L16 - Lepidosauria (Tuatara, Lizards, and Snakes)

where do lepidosaurs sit on the amniote tree

departed from sauria (crown)

reptile stem - older textbook tree

• anapsid/parareptila as natural groups

• dipasida as a calde defined by two temporal openings

• turtles as anapsid parareptiles, outside diapsida

reptile stem - 2026 working tree

• “parareptila”, “Eureptila”, and “diapsida” not recovered as clades

• new clade Parapleurota

• turtles are archosumromorphs

parapleurota clade =

millrettids + neodiapsida

reptile stem - skull opening are

conditions, not clades

• teach the “diapsid skull” as a condition; the name clade is falling out of use

reptile stem - crown reptiles radiated

250 million years ago

• in the triassic, after the end-Permian extinction

Lepidosauria

(crown group) last common ancestor of Rhynchocephalia + Squamata, and all its descendants

lepidosauromorphs

(total group) - every saurian closer to lepidosuars than to archosaurs - adds extinct stem taxa

a fossil can be a

lepidosauromorph without being a lepidosaur; “oldest lepidosaur” claims depend on which node

triassic origins: the fossil anchors - megachirella

italy - stem squamate or stem lepidosaur

triassic origins: the fossil anchors - agriodontosauras

oldest crown lepidosaur, if megachirella is outside the crown

triassic origins: the fossil anchors - wirtembergia

next-oldest rhynchocephalian

triassic origins: the fossil anchors - taytalura

earliest-diverging leidosauromorph

jurassic origins: the fossil anchors -

squamate takeover

• squamates overtake rhnchocephalians in disparity and rate

K-Pg origins: the fossil anchors -

83% of squamate species lost

what did the first lepidosaur look like - taytalura

earliest-diverging lepidosauromorph; a tuatara-like skull is “the plesiomorphic condition for all leidosaurs”

what did the first lepidosaur look like - agriodontosaurus

akinetic skull, no palatal teeth, probable insectivore - fits the predicted ancestors

what did the first lepidosaur look like - tree thinking

the tuatara skull is close to ancestral; Squamata kinesis is the derived innovation, not something tuatara lost

triassic period - rhynchocephalians were the more

diverse and disparate (i.e., different body forms) lepidosaurs

jurassic - squamates

overtook them in disparity and rates of evolution

Cretaceous - squamate

radiation; ehynchocephalisns decline

K-Pg period - small bodies species

survived; diversity took 20 million years to recover

• 83% of squamate species were lost at the boundary

lepidosauria =

tuatara + squamates

lepidosauria (tuatara + squamates) synapomorphies

• skin shed as a unit

• caudal autonomy

• epiphyses

• thyroid fenestra

• ankle

• kidney

• transverse cloacal

lepidosauria (tuatara + squamates) synapomorphies - skin

shed as a unit - cyclic alpha/beta keratin generations

lepidosauria (tuatara + squamates) synapomorphies - caudal autonomy

fracture planes within tall vertebrae

lepidosauria (tuatara + squamates) synapomorphies - epiphyses

secondary ossification centers; determinate growth

lepidosauria (tuatara + squamates) synapomorphies - thyroid fenestra

in the pelvis

lepidosauria (tuatara + squamates) synapomorphies - ankle

fused astragalocalcaneum; hooked 5th metatarsal

lepidosauria (tuatara + squamates) synapomorphies - kidney

sexual segment in males

Squamata only synapomorphies -

• paired hemipenes

• lower temporal bar lost

• vomeronasal system

Squamata only synapomorphies - paired hemipenes

tuatara have none

Squamata only synapomorphies - lower temporal bar lost

mobile (streptostylic) quadrate

Squamata only synapomorphies - vomeronasal system

well developed; tongue shape cares 9deeply forked in snakes and monitors)

squamata =

snakes and lizards

synapomorphies common error - hemipenes and a kinetic skull are often listed for

lepidosauria

• test- does the tuatara have it? it has neither

tuatara temporal bars

upper and lower bars complete

tuatara temporal bars results

akinetic; fixed quadrate (lower bar probably regained)

lizards temporal bars

lower bar lost

lizards temporal bar results

streptostylic quadrate; many add a frontal-parietal (mesokinetic) joint

snakes temporal bars

upper and lower bars lost

snakes temporal bars result

most kinetic skull; loose quadrate, palate, and jaws

amphisbaenians temporal bars

bars lost, bones fused

amphisbaenians temporal bars results

secondarily akinetic - the head is a digging ram

temporal bars trend

losing bones bough flexibility for handling large or awkward prey

temporal bars not a ladder

amphisbaenians revered it for burrowing

hemipenes is the

squamate signature

hemipenes - paired organs

stored inverted in the base of the tail; one is everted through the vent during mating

hemipenes - sperm transport

along an external groove (sulcus), not an enclosed tube

hemipenes - ornamented

spines, ridges, and lobes vary along species -

• long used in snake and lizard taxonomy

hemipenes - tuatara

no intermittent organ; mating by cloacal apposition

lineages - squamata =

12,252 of 12,640 reptiles; “lizards” are 65%, snakes 35%

lineages - lizards are paraphyletic

snakes and amphisbaenians are nested inside them

lineages - counts carry a date

200 new reptile species described per year

rhynchocephalia is only

one species

rhynchocephalia - acrodont teeth

fused to the jaw and never replaced; enlarged palatine row - denture teeth shear between two upper rows

