About the SA Blog Network

Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct
Tetrapod Zoology Home

Welcome to the Squamozoic!

The views expressed are those of the author and are not necessarily those of Scientific American.

Email   PrintPrint

When the Mesozoic ended, it was inevitable that the lizards, snakes and amphisbaenians – the squamates – would inherit the Earth. For the last 65 million years, the world has been so dominated by squamates that we term this stage in the planet’s history the Squamozoic. What is life like, today, on Squamozoic Earth?

Panoply of assorted Squamozoic beasts. A volant khaodracian and marine seawasp, podarciforms, lubagub iguaniform and marine skink are shown at top; assorted varanidans, teiioids and iguanians are shown at middle; an acanthodactylid and anopsidian amphisbaenian are at bottom. Image by Darren Naish.

Purely because we have to start somewhere, we begin with a scene from arid northern Africa. Some of the smaller squamates shown here – the viper, the social lacertidans, the duck-snouted acanthodactylid – might, we could speculate, have lived in a world where animals other than squamates (turtles, frogs or crocodyliforms, perhaps) formed the megafauna. But the millions of giant uromastyxians that live in these environments so control the flow of energy (Squalisenberg 2010) that it’s difficult to understand how other groups might ever get an evolutionary toehold. This sort of community structure (where big species are super-numerous) is not typical across the Squamozoic world, however.

Three herbivorous north African gigamastigures (one with lacertidan commensals) as well as lacertidans and a colubroid. Gigamastigures range from 2-4 m in length. Image by Darren Naish; colouring by Tim Morris.

Giant species several metres long evolved rapidly among numerous lineages shortly after the end of the Cretaceous and, today, the obvious animals in terrestrial, freshwater, marine and even subterranean ecosystems worldwide are all squamates (lissamphibians, turtles, crocodyliforms and birds make up the remainder of the world’s tetrapod fauna). Several squamate clades – including Iguania, Varanida, Serpentes, Lacertida and Scincida – include species more than 4 m long. There are no truly giant terrestrial animals in the Squamozoic world, but some marine snakes, anguimorphs and lacertidans are enormous (Naish 2012).

Pardemblia (the eublephariform at top left), Mastinochops (the crotaphytid below it) and Megamo (the large heloderm at right) are large, but they’re very much outnumbered by the small and mid-sized taxa from the same habitat. Click to enlarge.

Indeed, by far the majority of squamate species are small animals, less than 50 cm long. Many Squamozoic communities are dominated by dense assemblages of small lizard, snake and amphisbaenian species where big species are few and where the small ones greatly outnumber the large ones in biomass. This is illustrated in the adjacent assemblage of species from the North American south-west: there are large, predatory eublephariform gekkotans, erect-legged crotaphytid iguanians and macropredatory heloderms here, but they’re far outnumbered by smaller iguanians, gekkotans, teiioids and others (Naish 2012).

Back to that African scene… uromastyxians are present throughout the grasslands, semi-deserts and deserts of Africa, the Middle East and Asia. Like most other Old World iguanians, they belong to Acrodonta, the other main lineages of which are the chamaeleoniforms, hypsiluropithecines, ozdracians, khaodracians, and agamiforms. The highly simplified cladogram below depicts the relationships among most of the major iguanian groups.

Highly simplified 'consensus' cladogram for Iguania, with fossil taxa not shown.

Three big-bodied uromastyxian species are shown in that scene above. Here is where we see some of the most elaborate squamofibers in the Squamozoic world: the incredibly long, branched tail fibers present in various of the gigamastigures. Thick squamofiber coats are present in the endothermic uromastyxians and agamiforms of the cool Asian continental interior and also in many of the volant khaodracians of Eurasia. Elsewhere in Iguania, an insulatory squamofiber coat is present in various leiolepidans too. Bodies preserved in permafrost show that cold-adapted members of these groups were especially abundant during the cool periods of the last few million years.

Where the gerrhosaurians and macromabuyas are

Moving now to Sub-Saharan Africa, we here see some of the diversity present in the large and mid-sized gerrhosaurians and skinks, most notably the macromabuyas. Arid and rocky places here are inhabited by huge numbers of lacertidans, small agamiforms and skinks. Gerrhosaurians are armour-plated insectivores and omnivores, the biggest of which are ‘pseudo-ankylosaurs’ with heavily keratinized upper and lower jaws and a continuous dermal armour composed of interlocking rectangular scutes. This armour covering means that adults are effectively impervious to predation from macromabuyas.

Macromabuyas, gerrhosaurians and other taxa from southern Africa. Modoko dragon at top right; Clubtail lumpsteiner at top left. Lacertidans (like the adolfine at bottom left) and agamiforms (like the agamopsians at bottom right) make up much of the small squamate fauna. Image by Darren Naish; colouring by Tim Morris.

However, the fossil record shows that giant sabretoothed leiolepidans previously occurred across Africa (they are presently restricted to North America and Eurasia): it is likely that these were big enough, and formidably toothed enough, to tackle even the largest of gerrhosaurians (Squantón & Squalobart 1999). Modern biologists may be either thankful or sad that these awesome predators are no more.

Macromabuyas include some of the Squamozoic’s most awesome and notorious terrestrial predators. These giant, fast-moving mabuyidan skinks use raking claws, stout jaws and massive, ziphodont teeth to subdue other squamates. The Bluetail macromabuya is an especially gracile, long-limbed member of the group that routinely predates on small skinks while the Modoko dragon is a heavy-bodied macropredator, one of its most distinctive features being the peculiar square-shaped cross-section of its body.

Hard to believe that this is the skull of a mabuyidan skink! Skull of a lumpsteiner, with external osteoderms removed.

Not all members of the macromabuya radiation are predatory. The Clubtail lumpsteiner is another large, armoured herbivore, its upper surface and sides flanked with massive, lumpy scales arranged in longitudinal rows. Its enlarged labial scales have fused to form beak-like structures, reinforced with sequestered metals, used in cropping herbs and grasses. The scene above also shows a mother and juvenile stub-footed leptosiaphine skink and long-bodied adolfine lacertidan at lower left. A harem of social agamopsians are present at bottom right. Profound sexual dimorphism is an obvious feature of the latter, the brightly coloured males being substantially different in size, shape and proportions from the small, drab females.

