Chapter   11 - LEPIDOSAURS

SUBCLASS DIAPSIDA (Upper Pennsylvanian - Recent)

Stem Diapsids

Family Protorothyridae (Upper Pennsylvanian - Lower Permian)

small, lizard-like, no temporal fenestra, but included in Diapsida

Hylonomus [fig 8-23a] and Paleothyris are among the oldest and most primitive of the Reptilia

Order Araeoscelida (Upper Pennsylvanian -Lower Permian)

two temporal fenestrae, includes Petrolacosaurus [fig. 8-23e, 14-13a], the oldest reptile with a diapsid skull [Fig. 11-5a].

 

Infraclass LEPIDOSAUROMORPHA ("scaly reptiles")

1. There are several extinct orders, and two extant orders (tuataras and the lizards and snakes)
2. all with diapsid skulls.
3. Sprawling posture

SAUROPTERYGIA [fig 14-21]

1. mainly Mesozoic, marine lepidosaurs
2. euryapsid, lower temporal arch lost; elongate neck (>7 cervicals)

Order Plesiosauria (Upper Triassic - Cretaceous)

1. increased size of limbs
2. fore and hind limb similar in size
3. limbs uses as "wings.

Superfamily Plesiosauroidea

• long-necked; small skull

Superfamily Pliosauroidea

• short-necked; large skull

Order Ichthyosauria (Lower Triassic - Upper Cretaceous)

• fusiform body
• fin shaped extremities
• vertical tail fin
• ovoviviparous

Superorder LEPIDOSAURIA

Lepidosauria includes the Sphenodontida and Squamata.

Characteristics

1. Caudal autotomy ("self amputation") by means of fracture planes in tail. Tail regenerates; future autonomy can only occur in fracture planes anterior to the regenerated tail.  [Box 11.1; fig 11-9]
   • common in some lizards; a predator escape mechanism
2. Transverse cloacal slit
3. fenestrated pelvic girdle [= thyroid fenestra], a broad opening between the pubis and ischium
4. epiphyseal ossifications: determinate growth

Order SQUAMATA

• Squamata includes lizards and snakes
• ca. 6,000 sp.
• paired hemipenes in males

• Characteristics of skull [fig 11-5]
  1. lower temporal bar lost; streptostylic jaw articulation (quadrate free from lower temporal arch as a result of loss of the quadratojugal)
  2. mesokinetic joint between frontal and parietal

Suborder LACERTILIA [SAURIA]

• lizards
• 25+ families [Table 11.1, 11.2]; 4,000 species
• late Jurassic - Recent
• Characteristics
  1. middle temporal bar present [fig 11-5c, d, e]
  2. Diet
     1. Mostly insectivorous
     2. Phrynosoma is a highly specialized ant eater
     3. Some herbivorous: e.g. Iguana and relatives; marine iguana feeds on seaweed
     4. Some carnivorous: e.g. Family Varanidae
  3. Size
     1. Mostly < 20 grams
     2. Komodo dragon (worlds largest living lizard, 3 m. long; 75 kg)--ambush predator
     3. Megalania-extinct (Pleistocene) from Australia 5.5 m. long; 1000 kg
  4. Most species of lizards are diurnal but some are nocturnal:
     1. Nocturnal species include Gekkonidae
     2. Diurnal species include Anolis [Iguania: Polychrotidae] among many others
  5. Most lizards are terrestrial but a few are aquatic
     1. aquatic lizards include the marine iguana of the Galapagos Islands and the extinct Mesozoic marine lizards called Mosasaurs.

More examples  

• Iguanians-brightly colored lizards with ornamental structures such as crests, frills, and throat fans around head and neck for visual displays.  The group is diagnosed by having the frontal bones fused to each other.  Includes are three families: Iguanidae, Agamidae, and Chamaeleonidae.
• Chamaelionidae:  chameleons - catch prey with very long, sticky tongue that reaches lengths greater than that of their own entire body; change color to match environment. zygodactylous--grip branches; prehensile tail; eyes move independently
•   The Iguanidae include eight subfamilies [Table 11.2], including

• Scincidae: 1090 species; skinks - small snake-like; slender bodies with very small limbs
• Anguidae--include some legless forms that forage in leaf litter; e.g., glass lizard Ophisaurus
• Teiidae--whiptails such as Cnemidophorus 
• Gekkonidae
  • geckos - small, agile, mostly nocturnal species with modified scales on the bottom of their toes which allow them to walk upside-down and on vertical surfaces

Suborder AMPHISBAENIA- "worm" lizards [fig 15-2] some consider the 150 species of amphisbaenians lizards (e.g. Pough). tropical Characteristics the reptilian version of caecilians look like earthworms (annulated body) mostly legless, burrowing Rhineura floridana Florida Worm Lizard. Bipes from Mexico has well-developed forelegs amphi = double, baen = walk. move both forward and backward with equal efficiency specialized akinetic skull [fig 13-24g] single upper median tooth forms nipper with two lowers feed on invertebrates and small vertebrates--mainly insects

Bipes canaliculatus in CalPhotos.  Photograph by Adam P. Summers, Museum of Vertebrate Zoology

• Characteristics
  1. very kinetic skull
     1. upper temporal bar lost
     2. varanoid lizards, with an intramandibular joint, are probably ancestral to mosasaurs and snakes
     3. jaw loosely connected to skull by ligament-can stretch to open gape larger than diameter of body.
     4. lower jaw not fused at midline-held by ligament
  2. limbs lost - vestigial hind limbs in male Boidae
  3. accessory zygopotheses on vertebrae
  4. all insectivores or carnivores
  5. no eyelids - clear scale covers eye
  6. no ear openings

Scolephidia

• burrowing snakes (Typhlopidae and Leptotyphlopidae) with rudiments of a pelvic girdle; skull bones solidly joined
• 300 spp.

