550 species are the oldest out of 18 families that are known today. They are the most significant representatives of fauna at Madagascar, Fiji, Tonga, and Galapagos islands. They are active during the day and they live either on the land or on trees. Iguanas feed mostly on insects. Herbivorous iguanas from Galapagos Islands are exceptionally interesting since they feed on marine algae. They are the only lizards that enter the sea in their search for food.

Among numerous families we can also single out Agamidae with about 315 species, which are similar to iguanas, in particular when it comes to different scale crests, buns, and fans underneath their throat that can be inflated as well as intensive colours of males.

Chameleons are attractive lizards with two or three fingers, long tails, and tongue that they use to catch insects at a large distance. The eyes are raised on small handles and they can move independently one from the other. They are also known for fast changes of their body colour.

Geckos (or gekkos) have specific elongations on their fingers and sharp claws that enable them to move even along completely flat surfaces. Many geckos live in colonies. Contrary to most lizards, they have an emphasised vocalisation ability.

Varans are very old lizard group and some representatives can reach the length of up to three meters.

Most of our lizards belong to Lacertidae family that inhabits Europe, Africa, and Asia. The most numerous among them are Scincidae with more than 1,200 species. The rarity among them is Prasinohaema virens, which is unique among vertebrates because they have a green pigment in their blood.

Contrary to snakes, the number of poisonous lizards is very small, i.e. there are only two: large gila and Mexican lizard, but it has never been confirmed that either of them was the cause of death in humans.

Snakes are particularly well-known and the least favoured among people. The limbs in most of them are completely reduced and they move via contractions of body and tail muscles assisted by ventral scale plates. They are widely spread. They can be found in different biotopes of almost all parts of the world. Snakes cannot be found in arctic and Antarctic regions, at Island, in Ireland, New Zealand and some smaller ocean islands. Today, the snakes are classified in 15 families with about 3,000 species. They are also specific among vertebrates by the fact that some of them have glands creating poisonous substances. The poison is applied through specifically adapted teeth that are located on jaw bones.

Typhlopidae family includes small warm-like snakes that spend most of the time under the ground so that their eyes are stunted and they feed mainly on ants and termites.

Boidae mainly inhabit tropical and sub-tropical regions all around the world, as well as the western parts of North America. They are rarely smaller than one meter, while anacondas and pitons can reach the length of up to 10 m. In addition to that, pitons also show the amazingly complex behaviour patterns during egg keeping.

Colubridae family is the most numerous (about 1,400 species) and most widely spread. The largest number of our and European snakes belongs to this family. They cannot be found at Antarctic and at several ocean islands. They are prevailingly terrestrial although they can also be freshwater and marine animals. The largest number of these snakes is without the poison carrying apparatus.

Elapida family lives in tropical and sub-tropical regions. This group also includes many poisonous species such as cobras, coral snakes, and mambas.

Hydrophidae snakes are the most numerous in the south-west Pacific and to the east from Australia. They have a poison carrying apparatus and they can survive under water for several hours.

Viperidae and Crotalidae are groups of poisonous snakes. Viperidae can be found in Africa and Euro-Asia, from England to Japan, while Crotalidae are the most numerous in America where they are considered the most dangerous.

The fourth order of the reptile class includes crocodiles (Crocodylia) that are very similar to ancient, prehistoric lizards from the Mesozoic period. Crocodiles that live today represent the last remains of this ancient group of reptiles, which is obviously declining in number. It is considered that 27 living species from three families represent only 10% of the total number of species of these reptiles known in fossil stadium. They are all carnivorous and are known for their amphibian way of life. Their skin contains wide scale plates covered with strong thick skin. Their body is large and it ends with side flattened tail that is exceptionally favourable for swimming. Their skull is wide and flat. Interestingly enough, a larger number of skull bones are pneumatised, i.e. intertwined with air filled spaces that are connected to the middle ear and nasal cavity. Their mouth is filled with a large number of strong conical teeth.