North American benthamander (at top) and African Flathead lurkerskink (below): both are giant aquatic skinks, but are they close relatives demonstrating a trans-Atlantic crossing, or convergently similar members of disparate skink clades?

Leptosiaphines are another skink group that have radiated at large body size: most are terrestrial forms but giant aquatic species like the Flathead lurkerskink are present across Africa as well. Superficially similar, flat-headed, skulking aquatic predatory skinks of lakes and rivers, most notably the enormous (4 m long) Benthamander, are also present in North America. These lizards might also be part of Leptosiaphina. If so, we have to assume that they crossed the Atlantic at some point. Ocean crossings have occurred frequently throughout squamate evolution, but with the majority of such events involving small species that made these crossings via rafting (e.g., Austin 1999, Carranza & Arnold 2003, Gamble et al. 2008, Vidal et al. 2008).

African varanidans: exanthos and exanthungulates

African and Australasian varanidans. From top to bottom: Short-faced exantho, Whedon's snouted stretchwaran, Crested ardewaran. Images by Darren Naish; colouring by Tim Morris.

Exanthematicine varanidans also have a major presence in sub-Saharan Africa. The species shown here – the Short-faced exantho – is a savannah-dwelling browser that hides in burrows, using its spiky tail plates to block the entrance. Exanthematicine species similar to the Short-faced exantho are close to the ancestry of the exanthungulates, a cursorial, long-limbed lineage of social, herding grazers and browsers (Squinage 1971).

Exanthungulates use a beak formed from constantly growing, chisel-like teeth fused to keratinized labial scales to crop vegetation; enlarged guts containing large fermentation chambers enable them to digest the large quantities of grasses and forbs that they eat. It appears that long-distance visual communication has driven the evolution of elaborate keratinous cranial crests, squamofiber plumes and other structures in these elaborate animals (Squinage 1971). Long spikes on the forelimbs are used in mating battles and in fending off predators.

Australia: land of squamates! Err… just like everywhere else

This scene from Squamozoic Australia depicts the three main groups that dominate the continent: ozdracians, gekkotans and varanidans. Hundreds of small- and mid-sized species pack habitats across Australia. Ozdracians include long-tailed, gracile climbers like the diporiphorans, large predators like the Uberpogona shown here, and innumerable small, desert- and scrubland-dwelling generalists, insectivores and burrowers. The specialised myrmecophage Gigamoloch (shown here at bottom right) is also a member of this group.

An Australian desert scene. Top left: Uberpogona, an ozdracian. To its right: Perentotron, one of the world’s biggest varanidans. Lower left: diplodactyliform gekkotans, legless leristan skinks and a small python. Lower right: Gigamoloch. Image by Darren Naish; colouring by Tim Morris.

Several khaodracian and ptychozwoom groups have invaded Australia from the north, and hundreds of species belonging to both groups are present across south-east Asia and northern and eastern Australasia. Most members of both groups are insectivores or nectarivores, but specialised frugivorous, gum-feeding and avivorous species have evolved as well. The low diversity and cryptic habits and forms of modern birds are presumed to represent co-evolution with these abundant, often predatory, gliding squamates (Kosemen 1995).

Skulls of an exudate-feeding khaodracian at top and predatory, bird-eating khaodracian below. At right (and not to scale), an Australasian tube-snouted ptychozwoom (c. 1 m long).

Ptychozwooms are gliding gekkotans that use large membranes and limb and tail flaps to cover distance in the treetops. As with so many Squamozoic groups, species span the full range from small (less than 10 cm total length) to over 1 m long. The long jaws and tongues of tube-snouted ptychozwooms, like the animal shown in the adjacent illustration, allow the exploitation of flowering plants and it is believed that tubular flowers present across Australasia and tropical Asia have co-evolved with these gekkotans.

Leglessness in the Squamozoic

Giant Squamozoic moreliid python (10 m long) consuming a large diporiphoran (3 m long). As is typical for such scenes, insectivorous gekkotans, snakes and other squamates are in close association: they take advantage of the ticks and other parasites that flee the body of the prey squamate.

Legless skinks and a small python can be seen at bottom left of the colour scene above. Limbless lizards occupy numerous niches and habitats in the Squamozoic world and far outnumber snakes; it is thought that snakes have remained relatively low in diversity and disparity due to the early Cenozoic extinction of mammals, and indeed the fossil record shows that colubroid snakes went through a so-called ‘dark phase’ about mid-way through the Squamozoic from which few lineages recovered (Squage & Squazyndlar 2005). Had mammals radiated extensively at small body size, it is theorised that colubroids might have exploded in diversity to take advantage.

There are, of course, numerous colubroids that prey on other squamates as well as on frogs and invertebrates, and sand boas, pythons and other constricting macrostomatan snakes are prevalent in most environments. Giant pythons and boas occur on all continents. Specialised dentitions that allow these predators to grip and swallow smooth-scaled skinks are common. Hydrophoid and palaeophiid sea snakes evolved early in the Squamozoic and thousands of species occur in warm seas worldwide.

Snakes, skinks, dibamidans, anguimorphs and gekkotans are far from the only Squamozoic squamates with reduced or absent limbs. Subterranean amphisbaenians of many lineages occur worldwide and occur in enormous numbers: in some tropical Indian and African habitats, it is estimated that subterranean amphisbaenians equal or exceed terrestrial squamates in biomass. These ecosystem engineers range in size from less than 10 cm to over 4 m in the case of the absurd South American anopsidian Graboidus, a terrifying predator that conceals itself in tunnels constructed in soft sediments near watercourses before bursting upwards to grab prey items (Gans 1967).

Graboidus doing what it does best: bursting upwards from the soft sediment at the edges of riverbanks to ambush large prey animals (here eublephariforms). Several field biologists have mysteriously disappeared while studying these animals in the wild.

In parts of central Asia, large trogonophidan amphisbaenians exert so much predation pressure on surface-dwelling species that they may even subdue their numbers; an apparent consequence of this is that several trogonophidans here have seemingly become specialised predators of other trogonophidans (Mufasa 2001).