Booidea

• boas and pythons
• vestiges of a pelvic girdle and hind limb, teeth on premaxillary
• 80 spp
• reticulated pythons and anacondas reach lengths of up to 10 m.

Caenophidia (Colubroidea)

• maxilla more or less free

Colubridae: 1600 spp. Masticophis flagellum testaceus western coachwhip, red color phase, photograph © 2001by Dr. Allan H. Chaney,  used  with permission
Viperidae: 20 spp. Crotalus atrox Western diamondback rattlesnake photograph © 2001by Dr. Allan H. Chaney,  used  with permission
Elapidae: 250 spp Micrurus tenere Texas coral snake.  Photograph © 2001by Dr. Allan H. Chaney,  used  with permission

Snakes have two highly specialized means of capturing prey.

1. Some species are constrictors, which means they dispatch their prey by suffocation.
2. some species use venom to poison their prey. Venoms are classified as to their mode of action as hemotoxic and neurotoxic.
   1. Neurotoxic - acts on nervous system; primarily on optic and phrenic (diaphram) nerves, therefore causing blindness and respiratory paralysis respectively.
   2. Hemolytic - breaks down blood cells, vessels, and causes bleeding into internal body cavities.
   3. Venoms serve both to dispatch (and partially digest) prey as well as an antipredator mechanism.

Venomous snakes include

1. Family Viperidae (rattlesnakes and their relatives)
• all vipers have a pair of long retractable hollow fangs in the front of the mouth, for injection of venom into prey

• Subfamily Crotalinae - pit vipers - (see also below under heat detection) all of the familiar N.A. poisonous snakes are pit vipers.   include species of rattlesnakes, the water moccasin, and the copperhead (also in Central and South America and Asia). 12 out of 8000 bites from pit vipers reported yearly in the U.S. result in death

  Agkistrodon contotrix copperhead.  Photograph © 2001by Dr. Allan H. Chaney,  used  with permission

2. Family Elapidae - cobras, mambas, and coral snakes
• have short short, nonretractable, grooved (enclosed by folding) fangs in front; venom must be injected by chewing
• 200,000 bites from cobras yearly in India result in 9000 deaths
• predominant family in Australia
• Subamily Hydophiinae - includes the highly poisonous sea snakes

3. Family Colubridae-large family of mostly the familiar, non-venomous snakes

• includes mildly poisonous snake species  which have fangs in the rear of the mouth - use their venom, usually, to tranquilize prey that has been caught by constriction
• the lyre snake Trimorphodon, the cat-eyed snake, Leptodeira, the black-striped snake, Coniophanes, and the night snake, Hypsoglina,   occur in Texas
• at least two African species (e.g., the boomslang) are large enough to be potentially lethal to humans.
4. the skull is highly specialized and allows for a special feeding apparatus that enables them to consume prey much larger than their own diameter [fig 15.6]
1. the 2 halves of the jaw are joined only by muscles and skin, allowing them to be dislocated so that large prey can be swallowed
2. also, many of the skull bones are loosely held together so that the entire skull can flex asymmetrically to enhance the swallowing process
3. to allow the snake to keep breathing during swallowing, the tracheal opening is pushed forward between the two jaw halves into an area where air can enter

Locomotion [fig 15.4]

1. numerous vertebrae, rigid ribs, and strong vertebral column allow quick, efficient lateral movement through undulations of the body wall muscles.
2. locomotion by:
   1. lateral undulation - S-shaped movement by exerting force against surface irregularities such as rocks, plants, and other features of the terrain
   2. concertina movement - enables some snakes to move through a narrow passage, such as burrows or when climbing trees by moving with in and using irregular channels in the bark
      • anchors posterior part of body then extends head, then anchors anterior part of body and draws body up forming new loops then anchors posteriorly
   3. rectilinear movement - in a straight line, such as when stalking prey; use ventral scales in an alternating pattern of movement, contraction, fixation, and stretching to achieve movement
   4. sidewinding - used by desert vipers to move across loose sandy surfaces with minimal surface contact; raises body in loops, touches ground at only 2 or 3 points.

chemical sensation in snakes by:

1. Jacobson's organs - pair of pit-like structures in the roof of the mouth; lined with olfactory epithelia that are highly folded to provide increased surface area for efficient detection of volatile chemicals
   1. the forked tongue picks up chemical from the air and conveys them to the mouth where the tips are inserted into the Jacobson's organs
   2. sensory information is then transmitted to the brain where the chemical scents are identified.

Pit Organs: heat sensitivity and detection

1. Family Viperidae - Subfamily Crotalinae - pit vipers - possess heat-sensitive pits on their heads between the nostrils and eyes
2. the nerves in the pits respond to radiant energy in the long-wave infrared region of the spectrum (5000 - 15,000 nm). they are especially sensitive to the heat of warm-bodied birds and mammals. may be able to detect temperature changes of only 0.003 C

Reproduction in lizards and snakes

1. most squamates are oviparous - lay shelled, fertilized eggs from which the young hatches
2. some species are ovoviviparous - eggs hatch inside of mother, e.g., all pit vipers
3. viviparous - embryo develops in a placenta-like structure inside of mother's uterus
   1. viviparity has evolved at least 45 times among lizards (e.g., skinks) and 35 times among snakes
4. parthenogenesis [fig 15-13]
   1. all female species: 6 families of lizards (e.g. Cnemidophorus); 1 snake (typhlopid)
   2. usually arise from a hybrid cross between two closely related species; most are diploid, some are triploid
   3. usually inhabit habitats formed by disturbance (e.g., fires, floods); can rapidly recolonize since they have 2x the reproductive potential of sexually reproducing species. They are "weeds."  

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