Although a large number of species lives in tropical rivers of all the continents except for Europe, the largest number of these large Crocodilus porosus reptiles (up to 10 m) lives in salt water, and they can be found even up to 300 km away from the nearest land.

Alligators and caimans (Alligatoridae) are different from crocodiles by the fact that their fourth tooth cannot be seen when their jaws are closed. Alligators reach the maximum length of up to 5.5 m. Caimans have the widest skull among the contemporary crocodiles and they inhabit Mexico, South America, and Caribbean.

Contrary to alligators and caimans, which inhabit freshwater eco-systems, the Crocodylidae family lives in salt waters. They are widely spread in Indo-Pacific region and they can also be found in Indo-Australian archipelagos, all up to north Australia. They also inhabit the seas along the shores of south Florida, Caribbean, and north part of South America. The best known among them is the Nile crocodile (Crocodylus niloticus), which is widely spread around Africa, as well as at Madagascar island.

The Gavialis gangeticus specie is the only living representative of gavial family today. It used to inhabit large rivers of India and the surrounding regions, but the area it inhabits today is much smaller. Their snout is narrow and elongated, which is the adaptation to the fact that they feeds on fish they hunt by sudden side head swiping. They reach the length of more than six meters.

Similar to amphibians, this group is generally endangered due to the loss of their habitats, global climate changes, general pollution of eco-systems, hunting and poaching, settling of alochthonous (foreign) species. Due to all that, many species are extremely endangered and some have been irreversibly extinct.

Vodozemci i gmizavci su, krajnje neopravdano, dve najmanje omiljene grupe životinja na planeti. Obe ove grupe u svesti većine ljudi stvaraju osećaj odbojnosti i straha. Osnova ovih predrasuda leži u nepoznavanju njihovih stvarnih vrednosti i uloge u prirodi kao i u preuveličavanju opasnosti koja može da nam preti od njih.

Vodozemci su jedna od najznačajnijih karika u lancu razvoja kopnenih kičmenjaka, s obzirom na to da su upravo oni prvi kopneni kičmenjaci na zemlji. Razvili su se od davno izumrlih riba iz grupe šakoperki i „zaslužni su“ za probijanje barijere između vodene i vazdušne sredine.

Kroz istorijski razvoj dug više od 500 miliona godina zadržali su svoju iskonsku povezanost sa precima preko neraskidive veze između vodene sredine i njihovog načina razmnožavanja. Pošto polažu nezaštićena jaja u spoljašnju vodenu sredinu, njihovim larvama, koje dišu na škrge, neophodna je dovoljna količina vlage za potpuni preobražaj – metamorfozu. U larvenoj fazi pre metamorfoze mladi vodozemci su zavisni od vodene sredine dok se u odraslom stadijumu često udaljavaju daleko od vode. U periodima parenja „zov vode“ vraća ih na mesta gde su „se rodili“ da ponovo započnu novi ciklus života.

S druge strane, uloga gmizavaca je takođe veoma značajna jer je to prva grupa kičmenjaka koja se u potpunosti odvojila od vodene sredine i „izašla na kopno“. Nakon unutrašnjeg oplođenja gmizavci polažu posebno zaštićena jaja na kopnu, iz kojih izlaze potpuno formirane mlade jedinke. Zaslugu za ovakav fundamentalan raskid sa vodenom sredinom ima pojava amniotskog jajeta u kome embrion diše, „pliva“ i hrani se.

Vodozemci i gmizavci kao prastare grupe kičmenjaka svoj procvat doživele su pre više stotina miliona godina – vodozemci pre oko 300, a gmizavci pre oko 200 miliona godina.

I pored toga vodozemci sa nešto manje od 6000 i gmizavci sa oko 6500 recentnih vrsta i danas čine neodvojivi deo celine bogatstva živog sveta na zemlji.