Graboidus, anopsidian mega-amphisbaenian from South America. One of several terrifying and awesome Squamozoic macropredators.

A fundamental constraint prevents limbless subterranean animals from remaining viable above a body size of a metre or so, and Graboidus and other large amphisbaenians have seemingly re-evolved giant, pentadactyl clawed forelimbs for use in locomotion and prey dismemberment. Graboidus is parthenogenetic and viviparous. Remarkably, the babies reside in deep pockets on the sides of the parent and are nourished by an epidermal paste secreted on the inside walls of the pockets.

Varanidans and the Indopacific islands

Hypsiluropithecus: best known and most flamboyant of the hypsiluropithecines. Image by Darren Naish; colouring by Tim Morris.

Varanidans in Australia include a number of peculiar and even spectacular mid-sized, large and gigantic forms. One of the largest is the Perentetron: a long-tailed endurance predator that stands tall above the ground and has a particularly long, robust neck and massive dewlap. Longirostrine varanidans appear to have started their history as long-snouted predators adept at reaching gekkotans and other prey hiding inside rocky crevices: the mode of life still practised by snouted stretchwarans. Within this group, the remarkable ardewarans are facultatively bipedal waders that pluck fish from shallow water. They are also excellent swimmers and have covered large distances at sea, there being a radiation of island-endemic forms throughout the Indopacific region. Large iguaniforms also occur across the islands of the Pacific. Grabwarans are intelligent, omnivorous varanidans that use their elongate, dextrous hands and flexible arms to select plant and animal prey as well as to extract objects from cavities.

Australasia and south-east Asia are inhabited by the hypsiluropithecines, a group of aboreal frugivorous and herbivorous iguanians (closely related to ozdracians) with grasping hands, big bony casques and enlarged dewlaps. Like many iguanians, hypsiluropithecines are ornate, flamboyant and highly visual animals that use these enlarged bony and soft-tissue structures to advertise sexual maturity and social status. Interspecies alliances occur between frugivorous ptychozwooms, various hypsiluropithecines and ground-dwelling varanidans as they forage while keeping watch for grabwarans and other large, predatory varanidans.

The Neotropics: realm of anoliforms and teiioids

Diverse body shapes in Squamozoic teiioids: dracaenasaur and sprinter teiioid to scale. Image by Darren Naish, colouring by Tim Morris.

The South American tropics are dominated by the mostly terrestrial teiioids and mostly scansorial and arboreal iguanians. Cool-adapted, endothermic members of both groups occur in the south of the continent. Among the biggest and most impressive of South American squamates is the dracaenasaur, a big (3 m long), omnivorous teiioid, the elaborate dorsal armour of which is thought to have evolved in step with the predation pressure exerted by the predatory teiioids that also occur in South and Central America. Dracaenasaurs have awesome crushing power in their short, deep jaws and feed on molluscs, turtles, fruit and seeds.

The treetop Amazonian scene featured below shows anguimorphs (the prehensile-tailed alligator lizard shown at bottom left) and snakes (the bright yellow eyelash viper at bottom right), but is otherwise iguanian-dominated. The hyperbasilisk at top left is another highly flamboyant Squamozoic iguanian; males use their enormous and boldly coloured tail, dorsal and head crests and iridescent dewlaps to advertise their presence as mature territory-holders. They’re lek breeders; females are ornamented too but are far less flamboyant.

Amazonian treetops: hyperbasilisks, arbwanas and scansor anoles make up part of the large animal fauna; smaller anoliforms, anguimorphs and colubroids are more abundant. Image by Darren Naish; colouring by Tim Morris.

Iguaniforms inhabit the treetops too. The Amazonian arbwana (two individuals are shown at bottom right, building a communal stick shelter) is a highly social, big-brained iguaniform that lives in family groups and practises extended parental care. Arbwanas live in complex societies, use 30 to 40 different postural signals to communicate intentions and construct shelters and breeding dens (Squrghardt 2004). They live in clans of mostly related individuals, defending territories being based around safe breeding sites where centuries-old breeding burrows are used by generations of lizards and protected by caimans that have a mutualistic relationship with the arbwanas (Squrghardt 2004). Hatchling arbwanas live in crèches, protected and cared for by older siblings.

Tree-climbing squamates from the American tropics. From back to front: Chamaeleonic sloth anole, Great sailbacked anole, Amazonian arbwana.

Anoliforms are hyperdiverse in the Amazonian treetops, with species ranging from less than 10 cm in total length to others 2 m long (Losos 2009). In the scene above, a bluish crown giant anole makes a grab for a smaller species while (at back right) two giant scansor anoles advertise their treetop territory with their bright blue dewlaps and large tail flags. Scansor anoles have enlarged hindlimbs and are adept leapers, but the majority of large anoles are quadrupedal climbers or clingers. Big, omnivorous sloth anoles occur across the Americas (Losos 2009).

Squamate superiority: destined for success… or just lucky?

Structural extremes in Squamozoic squamate skulls (drawn to scale). At top: Great prow-nosed podarciform; below left: tube-snouted ptychozwoom; below right: Terrible terrameleon.

Sprawling and semi-sprawling gaits are common in small and mid-sized limbed squamates, but erect and semi-erect gaits have evolved independently on numerous occasions, sometimes in step with endothermy. Among iguanians, erect-limbed lineages are present among uromastyxians, chamaeleoniforms, hoplocercans, anoliforms, iguaniforms and leiolepidans; many varanidans are erect-limbed too and there are also erect-limbed gekkotans and skinks.

The left hindlimb skeletons of two Squamozoic varanidans compared, both in lateral view. (A) The small varanidan Shakirapops showing separate rotating joints between the unfused tibia and fibula and astragalocalcaneum. (B) Giant, digitigrade varanidan Outbackodon showing partially fused tibia and fibula, hinge-like tibiotarsal and tarsometatarsal joints, and prop-like metatarsal V.

