IntroductionReptiles evolved in theLate Carboniferous with the widely accepted date of 315 Ma. The oldest identifiedreptile is the Hylonomus lyelli (Sigurdsen & Carroll, 2016).
To this date,it is the oldest unquestionable reptile to have evolved, developing from reptiliomorphs.However, there are reptile fossils that pre-date Hylonomus. Westlothiana lizziaeis one, which when discovered was classified as the oldest, evolving 350 Ma (Carroll,1982).Reptiles are air-breathingvertebrates descended from tetrapods and allowed the evolution of mammals andbirds. Reptiles belong to the group Amniota along with birds and mammals(Benton & Spencer, 1995). The main characteristics of amniotes are theireggs and the ability of internal fertilisation (Reisz, 1997). Reptiles do not maintaina constant body temperature (Spelman, 2012). They are ectotherms (Shine, 2005),that is, they move into sunlight to warm up and use shade and water to cool(Spelman, 2012).
When temperatures get too low, reptiles become inactive byslowing down their metabolism, a process reptiles have control of (Spelman,2012). The ability to regulate their body temperature is used to simply reproduce,collect and digest food (Huey, 1982). The ability of reptiles to alter their metabolicrate means they can go for long periods of inactivity followed by short periodsof activity (Shine, 2013). The control of their metabolic rates means reptilesuse less of the environment resources than mammals and therefore consume lessenergy (Huey, 1982). The slow metabolic rates allow reptiles to feed only on afew types of prey which may be available only at certain times of the year(Shine, 2005).
The purpose of thisreport is to outline how reptiles evolved. More specifically, I focus on the keyevolutionary advancements that were essential for the evolution of reptiles andthe point which reptiles evolved. Evolutionaryadvances leading to reptile evolutionThe key evolutionarysteps essential for reptiles to evolve was the transition of life from water toland along with the development of lungs and subsequent loss of gills, thusallowing animals to breathe on land. Additionally, the development of limbs toaid in movement across the terrestrial landscape and the ability to either layeggs on land without drying out or being able to give birth.FishDuring the Devonian (400Ma), bony fish were dominant in oceans.
The two large groups were the lobefinned fish and the ray finned fish. A key feature of lobe fins was that, alongwith gills, they possessed lungs, allowing them to breathe in air and water(Lambert et al., 2003). This became highly advantageous when water bodiesbecame stagnant resulting in the reduction of oxygen levels in the water, becauselobe and ray fins could swim above the water level and breathe in atmosphericoxygen (Romer, 1957). Due to the development of lungs, the descendants ofOsteolepis (descendants of lobe fins) began developing limbs from some of theirfins whilst other fins began shrinking (Lambert et al., 2003).
Through theevolution of limbs, it enabled the fish to move to other water bodies when thecurrent one dried out during seasons of drought (Romer, 1957).TetrapodsTetrapodsevolved from lobe fins. The early tetrapods still lived in the water and theirlimbs were used to travel faster in water due to their paddle shape (as seen inFigure 1). They still possessed gills and tail fins along with distinct digits whichdeveloped from fin bones (Lambert et al., 2003). The Elginerpeton from Scotland(as seen in Figure 1) is an early tetrapod possessing the paddle morphology. Eventuallythe tetrapods migrated to the terrestrial landscape evidenced by footprintsfound in Australia from the Late Devonian.
Reptiliomorphs (tetrapodsmore related to amniotes than to amphibians) (Sigurdsen & Carroll, 2016) arethe ancestors of amniotes but are also amniotes (Lambert et al., 2003). Once onland, their skeletons needed to adapt to the increased gravity, allowing them tocarry their own weight. From tetrapods, amphibians evolved who could live onland but could not leave the water permanently.Reptiles evolved from amphibians,through the evolution of the amniotic egg.
From amphibians, there was a split,namely into synapsids and sauropsids, with the sauropsids splitting into anapsidsand diapsids.Synapsids are animals witha single temporal opening in the skull roof (Figure 2 (B)), and includes,mammals. Whereas sauropsids and consequently anapsids had no temporal openingsin the skull roof (Figure 2 (A)), diapsids, had two temporal openings on eitherside of the skull roof (Figure 2 (C)) (Sues, 2016; Sigurdsen& Carroll, 2016). Reptiles, birds and dinosaurs evolved from sauropsids.
Amniotic eggThe main evolutionaryadvancement that allowed the rise of reptiles was the hard-shelled amniotic egg(Figure 3). Amniotic eggs evolved from aquatic animals as a way of protectingthe eggs from aquatic predators (Carroll, 1969). The amniotic egg evolved fromamphibian eggs by the addition of the chorion and the allantois, with the developmentof the amnion following, forming the amniotic egg (Sigurdsen & Carroll,2016). The primary reason why amniotic eggs evolved was to be protected indroughts which were common in the Paleozoic. As water bodies evaporated,amphibian eggs dried up and desiccated thus being destroyed, whilst amnioticeggs remained unaffected (Romer, 1957). The egg contains four membranes thatsurround the embryo: yolk sac, allantois, amnion and the chorion (Reisz, 1997; Benton& Spencer, 1995). The hard shell, although porous, protects the embryo from”mechanical shock” (Romer, 1957) and stops the egg from drying out, but stillallows gas exchange with the surrounding environment (Benton & Spencer,1995). The albumin (egg whites) provides the embryo with water and protein,whilst the large amount of yolk enables the embryo to grow (Romer, 1957).