Klasa vodozemaca (Amphibia) danas naseljava skoro celokupnu teritoriju planete sa izuzetkom najhladnijih i najsuvljih predela kao i nekih okeanskih ostrva, a nema ih, naravno, u morima i okeanima. Današnji vodozemci svrstani su u tri reda.

Cecilije (Gymnophiona) po broju taksona najmanja su grupa vodozemaca. Postoji oko 170 vrsta u šest porodica. Njihova rasprostranjenost je cirkumtropska i nalaze se u Južnoj Americi, Africi, na Sejšelima i u Indiji. Ovo su tropski vodozemci izduženog tela sa redukovanim ekstremitetima, a kod nekih grupa i repnim regionom. U koži poseduju prstenaste nabore u kojima se često nalaze koštani elementi. Na čvrstoj, zatupastoj glavi nalaze se sitne oči presvučene kožom ili kostima. Po načinu života mogu biti akvatični, terestrični i fosorijalni. Nalaženi su i na 3000 m nadmorske visine, u predelu Anda.

Caudata ili repati vodozemci imaju izduženo, valjkasto telo sa dugačkim najčešće bočno spljoštenim repom. Sa retkim izuzecima imaju dva para kratkih ekstremiteta postavljenih sa strana tela.

Ovu grupu karakteriše i veoma interesantna biološka pojava pedomorfoze. Pedomorfni oblici repatih vodozemaca zadržavaju celog života larvene karakteristike (spoljašnje škrge, na primer), postaju polno zreli i razmnožavaju se kao larve. Pravi uzroci ove pojave još nisu do kraja razjašnjeni, ali se smatra da najveću ulogu mogu imati neki spoljašnji faktori u sadejstvu sa promenama u unutrašnjoj hormonalnoj regulaciji metamorfoze.

10 porodica sa oko 520 vrsta naseljava različita akvatična i vlažna terestrična staništa. Uglavnom su ograničeni na severnu hemisferu i najmnogobrojniji su u Severnoj i srednjoj Americi. Najjužniji delovi do kojih dopiru su središnji delovi Južne Amerike.

Osnovna karakteristika bezrepih vodozemaca (Anura) jeste specifičan način kretanja: skokovi u suvozemnim uslovima i plivanje, odnosno ronjenje u akvatičnim sredinama. Prilagođenost ovakvim načinima kretanja ogleda se u redukciji repnog regiona i izduživanju zadnjih ekstremiteta. Ova izuzetno mnogobrojna grupa sa preko 5000 vrsta ima skoro kosmopolitsku rasprostranjenost. Izostaju u fauni Arktika, Antarktika, nekih okeanskih ostrva i izuzetno aridnih pustinjskih predela. Najveći diverzitet vrsta Anura jeste u tropskim regionima (više od 80% ukupnog broja vrsta). Mali broj vrsta žaba (Rana temporaria, npr.) prelaze i arktički polarni krug, izdržavajući u stanju hibernacije temperature do - 6^OC.

Mogu se naći u veoma raznolikim staništima. Žive i u kopnenim i u vodenim staništima, a postoje i vrste koje veći deo života provode na drveću ili ukopane pod zemljom. Akvatične larve (punoglavci) pretežno su oblog i kratkog trupa, sa unutrašnjim škrgama. Većina larvi se hrani biljnom hranom. Kod nekih grupa punoglavci se hrane uginulim životinjama, dok su poznati i slučajevi karnivorske ishrane ili korišćenja planktona (filtracione ishrane).

Odrasle žabe su, najčešće, karnivori i hrane se beskičmenjacima, najviše insektima, glistama, puževima, paucima, stonogama, čak i drugim kičmenjacima. U ovoj grupi nije redak i kanibalizam. Njihov plen je svaka životinja koju mogu da savladaju i progutaju. Zanimljivo je da postoje i potpuno herbivorne vrste kao što su jedna vrsta iz porodice Hyliidae, Hyla (Xenohyla) truncata, koja se hrani voćem, i žaba vrste Euphlyctis hexadactylus, čiju ishranu čini vodena vegetacija.