Some of these animals – like uromastyxians and many varanidans – are facultatively erect-limbed, with a wide range of limb postures being possible due to the nature of their shoulder and hip joints and the retention of a flexible ankle joint (Squarig 1980). In some, juveniles sprawl while adults walk habitually with erect limbs and narrow gaits. In various hoplocercans, leiolepidans, giant varanidans and others [see adjacent diagram, comparing a small varanidan with a giant one], the ancestrally flexible ankle region has been replaced by a hinge-like joint that prevents foot rotation. Some big-bodied cursorial and semi-cursorial squamates are also variable with respect to foot posture, with digitigrady being used when the animals move quickly and plantigrady or semi-plantigrady being used at slow speeds.

Squamates can be considered superior with respect to lissamphibians, turtles and crocodyliforms due to their ability to adapt to diverse habitats and surfaces. It’s thought that the remarkable evolutionary plasticity of squamate limb posture allowed these animals to rise quickly to ecological dominance following the Cretaceous-Paleogene extinction event; indeed, this hypothesis of morphological superiority has commonly been considered the main reason for the success of squamates over other animal groups (Squomer 1965, Squarig 1980).

More recently, however, it’s been argued that squamates were essentially lucky: able to dominate environments so quickly simply because the lineages present at the end of the Cretaceous were mostly saved from extinction (Squenton 1983). Dinosaurs and synapsids, of course, did not fare so well. It’s fun to wonder how different things might have been had squamates been less fortunate across the Cretaceous-Paleogene boundary.

Big teiioids

South America: land of anoliforms and teiioids. A teguyan and smaller teiioid are at left; at right, a sprinter teiioid attacks a herbagwana while a diversity of anoliforms (including a large sloth anole) are in the tree. Image by Darren Naish; colouring by Tim Morris.

This Patagonian grassland scene features various of the big-bodied teiioids and iguanians present in this part of the world. Anoliforms large and small can be seen in the tree (including a lounging sloth anole) while a giant, erect-limbed, herbivorous iguaniform is under attack from a teiioid. Several remarkably gracile, long-limbed teiioid species – informally grouped together as the sprinter teiioids – are swift pursuit predators of other squamates.

Teiioids of two different species are present at left. The teguyan (the large, yellow animal) is one of several teiioids that stands and even walks bipedally on occasion. The presence of thick-based, heavy tails, the ability to switch between digitigrady and plantigrady, and proportionally short forelimbs have seemingly encouraged the evolution of facultative bipedality in several iguanian, varanidan, lacertidan and teiioid lineages. Semi-bipedal teguyans are superficially similar to arbwanas in being big-brained and in possessing partially opposable fingers.

Madagascar: land of chamaeleoniforms

Some of Squamozoic Madagascar’s most spectacular squamates, drawn to scale. All are iguanians. (A) Sailbacked chalarodont. This is a male: females lack a sail entirely; it’s one of many iguanian ornaments that has seemingly evolved under sexual selection pressure. (B) Superspinose giant pholidomeleon, one of the largest of the chamaeleoniforms. (C) Conehead terrameleon. (D) Terrible terrameleon, formidably armed predator with long, raptorial hands and spear-tipped tongue-canon.

Madagascar is chamaeleoniform capital of the world, with hundreds of species occupying terrestrial environments as well as all levels of the woodlands and forests. Many are tiny and insectivorous but there are species occupying all body sizes up to 4 m. Terrestrial species are predated upon by other chamaeleoniforms (especially by the terrameleons: read on), sailback chalarodonts and large gerrhosaurians.

Cryptic morphologies and elaborate spinose ornament have evolved in step with these predation pressures, the most extreme example being represented by the giant pholidomeleons. The several large species (between 2-4 m long) of this group possess hand-shaped clusters of spines along their dorsal midlines, long spikes on their tails, and additional spike clusters on the snout, lower jaw, limbs and flanks. Some are omnivorous, foraging for small chamaeleoniforms in leaf litter as well as edible plant material, though several are specialised myrmecophages. Adults are impervious to predation; juveniles are not, but the extended parental care in these viviparous squamates means that small individuals are defended by adults.

The most remarkable of all chamaeleoniforms, however, are the terrameleons. From quadrupedal, terrestrial brookesiid ancestors, a lineage of short-bodied and short-tailed predators evolved. As body size increased, the importance of the prehensile forelimbs in apprehending and subduing prey increased such that the most modified members of the group are erect-bodied bipeds with stupendously long arms and grasping hands.

These are probably the most bizarre and freakish animals of the Squamozoic world.

Seas of the Squamozoic

Simplified cladogram showing hypothesised relationships among the marine diploglossines. Seawasps are but 1-3 m long, while bathysaurs, sensopsians and gulping terrors all include species that exceed 10 m in length.

Finally, what of the seas? Numerous squamate lineages independently invaded the marine realm during the Squamozoic, some by taking to life at sea directly from beaches and estuaries, and others following adaptation to life in freshwater. Several skink lineages have taken to the seas, including the deep-diving, crevice-feeding fentebeches. Also important in the seas is the diploglossine clade that includes seawasps, the often spoon-jawed bathysaurs and big-toothed merdodonts, and the big, long-bodied sensopsians and gulping terrors.

Among iguanians, a radiation of omnivorous and herbivorous iguaniforms termed the lubagubs graze in herds on marine algae throughout the shallow coastal waters of the world. Strongly curved forelimb claws are used by some species to cling to seafloor rocks while foraging at depth. Marine elapids evolved early on and the Squamozoic seas are filled with untold millions of seasnakes large and small. Marine predators like seawasps and bathysaurs feed as much on seasnakes as they do on fish and cephalopods.

One of the most important marine groups are the podarciforms, an ancestrally Mediterranean lacertidan clade that includes amphibious, shallow marine and fully pelagic species. Obvious features of these lizards include their pointed, prow-like rostra, enlarged, mirror-like belly scales and strangely folded limbs, partially concealed by massive flanges that have grown outwards and downwards from the area just in front of the limbs. In the wholly pelagic dragonesque podarciforms, the forelimbs are absent and replaced by shiny flanges used only in display; the hindlimbs have also been replaced, this time by hook-tipped flanges that mimic limbs. Mirror-like scales line the sides of the neck as well as the body and tall display fins are present over the tail base and along much of its length.

