Thursday, February 27, 2014

Laboratory #5:
Lepidosauria: Serpentes & Ophidia
 
By: Meghan Kelley and Katie Sagarin


 
INTRODUCTION:

Squamata is composed of lizards, within Sauria, and snakes, within Serpentes, which together make up about 8000 different species that have been classified and are found on every continent, except Antarctica.  While lizards and snakes may appear different, snakes are actually a group of highly derived lizards, including specialized physical morphology and physiological functions, such as advanced chemosensory perception and envenomation of prey. Serpentes is further divided into two infra-orders: Scolecophidia and Alethinophidia. Scolecophidia are a smaller, more basal group whose members are fossorial and have akinetic skulls. Alethinophidia contains all the more well-known “typical” snakes, such as Pythonidae, Boidae, Colubridae, Elapidae, and Viperidae, which are represented in the photos above in order from left to right, respectively.
 
GENERAL MORPHOLOGY:
The closest relative to snakes are the varanid lizards, which have elongate necks, forked tongues, and streptostyly, a type of jaw articulation that allows the mouth to be opened wide.
 
 

 
Figure 1: The top picture is a varanid lizard skull, which has a reduced number of bones and fusion of many skull regions, similar to a snake's skull, shown in the bottom two images.  An additional similarity between the varanids and snakes articulation is that the quadrate (circled in red) drops down and re-positions more vertically, in order the fully open the mouth via streptostyly.  The bottom right picture shows the vertical position of the quadrate bone.
 In comparison to varanids, snakes have lost their limbs and modified their skulls even more extremely. The rami of the jaw do not fuse, allowing the jaw to be expanded laterally. Many snakes also use ratchet feeding, moving one side of their skull at a time to pull their prey into their throat. Because snakes tend to eat prey larger than their heads, many have a protrusible trachea which allows them to breathe while their mouths are full, as seen in Figure 2.


Figure 2: In the top diagram, the venom gland and duct (red), the protrusible trachea, and the chemo-sensing forked tongue, that brings in scent to the Jacobson's organ, are labeled. On the lower jaw of the snake skull in the lower picture, it is clear that the rami, or two mandibular regions, are not fused.

VENOM DELIVERY & DENTITION:
Snakes can have some combination of four tooth conditions. The most basic is aglyphous teeth, which point straight down in the mouth and are generally conical or laterally compressed. Opisthoglyphous teeth point posteriorly and are generally positioned at the back of the maxilla to keep prey in the mouth. Proteroglyphous teeth are the most anterior on the maxilla and are hollow for venom delivery, and are only found within the family Elapidae. Lastly, solenoglyphous teeth are hollow, retractable teeth on a kinetic maxilla, perfect for venom injection. They are only found within the family Viperidae. All four teeth types are pictorially represented in Figure 3, and the potential evolution of these derived teeth is shown in Figure 4.
 
 



Figure 3: These are the four types of teeth that developed in different snake groups, shown with their groove or channel of venom delivery, indicating that proteroglyphous and solenoglyphous are the more efficient venom delivery systems.
Figure 4: The four teeth conditions in snakes phylogenetically developed at different rates with the deriving of modified salivary glands that became venom glands. "Glyphous" refers to the channels or grooves in some teeth types that function as a delivery system for venom. a) This is a color-coded phylogenetic tree of approximate development of differing teeth types. Front-fanged in this figure refers to the relative position of either proteroglyphous (Elapidae) or solenoglyphous (Viperidae) fang types.  Aglyphous, or non-grooved teeth, are considered the basal condition of teeth in snakes and are found to some extent in all snakes.  b) These are lateral views of the four teeth conditions; white circles represent the fangs that are present. c) This is a palatal view, in which the red region is the maxilla region, and the fangs are circled in black. This image is taken from Vonk et al., 2008.
FAMILY CHARACTERISTICS:

-Pythonidae: Contains 9 genera, distributed throughout sub-Saharan Africa, and south and southeast Asia to Australia, spanning deserts and forests alike. They are large to giant snakes with teeth on their premaxilla (except in Aspidites). Many have infrared receptors in labial pits which allow them to perceive body heat signatures. All species of Pythonidae are oviparous, and most coil around their eggs to incubate them. Some species have reportedly been known to eat tigers.

Ohio species: NONE

-Boidae: Contains 12 genera, distributed throughout western North Africa to South America and the Caribbean, up the west coast of North America, and southwestern Asia/the Middle East. Boas are small to large, but contain the largest species of snake, Eunectes murinus, the anaconda. Most have infrared receptors in inter-labial pits (Figure 5), and cloacal spurs, which are vestiges of  hind limbs.
 