Nažalost, vodozemci su jedina osnovna grupa kičmenjaka koja je, kako se to danas smatra, globalno ugrožena. Smatra se da je na celoj planeti čak više od 30% ukupnog broja (više od 1850) vrsta vodozemaca ugroženo, od kojih je jedan broj i nepovratno nestao. Iako konkretne brojke o nestalim vrstama nisu poznate, procenjuje se da je sadašnji stepen nestajanja veći od ikada spoznatog u poslednjih 100 000 godina. Procenjuje se i da je do kraja prošlog veka  preko 500 vrsta žaba i repatih vodozemaca u opadanju po brojnosti ili su svrstane u kategoriju sa specijalnim konzervacijskim statusom.

Klasa gmizavaca (Reptilia) slično vodozemcima predstavlja specifičnu grupu kičmenjaka koja na sebi svojstven način obeležava celokupni razvoj živog sveta na planeti. Od ukupno 12 grupa gmizavaca, čiji je najveći broj dugi niz godina (skoro 160 miliona godina) vladao planetom, danas su ostale svega četiri reda.

Prvi red – kornjače (Testudines) odlikuju se zdepastim telom koje je zaštićeno koštanim oklopom otvorenim sa oba kraja, za koji se vezuju i delovi skeleta (kičmenica, rebra itd.). Kornjače nemaju zube, već su im vilice presvučene rožnim navlakama u obliku kljuna. Kopnene kornjače se hrane pretežno sočnim biljnim delovima, dok su morski i slatkovodni predstavnici mesojedi. Danas poznajemo oko 240 vrsta svrstanih u 12 porodica. Među poznatije porodice spadaju i terestrične kornjače Testudinidae, pretežno tropskih i suptropskih predela, ali se mogu naći na svim kontinentima izuzev Australije i Antarktika. Zmijovrate slatkovodne i kopnene Chelidae naseljavaju Australiju, Novu Gvineju i Južnu Ameriku. Slatkovodne Emydidae žive u potocima, rekama, jezerima, barama svih kontinenata izuzev Australije i podsaharskog dela Afrike. Marinske su grupe kornjača porodica Dermochelyidae i Cheloniidae. Jedina vrsta porodice Dermochelyidae jeste kornjača lauta (Dermochelys coriacea), najveća morska kornjača. Može dostići dužinu od 2,5 metra i težinu preko 900 kilograma. To je i najšire rasprostranjena vrsta gmizavaca uopšte. Naseljava uglavnom tropska i suptropska mora, dopirući na sever do polarnog kruga, a na jug do Novog Zelanda.

Drugi red Tuatare ili Sphenodontia predstavljaju malu grupu primitivnih gmizavaca. Današnji predstavnici su preostali potomci nekad (pre oko 220 miliona godina) veoma raširene grupe gmizavaca. Mogu se naći samo na Novom Zelandu i nekim obližnjim ostrvima.

U treći red – Squamata spadaju gušteri, zmije i amfizbene. Sa oko 6400 daleko su najmnogobrojnija i najrasprostranjenija grupa gmizavaca.

Amfizbene su mala grupa potpuno fosorijskih gmizavaca koji nastanjuju delove severne, srednje i južne Amerike, Afrike, južne Evrope kao i Indije i jugozapadne Azije. Danas je poznato oko 160 vrsta. Interesantno je da one često puze i unazad, po čemu su i dobile ime („oba smera“). Zbog te osobine u mitologiji staro- i srednjovekovnih pisaca zauzimaju značajno mesto među izmišljenim čudovišnim stvorenjima, kao zmija amfizbena koja ima glavu na oba kraja tela.