A Squamozoic Pacific scene: a lubagub group and fentebeche at top right; two dragonesque podarciforms and a prow-nosed podarciform in the centre; seasnake swarm at bottom left. Image by Darren Naish; colouring by Tim Morris.

This was the Squamozoic

This brief tour of the modern Squamozoic world should serve as something as an introduction to its many, diverse inhabitants. In lacking gigantic terrestrial species, the Squamozoic world has an utterly different feel from the Mesozoic one. But its many elaborate, bizarre animals are no less fascinating, and the densely packed nature of many environments means that animal life seems richer and more abundant. It also seems like a somehow more egalitarian world: the erect gaits, endothermy, behavioural and ecological flexibility and relative intelligence of so many Squamozoic species are scattered across all extant lineages. Truly, this is the greatest time to be alive; to be a squamate in the Squamozoic.

Credits: thanks to Tim Morris for colouring many of the scenes used above. Tim also invented the term ‘Squamozoic’ in the first place. Matt Wedel made initial comments that inspired the initiation of the Squamozoic. C. M. Kosemen provided continuous advice and inspiration. Will Naish discovered Megamo, a North American heloderm.

For previous Squamozoic-themed articles at Tet Zoo, see…

Refs – -

Austin, C. C. 1999. Lizards took express train to Polynesia. Nature 397, 113-114.

Carranza, S. & Arnold, E. N. 2003. Investigating the origin of transoceanic distributions: mtDNA shows Mabuya lizards (Reptilia, Scincidae) crossed the Atlantic twice. Systematics and Biodiversity 1, 275-282.

Gamble, T., Bauer, A. M., Greenbaum, E. & Jackman, T. R. 2008. Out of the blue: a novel, trans-Atlantic clade of geckos (Gekkota, Squamata). Zoologica Scripta 37, 355-366.

Gans, C. 1967. A check list of recent amphisbaenians (Amphisbaenina, Reptilia). Bulletin of the American Museum of Natural History 135, 63-105.

Losos, J. 2009. Lizards In An Evolutionary Tree. University of California Press.

Kosemen, M. C. 1995. Why are there so few birds? Is it because gliding squamates prevent them from becoming numerous and interesting? I bet it is. This is the Squamozoic, donchaknow. Squamozoica 45, 348-358.

Mufasa, K. 2001. Trogonophidan amphisbaenians eat surface-dwelling squamate prey; trogonophidans take to eating other trogonophidans. It’s one of those circle-of-life kind of things. Squamarama 40, 340-354.

Naish, D. 2012. Field Guide to the Squamozoic. Squamozoic Press, Milton Keynes.

Squage, J.-C. & Squazyndlar, Z. 2005. Latest Oligocene-Early Miocene in Europe: dark period for colubroid snakes. C. R. Palevol 4, 428-435.

Squalisenberg, C. 2010. The Monstersaur’s Tooth: Keystone Predators, Trophic Cascades and Biodiversity. Island Press, Washington.

Squantón, M. & Squalobart, À. 1999. Neck function and predatory behavior in the scimitar toothed leiolepidan Gorgolacerta latidens (Squowen). Journal of Vertebrate Paleontology 19, 771-784.

Squarig, A. J. 1980. Differentiation of lineages among Squamozoic tetrapods. Mémoires de la societé squamozoique de France 139, 207-201.

Squenton, M. J. 1983. Squamate success in the Squamozoic: a noncompetitive ecological model. The Quarterly Review of Biology 58, 29-55.

Squinage, B. 1971. Exanthungulate Biology and Diversity: an African Radiation. Oxford University Press, Oxford.

Squomer, A. 1965. Squamate superiority and the awesome adaptability of the squamate hindlimb and foot and why it rules and why other animals are lame and why they don’t rule the world. Squamarama 29, 1-23.

Squrghardt, G. 2004. Arbwana research: looking back and looking ahead. In Alberts, A. C., Carter, R. L., Hayes, W. K., Martins, E. P. (eds) Arbwanas: Biology and Conservation. University of California Press (Berkeley), pp. 1-12.

Vidal, N., Azvolinsky, A., Cruaud, C. & Hedges, S. B. 2008. Origin of tropical American burrowing reptiles by transatlantic rafting. Biology Letters 4, 115-118.

Darren Naish About the Author: Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Southampton, UK). He mostly works on Cretaceous dinosaurs and pterosaurs but has an avid interest in all things tetrapod. His publications can be downloaded at He has been blogging at Tetrapod Zoology since 2006. Check out the Tet Zoo podcast at! Follow on Twitter @TetZoo.

The views expressed are those of the author and are not necessarily those of Scientific American.

Rights & Permissions

Comments 48 Comments

Add Comment
  1. 1. CS Shelton 5:11 am 04/1/2013

    I was looking forward to the funny, but now I can’t wait to read this instead! Back to the top with me…

    Link to this
  2. 2. Hai~Ren 5:24 am 04/1/2013

    The references are hilarious. Took me a while to realise that some of them are parodies of genuine papers and authors. ;)

    Link to this
  3. 3. Pristichampsus 7:15 am 04/1/2013

    Absolutely squamotastic. Glad we could collaborate in such a productive way, or that I at least could be employed to such effect, and have exposure on the best zoology blog around.

    Link to this
  4. 4. josimo70 9:40 am 04/1/2013

    What’s the etymology for merdodonts? Shit-toothed?

    Link to this
  5. 5. John Scanlon FCD 10:14 am 04/1/2013

    My library doesn’t seem to have Squamarama, do you know if it’s open access yet?

    Link to this
  6. 6. Heteromeles 11:13 am 04/1/2013

    I’m trying to remember the scientific name of that recently discovered species of Indopacific, gregarious sea snakes that swarm together to mimic an octopus. It’s one of the few known examples of group deception. Any help?

    Link to this
  7. 7. vdinets 11:43 am 04/1/2013

    John: all journals are open access. Since there is much less parental care in Squamozoic, the idea of private property didn’t develop, and neither did the concept of copyright.