 
Figure 5: Notice the conspicuous inter-labial pits laterally positioned from the nostrils, under the eyes.
 
Ohio species: NONE

-Elapidae: Contains 62 genera, distributed mainly in the southern hemisphere or near the equator. Elapids are proteroglyphous, and do not have infrared receptors. Elapids cover a wide range of lifestyles, from arboreal to fossorial to aquatic. This family includes coral snakes, sea snakes, cobras, and death adders.

Ohio species: NONE

-Colubridoidea: Contains more than 100 genera, distributed globally. Colubridae contains the most species of any family of snakes. Some colubrids have opisthoglyphous dentition, and have a variety of body forms, with almost no distinct physiology tying them together as a group.

Ohio species:
Opheodrys vernalis (Smooth Green Snake)

Opheodrys aestivus (Rough Green Snake)
 
Figure 6: Opheodrys aestivus, the rough green snake, is different from the smooth green snake because of its conspicuously keeled scales that give it a rough texture.
Coluber constrictor (Blue Racer)

Pantherophis vulpina (Fox Snake)
 
Pantherophis obsoletus (Black Rat Snake)
Figure 7: Top row from left to right includes a juvenile and an adult blue racer, while the bottom left is a fox snake and the bottom right is a black rat snake.  Some of the most important ways of telling confusing snake species apart is by including the type of habitat that they are found in; for example, black rat snakes and fox snakes are usually found in dryer forested habitats, sometimes arboreal, and blue racers are found in wet, possibly marshy terrestrial habitats.
Lampropeltis getula (Black Kingsnake)

Lampropeltis triangulum (Eastern Milk Snake)

Figure 8: On the left, the black kingsnake is represented, and the Eastern milk snake is shown on the right.
-Natricidae: Contains 38 genera distributed from North to Central America, Africa, and Eurasia through the East Indies. Many members of Natricidae are aquatic, both fresh- and salt-water. The New World species are all viviparous, while the Old World species are mainly oviparous.

Ohio species:

Nerodia sipedon pleuralis (Midland Water Snake)
Nerodia erythrogaster (Copperbelly Water Snake)
Nerodia sipedon sipedon (Northern Water Snake)
Regina septemvittata (Queen Snake)

Figure 9: Left is the Northern Water Snake, and right is the Queen Snake, both of which are very common, especially in the Midwest.
Thamnophis butleri (Butler's Garter Snake)
Thamnophis radix (Eastern Plains Garter Snake)
Thamnophis sauritus (Ribbon Snake)
Thamnophis sirtalis (Eastern Garter Snake)

Figure 10: This is the common Eastern Garter Snake; however, there is much variation in color and striping within this species and within the genus Thamnophis.
 
Storeria occipitomaculata (Red-belly Snake)
Storeria dekayi (Northern Brown Snake)
 

Figure 11: Red-belly snake is on the right, and the Northern Brown Snake is on the left. Both are photos taken in the field in the eastern U.S.

 
Virginia valeriae (Smooth Earth Snake)
 
-Dipsadidae: Contains 97 genera, distributed throughout the New World. Again, this is a highly diverse group that occurs in all habitats, except marine.

Ohio species:
Diadophis punctatus edwardsi (Northern Ringneck Snake):
 
Heterodon platyrhinos (Eastern Hognose Snake):
 
Carphophis amoenus (Worm Snake):



-Viperidae: Contains 37 genera, distributed globally except for Oceania, Madagascar, and the Arctic. Vipers have tubular, solenoglyphous teeth (perfect for venom injection), and triangular heads. Members of the subfamily Crotalinae have an infrared-sensing pits between their eyes and nostrils, allowing them to “see” heat signals stereoscopically. Other Viperidae taxa simply have infrared receptors beneath scale surfaces.

Ohio species:

Agkistrodon contortrix mokasan (Northern Copperhead: Moccasin)
Crotalus horridus (Timber Rattlesnake)
Sisturus catenatus catenatus (Eastern Massassauga):
 
PICTURES CITED:
Photos under title:
 
 
 
 
 
General Morphology:
 
 
 Venom Delivery and Dentition:
 
Vonk, F. J., J. F. Admiraal, K. Jackson, R. Reshef, M. A. G. Bakker, K. Vanderschoot, I. van den Berge, M. van Atten, E. Burgerhout, A. Beck, P. J. Mirtschin, E. Kochva, F. Witte, B. G. Fry, A. E. Woods, and M. K. Richardson. 2008. Evolutionary origin and development of snake fangs. Nature: 630-633.
 
Family Characteristics:
 
 
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