Broj vrsta guštera danas je procenjen na oko 4700. Široko su rasprostranjeni i nastanjuju veoma različita staništa. Mogu biti terestrični, arborealni, fosorijski, ali i akvatični. Velika većina su karnivori, ali ih ima i herbivora. Mogu biti mnogobrojni i u najsuvljim delovima planete, a penju se i do 5000 m nadmorske visine. Mnogi gušteri imaju sposobnost odbacivanja dela repa kada su napadnuti ili ugroženi. Odbačeni deo repa se još neko vreme pokreće, čime odvlaći pažnju napadača. Posle odbacivanja rep se regeneriše kao hrskavičav nastavak.

Od 18 danas poznatih porodica najstarije su iguane sa oko 550 vrsta. Oni su najznačajniji predstavnici faune Madagaskara, ostrva Fidžija, Tonge i Galapagosa. Aktivni su danju, žive na kopnu ili drveću. Većinom se hrane insektima. Izuzetno zanimljive su herbivorne iguane sa Galapagoskih ostrva, s obzirom na to da se hrane morskim algama. To su i jedini gušteri koji u potrazi za hranom zalaze u more.

Među mnogobrojnim porodicama interesantne su i Agamide sa oko 315 vrsta, slične  iguanama, naročito po različitim kožnim krestama, ćubama, lepezama ispod grla koje se mogu naduvati kao i intenzivnoj obojenosti mužjaka.

Hameleoni ili kameleoni su atraktivni gušteri sa dva ili tri prsta, dugim repom i jezikom kojim hvataju insekte i na velikoj udaljenosti. Oči podignute na male drške mogu da se pokreću nezavisno jedno od drugog. Poznati su i po brzoj promeni boje tela.

Gekoni imaju posebna proširenja na prstima i oštre kandže, koje im omogućavaju da se kreću i po potpuno ravnim površinama. Mnogi gekoni žive u kolonijama. Za razliku od većine guštera, imaju izraženu sposobnost oglašavanja.

Varani su veoma stara grupa guštera, a pojedini predstavnici mogu da dostignu dužinu i do tri metra.

Većina naših guštera spada u porodicu Lacertidae koji naseljavaju Evropu, Afriku i Aziju.

Najmnogobrojnije su Scincidae sa preko 1200 vrsta, a kuriozitet među njima je Prasinohaema virens, jedinstven među kičmenjacima po tome što ima zeleni pigment u krvi.

Za razliku od zmija, veoma je mali broj otrovnih guštera, svega dva: velika gila i meksički gušter, ali za njih nikad nije utvrđeno da su izazvali smrt kod ljudi.

Zmije su posebno dobro poznate i neomiljene među ljudima. Ekstremiteti su kod velike većine potpuno redukovani i kreću se kontrakcijama muskulature trupa i repa potpomognuti ventralnim rožnim pločicama. Široko su rasprostranjeni. Nalaze se u različitim biotopima skoro svih krajeva sveta. Izostaju u arktičkom i antarktičkom području, na prostorima Islanda, Irske, Novog Zelanda i nekih manjih okeanskih ostrva. Današnje zmije su svrstane u 15 porodica sa oko 3000 vrsta. Zmije su posebne među kičmenjacima i po tome što kod nekih postoje žlezdani organi koji stvaraju otrovni sekret. Otrov se unosi specifično izmenjenim zubima koji se nalaze na viličnim kostima.

U porodicu Typhlopidae spadaju male crvolike zmije koje najveći deo vremena provode ispod zemlje, oči su im zakržljale a hrane se uglavnom mravima i termitima.

Udavi ili Boidae uglavnom naseljavaju tropske i suptropske regione širom sveta kao i zapadne delove Severne Amerike. Retko su manji od jednog metra, dok anakonde i pitoni mogu da dostignu dužinu i do 10 m. Dodatno pitoni pokazuju i zadivljujuće složene obrasce ponašanja prilikom čuvanja jaja.