    Link to this
  8. 8. Tayo Bethel 4:24 pm 04/1/2013

    That was fun! Can we do it again? LOL

    Link to this
  9. 9. David Marjanović 5:33 pm 04/1/2013

    turtles, frogs or crocodyliforms, perhaps

    Crocodyliforms, definitely. Turtles, OK. But frogs? Is there a way to arrive at a herbivorous adult frog? And is the weird size limit perhaps real?

    Link to this
  10. 10. Metridia 6:13 pm 04/1/2013

    Squamates are far too lame to have taken over the biosphere after the archosaurs.

    Link to this
  11. 11. naishd 6:27 pm 04/1/2013

    Thanks for comments… much here that warrants response.

    With reference to comment 7… yes, all Squamozoic journals are indeed OA :) And you might like to wonder about the identity of the scientists who are studying the Squamozoic fauna.

    David (comment # 9): there are herbivorous frogs: Xenohyla, for example.

    More later. Still aiming to catch up after returning from the field.


    Link to this
  12. 12. Jerzy v. 3.0. 6:50 pm 04/1/2013

    Cool! I just wonder about two things:

    Would ectodermic nature of squamates lead to re-evolution of pelycosaur-like crests in many species?

    Would slow metabolism lead to great variety of species in tundra and deserts, much of which aestivate or hibernate? Imagine ecosystem of boreal forests, where all the large animals hibernate for winter. In late autumn, largest predators might have a great time, because they still keep their body heat and hunt prey which already hibernated. But in the early spring tables turn, and small predators warm up quicker and ambush sluggish megapredators in groups…

    Link to this
  13. 13. Jerzy v. 3.0. 6:50 pm 04/1/2013

    ectothermic, thermic, thermic!!! Sorry.

    Link to this
  14. 14. Jerzy v. 3.0. 6:52 pm 04/1/2013

    And one more – perhaps long grasping tongues of chameleons evolved into totally new, complex manipulative organs?

    Link to this
  15. 15. Jerzy v. 3.0. 6:55 pm 04/1/2013

    And still another one: imagine small typhlophid snake which evolved into a parasite, burrowing under the skin and into the fat and flesh of large sleeping animals, like a squamate candiru?

    Link to this
  16. 16. pmurphy98 7:39 pm 04/1/2013

    Ah yes, quite good -a nice, brief overview of the brilliance of this Squamozoic World we all live in.
    I’ve always thought it strange that the duck-snouted acanthodactylid was named as such. Given the inherent Squamate superiority of acanthodactylids, I always thought it would have made more sense to call ducks, “acanthodactylid-snouted anseriformes.”

    Also, is Whedon’s snouted stretchwaran named after Squoss Whedon, the creator of Squffy the Squampire Slayer?”

    Link to this
  17. 17. AlHazen 11:48 pm 04/1/2013

    So, it’s all been downhill since the Permian? (Sad Synapsid)

    Link to this
  18. 18. Squiddhartha 2:27 am 04/2/2013

    Note that the 1990 documentary on Graboidus, Tremors, is available for streaming on Netflix.

    It seems as though the feeding apparatus of the tube-snouted ptychozwooms might also be effective in insectivory, and you indicate there are insectivores in that group — are the tube-snouts exclusively nectar feeders?

    Link to this
  19. 19. CS Shelton 3:02 am 04/2/2013

    Jerzy- Your squamate candiru freaks me out.


    Link to this
  20. 20. CS Shelton 3:57 am 04/2/2013

    I like the anoliform lion tamirin guys. Cute.

    Hey, what is a squamofiber like, structurally? How did those evolve?

    Link to this
  21. 21. CS Shelton 4:10 am 04/2/2013

    The duperspinose giant pholidomeleon looks like it was reconstructed by David Peters. Er, Squavid Squeters. Sorry to mention him. I know that guy is a total squack, and ruining the Squamonet for zoologists.

    Link to this
  22. 22. naishd 6:28 am 04/2/2013

    Excellent comments, thanks everyone. Evidently, there’s much complexity to the Squamozoic world, not all of it mentioned in the article here. Some responses and thoughts…

    – Stuff not mentioned in article above: complex co-evolution with mites and other arachnids, the diverse lacertidan fauna of Eurasia, scale loss in various lineages, heloderms of Europe, herbivorous long-necked varanidans of Australasia and Oceania, African fat-tailed swimmergeckos, social twig teiioids…

    – ectothermy and dorsal sails (Jerzy’s comment # 12): melanism and expandable flank tissues (see the weird pleats on the flanks of the liolepidan in some of the images above) allow some big squamates from cool climates to collect and retain heat; dorsal sails are seen in the Squamozoic, but they’re more to do with sexual display (as they may well have been in sail-backed synapsids) and are not present in cool-climate forms. As for the boreal fauna: you are dead right that endothermic predators spend much time locating hibernating ectotherms. Parasitic scolecophidians are a nice touch, good work :)

    – pmurphy98 (comment # 16): yes indeed, Whedon’s snouted stretchwaran is named for Squoss Whedon, creator of Squffy the Squampire Slayer. I was listening to ‘Once More With Feeling’ while describing that species.

    – Squiddhartha (comment # 18): tube-snouted ptychozwooms do indeed eat insects as well as plant products; members of some lineages are arboreal ant- and termite-eaters.

    – CS Shelton (comment # 20): squamofibers started out as soft, flexible spines growing from the central keels of typical scales (as seen in, e.g., fence lizards, bush vipers and egernia skinks). In certain lineages they became elongate, overlapping and more complex, with either several or many branches growing along their length, or with many parallel spines emerging from the edge of each scale. Comment # 21: yes, the pholidomeleons do have a Squetersesque look about them. But we don’t need to talk about that.

    And – - seriously, that field guide might appear in print one day.


    Link to this
  23. 23. Finback 9:09 am 04/2/2013

    _Perentotron_ made my day, as a robonerd.

    Link to this
  24. 24. Tayo Bethel 9:25 am 04/2/2013

    Do any squamates in real life have grasping hands? … Just curious.

