This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
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?
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.
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).
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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.
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.
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.
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.
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
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.
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).
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
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).
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).
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
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
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.
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.
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?
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.
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
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
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
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.
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...
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