Porodica Colubridae je najmnogobrojnija (oko 1400 vrsta) i najrasprostranjenija. Ovoj porodici pripada najveći broj naših i evropskih zmija. Nedostaju na Antarktiku i na nekoliko okeanskih ostrva. Pretežno su terestrične, ali ih ima i slatkovodnih i morskih. Najveći broj ovih zmija nema otrovni aparat.

Porodica Elapida je rasprostranjena u tropskom i suptropskom pojasu. U ovu grupu spadaju i mnoge otrovne vrste kao što su kobre, koralne zmije, mambe.

Zmije mora (Hydrophidae) najmnogobrojnije su u jugozapadnom Pacifiku i istočno od Australije. Imaju otrovni aparat, a pod vodom mogu izdržati i nekoliko sati.

Viperide i Crotalide su grupe otrovnih zmija. Viperide se mogu naći u Africi i Evroaziji od Engleske do Japana, dok su Crotalide najmnogobrojnije u Americi, gde ih smatraju i najopasnijima.

Četvrti red klase gmizavaca krokodili (Crocodylia) veoma su slični prastarim gmizavcima srednjeg doba (mezozoik). Današnji krokodili predstavljaju poslednje ostatke ove prastare grupe gmizavaca, koja je u očiglednom opadanju. Smatra se da današnjih 27 vrsta iz tri porodice predstavljaju tek 10% od ukupnog broja vrsta ovih gmizavaca poznatih u fosilnom stanju. Svi su mesožderi i imaju amfibijski način života. Njihova koža sadrži široke koštane ploče presvučene snažnom rožnatom navlakom. Telo im je krupno i završava se snažnim bočno spljoštenim repom veoma pogodnim za plivanje. Lobanja im je široka i zaravnjena i, što je zanimljivo, veći broj kostiju lobanje je pneumatizovan, odnosno prožet vazdušnim prostorima koji su u vezi sa srednjim uhom i nosnim prostorima. U ustima imaju veliki broj snažnih koničnih zuba.

Iako veći broj vrsta živi u tropskim rekama svih kontinenata izuzev Evrope, najveći od ovih velikih gmizavaca Crocodilus porosus (do 10 m) živi i u slanoj vodi, a može se sresti i do 300 km daleko od najbližeg kopna.

Aligatori i kajmani (Alligatoridae) razlikuju se od krokodila po tome što im se četvrti zub ne vidi kada su vilice zatvorene. Aligatori dostižu maksimalnu dužinu od 5,5 m. Kajmani imaju najširu lobanju među današnjim krokodilima, a naseljavaju Meksiko, Južnu Ameriku i Karibe.

Za razliku od aligatora i kajmana, koji nastanjuju slatkovodne ekosisteme, porodica Crocodylidae živi u slanim vodama. Široko su rasprostranjeni u indopacifičkom regionu, a mogu se naći i u indoaustralijskom arhipelagu, sve do severne Australije. Nastanjuju i mora oko obala južne Floride, Kariba i severnog dela Južne Amerike. Najpoznatiji je nilski krokodil (Crocodylus niloticus), koji je široko rasprostranjen po Africi, ali i na ostrvu Madagaskaru.

U okviru porodice gavijala danas živi samo jedna vrsta Gavialis gangeticus. Nekada je  naseljavo velike reke Indije i okolnih područja, dok mu je danas areal mnogo manji. Njuška im je uska i izdužena, što predstavlja prilagođenost na ishranu ribama, koje love naglim bočnim trzajem glave. Dostižu dužinu i preko šest metara.

Slično vodozemcima, i ova grupa je uopšte ugrožena gubitkom staništa, globalnim klimatskim promenama, opštim zagađenjem ekosistema, lovom i krivolovom, naseljavanjem alohtonih (stranih) vrsta. Usled toga mnoge vrste su krajnje ugrožene, a neke su i bespovratno nestale.