Inaddition to this, the chorion (allows gas exchange and surrounds the embryo andyolk sac), amnion (liquid-filled sac surrounding the embryo and is theinnermost membrane (Sigurdsen & Carroll, 2016)), and the allantois (sacbeneath the shell, functioning as a lung through gas exchange via the shellpores, as well as acting as a bladder, storing waste products) are extramembranes which are distinctive of amniotic eggs (Romer, 1957), and provide protectionto the egg (Smithson et al., 1994). The amnion as well as being present inhard-shelled eggs is also present in animals that give birth (Sigurdsen &Carroll, 2016). These embryonic membranes however, are unlikely to be preservedas fossils and any eggs from the Carboniferous are rare to be found fossilised (Smithsonet al., 1994; Sigurdsen & Carroll, 2016).
The amniotic egg provided access forreptiles to spread across the terrestrial landscape without returning to waterfor reproduction (Reisz, 1997).The key differencesbetween reptiles and amphibians is the ability of reptiles laying hard-shelledeggs on land without them drying out whilst amphibians lay their eggs in ornear water, relying on the water to provide the embryo with oxygen and food(Sues, 2016). Reptiles do not have skin glands but developed scales in theepidermis made from keratin (Sues, 2016). The primary function of these scales isto prevent water loss (Sues, 2016), therefore, enabling reptiles to live awayfrom water. Environmentand climate needed for reptile evolutionThe Carboniferous isknown for its Coal Forests, primarily because Europe and North America were atthe equator from 318-299 Ma and the tropical temperature allowed the growth ofvast amounts of vegetation creating the rainforests (Sahney et al., 2010).
The rainforestsallowed more niches to become available in which animals could live in. Figure 4highlights the high levels of oxygen during the Carboniferous; peaking at 32%,much higher when compared to the present-day oxygen levels at 21% (NationalGeographic, 2012). In the Late Carboniferous, the temperatures started cooling gradually,shifting towards an arid climate due to the vast swamps taking in large amountsof carbon dioxide. The annual sequestration of carbon at this time was 13-47 x109 tonnes (Cleal, 2017), which drastically reduced the amount ofcarbon dioxide in the atmosphere and thus resulting in cooler temperatures,receding sea levels and glaciation. Ice caps formed over Gondwana and theglacial deposits of this ice cap are found in South Africa as tillites(Falcon-Lang et al., 2010). The climate cooling was advantageous for reptilesas they are more tolerant of colder temperatures than amphibians.
This climate factorfurther allowed their evolution.Whendid reptiles evolveThere are two proposeddates as to when and which animal reptiles evolved from. One is the Hylonomuslyelli which evolved 315 Ma and is the more widely accepted of the two. Theother is the Westlothiana lizziae which evolved 350 Ma. Although it was similarto reptiles, the morphology of Westlothiana was closer to that of amphibians.Early reptiles in general, were similar to modern primitive lizards; they weresmall in size (Carrol, 1982). To first identify whether a vertebrate is anamniote (including reptiles) the vertebrate must have an astragalus in theankle that is formed through the fusion of tarsals, along with ossifiedvertebrae with large pleurocentra (Sigurdsen & Carroll, 2016).Reptile trackways have beenlocated in the Lower Pennsylvanian Grande Anse Formation of New Brunswick, whichunderlies the assemblage in which the Hylonomus was found by 1 km, and so, thissite shows the earliest evidence of amniotes (Falcon-Lang et al.
, 2007). The trackwaysrepresent true tracks because the preservation was on the underside of thebedding plane and in convex hyporelief, along with transverse scale impressionswhich were interpreted to be made from keratin: a distinctive reptilian feature(Falcon-Lang et al., 2007). Table 1 shows the measurements of key features usedto compare amniotes (reptiles) and non-amniotes. The trackways found in NewBrunswick correlates with the measurements of amniotes rather thanreptiliomorphs. This means that the trackways represent early reptiles.
Thelithology in which the trackways were found are associated to an alluvial plainwith an eastward palaeocurrent flow (Falcon-Lang et al., 2007). This impliesthat early reptiles of the Carboniferous lived on dryland like dry river bedsor dry alluvial plains (Falcon-Lang et al., 2007). These trackways provideevidence for the existence of reptiles to coincide with the existence of theHylonomus.WestlothianalizziaeThe articulated skeletonof Westlothiana lizziae (Figure 5) found at East Kirkton quarry, near Bathgate,West Lothian, Scotland in 1988, was considered the oldest known reptile(Smithson & Rolfe, 1990), and is a holotype. The skeleton was found in theUpper Oil Shale Group of the Upper Viséan, and was considered the mostprimitive reptile, with it being 20 cm long, containing 48 simple conical teethin each jaw and having forelimbs shorter than its hindlimbs (Smithson et al.,1994).