rhynchocephalia - slow life

average lifespan 60 years (up to 100); growth to 35 years

rhynchocephalia - relict

mesozoic group; Sphenodontidae (wedge-tooth lizards) once very diverse group

tuatara biology =

cool, nocturnal, and island-bound

tuatara biology - name

maori for “spines/peaks on the beak”

tuatara biology - lizard-like, not a lizard

heavier skull; more primitive vertebrae; different rib form

tuatara biology - nocturnal and cool

active at body temperatures low for a reptile

tuatara biology - parietal eye

most pronounced of any living tetrapod; lens-, cornea-, and retina-like parts under scales; senses light intensity, not images

tuatara biology - range collapse

once on North and South islands; lost from the mainland after human arrival and introduced mammals

iguana problem

morphology (outgroup) vs molecules (middle) divergences

major squamate lineages - dibamidae members

blind lizards

major squamate lineages - dibamidae key traits

limbless, burrowing, reduced eyes

major squamate lineages gekkota members

geckos, pygrodids

major squamate lineages - gekkota key traits

setal toe pads in many; spectacle instead of eyelids in most; vocal

major squamate lineages - scincoidea members

skinks, cordylids, gerrhosaurids, xantusipds

major squamate lineages - scincoidea key traits

skinks most species-rich lizards; repeated limb reduction and vivipariety

major squamate lineages - lacetoidea members

lacertids, teiids, gymnopthalmids, amphisbarnians

major squamate lineages - lacetoidea key traits

amphisbenians = annular rings, burrowing

• bipes keep forelimbs

major squamate lineages - anguimorpha members

anguids, xenosaurids, helodermatids, varanids

major squamate lineages - anguimorpha key traits

heloderma venomoys (grooved teeth); mosasaur placement debated

major squamate lineages - iguana members

pleurodonta; acrodonta (agamids + chameleons)

major squamate lineages - iguania key traits

acrodont teeth in acrodonta convergent with tuatara

major squamate lineages - Serpentes members

snakes (4,249 species)

major squamate lineages - Serpentes key traits

limbless (vestiges in some); highly kinetic skull

“Toxicofera” - the clade, strongly supported

• iguana + angiomorpha + serpentes

• recovered by every major molecular dataset

“Toxicofera” - the venom hypothesis - contested

• single early origin of venom-secreting oral glands

• critics: iguana oral secretions may not count as venom

“Toxicofera” - keep them apart

accepting the clade does not require accepting a single origin of venom

iguana - tongue prehension

• most catch prey with fleshy, sticky tongue

• plant-eaters are secondary exceptions

chameleons have a

ballistic tongue, longer than the torso

other squamates - jaw prehension

• seize prey with the jaws

• tongue freed for chemical sampling (tongue-flicking)

  • old name = hard tongue

feeding: tongue capture vs jaw capture - tree checks

on the molecular tree iguana sits inside toxicofera, so tongue capture is derived and jaw prehension is ancestral or convergent

amphisbaenians (worm lizards) characteristics

• burrowers; hard to study

• rigid, fused skull used to tunnel

• skin in rings (annuli) - earthworm-like

• distinctive single median upper tooth

• most limbless; biceps keeps digging forelimbs

snakes vs lizards rules of thumb - snakes

• no moveable eyelids; no external ear openings

• no functional lung (left reduced or lost)

• single row of wide belly scales

• nested within lizards (colloquial)

snakes vs lizards rule of thumb exceptions exist

legless lizards (glass lizards) and lidless geckos

where did snakes come from? Natasha

• Patagonia; early snakes “big-bodies and big-mouthed”

• kept a cheekbone (jugal) and hindlimb, both lost later

where did snakes come from? Tetrapodophis

• announced 2015 as a four-legged snake

• reinterpreted as a dolichosaurid lizard

where did snakes come from? blind snakes

• scolecophidia is paraphyletic; Anomalephidaw is sister to all “typical” snakes

• lost 7 of 12 bright-light vision genes

where did snakes come from? verdict

both hypotheses remain open

• najash points to large0mouthed, land-dwelling early snakes; blind-snake vision genes suggest a burrowing phase

how snakes swallow large prey - rigid braincase

skull of roof and braincase fused into a solid cylinder that protects the brain

how snakes swallow large prey - mobile jaws and palate

quadrate, supratemporal, palate, and upper jaws move independently

how snakes swallow large prey - unfused chin

the two lower jaws are not joined in front, so ea

how snakes swallow large prey - ratchet swallowing

left and right jaws “walk” over the prey in turn

how snakes swallow large prey - limbs

reduced or absent; boas and pythons keep pelvis spurs (vestigial hindlimb), larger in males

snake venom delivery dentition type - aglyphous

no fangs; solid, sharp, backward-curved teeth

• boas, pythons, many “colubrids”

snake venom delivery dentition type - opisthoglyphous

enlarged reat maxillary teeth; solid or grooved

• rear-fanged “colubrids”