    Link to this
  25. 25. naishd 9:42 am 04/2/2013

    Do any squamates have grasping hands? Yes. Chameleons are the obvious ones; some varanids (specifically, Varanus beccarii) also grasp things by curling their manual digits round them – they’re capable of sophisticated, coordinated grasping behaviour and use this to extract prey from cavities in wood. See…

    Mendyk, R. W. & Horn, H.-G. 2011. Skilled forelimb movements and extractive foraging in the arboreal monitor lizard Varanus beccarii (Doria, 1874). Herpetological Review 42, 343–349.


    Link to this
  26. 26. BrianL 10:45 am 04/2/2013

    Is there any reason to presume that birds would be a small and cryptic clade in the Squamozoic? Surely the likes or convergent likes of *Sagittarius*,* Circaetus* or even *Mascarenotus* would quickly evolve as specialised hunters of squamates? In fact, in a world dominated by squamates in which large forms presumably evolved from (mostly) small ancestors in the Paleogene and in the absence of mammals, I’d expect dedicated avian squamativores to have grown in size, power and impressiveness accordingly in an evolutionary arms race. A pseudo-phorushacid would have a field day in your alternative South America, I’d think.

    Also, with flighted birds around would all those gliding nectarivorous and insectivorous lizards be all that competitive, at least at first? I certainly don’t buy them being competitive enough to drive birds into cryptic niches!

    I presume this is all artistic license in order to make the Squamozoic as squamatic as possible?

    I might be biased though, I far prefer a Ornithozoic over a Squamozoic. Nothing would beat a Testudinozoic, however.

    Link to this
  27. 27. Gigantala 10:56 am 04/2/2013

    I take it there are no genuinely volant squamates?

    Link to this
  28. 28. naishd 11:29 am 04/2/2013

    BrianL (comment # 26): after the Cretaceous-Paleogene event, all surviving birds were small. Squamates survived at high diversity across the boundary (this is where the Squamozoic timeline differs from our own, since real-world squamates were hit hard by the K-Pg event), and large-bodied anguimorphs and teiioids (already habitual raiders of archosaur nests) exploded in diversity once freed from competition with dinosaurs. These large, intelligent, adaptable predators subdued the evolution of large birds, mostly due to their ability to locate nests and devour eggs.

    So, ground-nesting, beach-nesting and cavity-nesting birds are rare in the Squamozoic for these reasons: birds are small and constrained to nesting high in flimsy branches.


    Link to this
  29. 29. vdinets 12:16 pm 04/2/2013

    Darren: some Squamozoic birds nest in trunks of live conifers and protect the nesting holes with resin flows, but most species either nest in high, cold mountains and commute daily to lower elevations to feed, or nest in very high latitudes and have chicks capable of migrating towards the tropics a few days after hatching. The parents’ ability to secret milk-like substance helps a lot. In summer, ice shields of the Antarctic and ice floes of the Arctic Ocean are covered with millions of bird nests.

    A side question: with so many island species of tortoises, how come none has ever evolved reduced or absent shell?

    Link to this
  30. 30. David Marjanović 1:03 pm 04/2/2013

    there are herbivorous frogs: Xenohyla, for example.

    “This is the only frog that eats fruit.” entry of X. truncata

    Squamates survived at high diversity across the boundary (this is where the Squamozoic timeline differs from our own, since real-world squamates were hit hard by the K-Pg event)

    …But if so, why has squamate evolution been so surprisingly similar to our timeline? In both timelines, colubroids, brookesiids etc. etc. etc. have appeared independently.

    with so many island species of tortoises, how come none has ever evolved reduced or absent shell?

    Maybe it’s simply because they’re all so young?

    Link to this
  31. 31. David Marjanović 1:04 pm 04/2/2013

    Furthermore, protection isn’t the only function of the tortoise shell. Another is some degree of thermal insulation.

    Link to this
  32. 32. Jurassosaurus 1:16 pm 04/2/2013

    Regarding grasping hands, diplodactylid geckos are also graspers, as exemplified by Strophurus elderi.

    A side question: with so many island species of tortoises, how come none has ever evolved reduced or absent shell?

    Likely because there was no selective pressure to reduce the shell. It’s not a very satisfying answer, but it’s probably right. That said, Chelonoidis nigra abingdoni has modified the shell so much that one could argue it is pretty reduced. Also, giant tortoises do have lighter shells overall than smaller tortoises, so there are signs of reduction in terms of bone deposition.

    Link to this
  33. 33. BrianL 1:22 pm 04/2/2013

    The *Cilindraspis* tortoises of the Mascarenes also had small, ‘lightweight’ shells. Would leatherback turtles also count as having reduced shells?

    Aside from thermal insulation and protection for adults, I’d think protection for juveniles may also be a reason not to reduce the shell too much in giant tortoises. Adults (at least in island populations) may have been nigh invulnerable to attack by most local predators, but juveniles were not. In theory, this could mean that shell-lessness might evolve far more quickly in a livebearing tortoise giving birth to larger, less vulnerable young.

    Link to this
  34. 34. Shuhray 2:48 pm 04/2/2013

    “Origin of tropical American burrowing reptiles by transatlantic sap.”

    Link to this
  35. 35. Jerzy v. 3.0. 6:52 pm 04/2/2013

    What about basilisk descendant which evolved into jacana-like creature?

    I also imagine social snakes, which hide in the grass and together snare and tie large prey…

    I might also imagine that inflatable air sacs evolved into complex and totally new organs.

    Maybe a badger-like lizard which inserts bits of its body under stones, logs, or maybe into cracked dead wood, and then contracts some muscles, prying open the hole in hydraulic way?

    Some big chameleon-like animals where courting males inflate their head crest four or five times, into the great shield, something between chameleon helmet and tragopan facial shield?

    Or a boreal lizard which warms itself by inflating two giant “fins” on the sides, catching sun.

    Or advanced chameleon which can not only change color, but inflate and deflate skin nodules to match texture of its surroundings, like some octopuses?

    And snakes whose venom evolved beyond poison to hyper-potent enzyme quickly dissolving prey tissues to goo. Which was apparently the original function of snake venom.

    Link to this
  36. 36. Heteromeles 7:44 pm 04/2/2013

    I’d suggest you might want to experiment with relocating the Chicxulub strike. Would the asteroid hitting in the deep ocean have affected more birds and fewer squamates? It appears that the early bird lineages are disproportionately shorebirds.