The site itself is dated to 335 Ma and is a part of the lower Brigantian,from the Viséan in the Lower Carboniferous (Sigurdsen & Carroll, 2016).The reasons why Westlothianawas considered a reptile is partly because the skull resembles those of otheramniotes: the large size of the parietal, and the lack of the intertemporal,squamosal notch and labyrinthine infolding of the teeth (Smithson et al., 1994).Westlothiana had a short tibia and fibula, characteristic of amniotes (of whichreptiles are part) (Smithson & Rolfe, 1990). The skull roofing bones had a smoothsurface typical of reptiles and did not have a dermal ornament which istypically found in tetrapods (Smithson et al., 1994). The skull itself resembledthat of both tetrapods and amniotes (Sigurdsen & Carroll, 2016). It alsoshared a trait with Late Carboniferous amniotes: the cheek was loosely attachedto the back of the skull table (Smithson et al.
, 1994). Westlothiana hadoverlapping oval shaped dorsal scales which were relatively thick and coveredthe back of the skull and parts of the tail, however, due to the thickness and heavylayering of the scales, it is classed as a primitive development which was notin amniotes (Smithson et al., 1994).
However, there is adebate over whether Westlothiana is a reptile because it has not been directly determinedthe type of eggs it laid (amphibian or amniotic). Key features of the skullwere not well preserved and so it was inconclusive what precise group Westlothianabelonged to, other than that it was not an amniote. To accurately place Westlothiana,more specimens are required (Reisz, 1997). With further research, it has beendetermined that Westlothiana is a reptiliomorph and is a stem group from amniotes(Smithson et al., 1994). It is a stem group because it does not have an oticnotch, gastrocentrous vertebrae or the pedal phalangeal formula similar to amniotes,but cannot be classified as an amniote because the structure of the palate andtarsus are too primitive than defined early amniotes (Smithson et al., 1994).
Also, Westlothiana has lamellae along the vertebrae which is not present inearly amniotes like the Hylonomus (Smithson et al., 1994). When Westlothianawas first described, it was stated to have two large proximal elements in eachtarsus: the astragalus and the calcaneum (Smithson, 1989), a distinctive featureof all amniotes.
However, further study showed a third proximal tarsal: tibiale,as seen in Figure 6. What was previously regarded as the astragalus andcalcaneum were actually the intermedium and fibulare respectively (Smithson etal., 1994).
Another feature which Westlothiana does not possess, is the pterygoidflange which is key in early amniotes (Smithson et al., 1994). Westlothiana is therefore,a reptiliomorph because it lacks the pterygoid flange and an astragalus andcalcaneum which are key features of amniotes (Smithson et al., 1994), instead,Westlothiana is a transition animal from amphibian to reptile, where is possessesboth amphibian and reptilian features.
HylonomuslyelliThe more widely regardedfirst reptile is the Hylonomus lyelli (Figure 7), found in the Westphalianstage of Joggins, Nova Scotia, Canada (Lambert et al., 2003; Sigurdsen &Carroll, 2016) by William Dawson in 1859 (Falcon-Lang et al., 2010) and wasclassed as the earliest known amniote and possessed slender limbs (Meyer &Anderson, 2013; Sigurdsen & Carroll, 2016).
The reasons being, because theHylonomus had a transverse flange on the pterygoid, an astragalus on the anklejoint, phalangeal formula of 23453 based on the manus and pes (Sigurdsen &Carroll, 2016). All of these characteristics are diagnostic for reptiles, thus,naming the Hylonomus as the oldest reptile. Some of the specimens found at this site werecomplete skeletons and in some cases, scales were found intact. Reasons for thepreservation of these specimens is from forest floor traps where the specimenswere found. The formation of these traps occurred by the smothering of treeroots via floods or salty water that results in the rotting and collapsing oftrees, leaving the stump, that over time, hollowed, forming a trap through therise in the forest floor by continued floods (Lambert et al.
, 2003). As aresult, this site only contains terrestrial fossils which were found insidehollow tree stumps of lycopods (Smithson et al., 1994). Although, anotherreason why vertebrates were found in hollowed tree stumps is because they mayhave been used as shelter, especially during forest fires, which inevitably,burned the animals. This is because charcoal was found on some of the specimens(Falcon-Lang et al.
, 2010).ConclusionReptiles evolved in theCarboniferous, approximately 315 Ma, with the first undisputed reptile beingthe Hylonomus lyelli, which was found in 1859 by William Dawson. Oldervertebrates that resembled reptilian morphologies have been regarded as transitionvertebrates from amphibians to reptiles (reptiliomorphs). The Westlothianalizziae being regarded as a reptiliomorph. The key evolutionary advancementthat allowed the evolution of reptiles was the amniotic egg, which enabled eggsto be laid on the land without drying, but still allowing oxygen and carbondioxide exchange with the environment and water retention.
This keyevolutionary step allowed life to transition out from the water towards land. Theentire terrestrial landscape therefore was eventually inhabited by reptiles throughtheir tolerance of low temperatures, slow metabolism and their ability to usetheir surroundings to alter their body temperatures. Through the evolution ofreptiles, dinosaurs, birds and mammals also evolved.