    Link to this
  37. 37. CS Shelton 1:27 am 04/3/2013

    Oh, forgot to ask. Can we get larger pictures sometime, maybe on your DA account? It’s hard to see some of the interesting little creatures.

    Link to this
  38. 38. naishd 5:47 am 04/3/2013

    More responses…

    – Gigantala (comment # 27): khaodracians are mostly gliders but some are capable of true flapping flight.

    – vdinets (comment # 29): yes, good call on those cryptic birds. Others have already answered the tortoise shell question. I think several factors are involved: (1) tortoises in predator-free environments are probably not old enough to have done any more than reduce the extent and thickness of the shell; (2) the shell has a role in intraspecific display, combat and thermoregulation, so persists even when not involved in anti-predator behaviour; (3) the fact that the shell is formed from expanded ribs located lateral to the limb girdles makes it difficult to lose. Having said all that, leatherback turtles have essentially lost the shell and yet they still retain the conventional turtle bauplan, so maybe that last point is moot.

    – David (comment # 30): I’m pretty sure that Xenohyla is no longer not the only fruit-eating frog – frugivory has been reported in one or two others. Note that deliberate eating of algae and leaves is known for the Indian ranid Euphlyctis hexadactylus (stomach contents include as much as 80% plant material).

    – Why are there colubroids, brokesiid chameleons etc. in the Squamozoic? Well, why wouldn’t there be? The stem-members of these lineages were already present prior to the K-Pg event, so much of Squamozoic squamate evolution parallels that of our timeline.

    – Jerzy (comment # 35): you’re good at this :)

    – CS Shelton (comment # 37): larger images are on my TetZoo tumblr.


    Link to this
  39. 39. David Marjanović 11:05 am 04/3/2013

    It appears that the early bird lineages are disproportionately shorebirds.

    …No. That was an idea from the early 1990s that hasn’t held up.

    BTW, what prevents squamates in our timeline from plundering more bird nests?

    the shell is formed from expanded ribs located lateral to the limb girdles

    The shell is formed from dermal bone. Most of it (the costal plates) forms on top of the developing ribs, so can arguably be considered an expansion of the ribs. The neural and marginal plates, however, are independent osteoderms (the neural plates have detached and shifted in the fossil of Odontochelys), and the plastron consists of the interclavicle, the clavicles, and a few pairs of gastralia.

    While there’s shell lateral to the shoulder girdle, that’s all dermal bone; the ribs don’t extend there, they stay caudal to the shoulder girdle – their plesiomorphic position.

    deliberate eating of algae and leaves is known for the Indian ranid Euphlyctis hexadactylus (stomach contents include as much as 80% plant material).


    The stem-members of these lineages were already present prior to the K-Pg event

    Wikipedia says the oldest chamaeleon is from the middle Paleocene, and doesn’t say if it’s a stem- or a crown-chamaeleon.

    Judging from Longrich et al. (2012, 2013, including supp. inf.), there aren’t any known Cretaceous (crown-)macrostomatans, thus no stem-colubroids.

    Link to this
  40. 40. David Marjanović 11:05 am 04/3/2013

    from the early 1990s

    And earlier, actually; 1980 at the very least.

    Link to this
  41. 41. naishd 12:36 pm 04/3/2013

    Turtle shells: yes, sorry, I should have clarified. Dermal bone built on top of expanded ribs.

    Re: Squamozoic colubroids and chameleons… I don’t mean to imply that there were stem-colubroids and stem-chameleons prior to the KPg event; rather, that snakes and iguanians ancestral to these forms were present. We might speculate that they would have evolved regardless of squamate fortunes post-Cretaceous.


    Link to this
  42. 42. BilBy 11:19 am 04/4/2013

    @Jerzy #35 “I also imagine social snakes, which hide in the grass and together snare and tie large prey…”
    Which one of the snakes would get to eat it? :)

    Link to this
  43. 43. Heteromeles 1:38 pm 04/4/2013

    @Bilby: the better question may be whether they can share prey by vomiting.

    A couple of separate questions. One is, what is the history of mosasaurs in the Squamazoic?

    Another one is, what is (are) the route(s) to endothermy in the squamozoic world? Aside from the varanid’s work-around for Carrier’s Constraint, it looks like it might be hard to get sufficient oxygen into a lizard without some substantial anatomic rearrangements. How do we get there from here?

    Link to this
  44. 44. Jerzy v. 3.0. 5:30 pm 04/4/2013

    I just thought that the better timeline for Squamozoic fantasy would be future Earth after global nuclear war, or maybe some chemical weapon which killed all the mammals and birds.

    Link to this
  45. 45. Jerzy v. 3.0. 5:34 pm 04/4/2013

    But many lizards move without bending their bodies. Phrynosoma for one. I guess endothermy would go together with more advanced locomotion…

    Link to this
  46. 46. Heteromeles 11:09 am 04/6/2013

    Does endothermy go with galloping (aka advanced locomotion)? There are many other possibilities that don’t necessarily require a stiff trunk. Some other possibilities:

    –Gigantothermy and active temperature control, as in leatherback turtles (

    –Heat from rumination in goat-sized herbivores and above.

    –Advances in monitor style gular pumping to fully take the place of a diaphragm.

    –independent evolution of a muscular diaphragm (I believe tegus have a proto-diaphragm?), with concurrent development of a stiffer torso and/or the potential to gallop.

    –obligate bipedality, which allows respiration to be decoupled from movement, as in humans.

    Any of these more likely than others? I didn’t see too many bipeds in the Squamozoic…

    Link to this
  47. 47. David Marjanović 3:41 pm 04/8/2013

    –Gigantothermy and active temperature control, as in leatherback turtles (

    Not sure if that actually counts as gigantothermy. Anyway, requires a very compact body shape.

    Link to this
  48. 48. aaronthemad 6:51 pm 04/13/2013

    So what intelligent, journal-publishing, globe-spanning species is/are the biologists studying the Squamozoic?

    Link to this

Add a Comment
You must sign in or register as a member to submit a comment.

More from Scientific American

Email this Article