“How the Viper strikes – One little helpful bone in the skull makes all the difference” by: Dr. Jim Mead
Ok – let’s side-step from the giant critters such as mammoths and bison for just a moment. Have you ever wondered how a pitviper (such as a rattlesnake or copperhead) can open its mouth so wide and then rotate out a huge, long set of fangs? A lot of this action has to do with one little bone in the skull called the ‘prefrontal’. Look at the photo taken by Sandy Swift of the skull of the non-venomous corn snake (Pantherophis guttatus). The front of the face is to the left. Note the green, darkened-in area to the left of the eye area – this is the prefrontal bone. On most snakes this bone helps form the orbit to protect the eye. It is well connected to the other bones of the skull and forms a fairly rigid orbit protection. You can get an idea of the shape of this little bone by the outline.
Corn snake skull. Top specimen is a view from above (dorsal) and the lower is a side view. The green area is the prefrontal bone.
Now take a look at the skull of the rattlesnake (Crotalus adamanteus) and after being impressed with the huge fangs, note the green, darkened-in area to the left of the eye area – again the prefrontal bone. The shape is different AND more importantly, it is loosely attached to the bones adjacent to it. When the mouth is wide open, the prefrontal bone maneuvers or rotates (sort of like a ball-bearing) both against the frontal bone (above the eye on top of the head), but also with the maxilla that holds the fangs. This double-joint allows the maxilla to really rotate back and permits the fangs to be projected down and out. Follow up the fang from the tip to where it connects to the bone (maxilla) – note the depression or pit. This is the heat sensory pit (loreal pit) and is why these vipers are called the ‘pitvipers’.
The rattlesnake with same views as above and showing the heat-sensory pit and the large fangs. The prefrontal bone helps the fang on the maxilla bone to rotate to the extreme which permits the fang to be well-placed for biting and envenomation. Photo by Sandy Swift.
The Family Viperidae contains two subgroups or subfamilies: Viperinae (Old World vipers) and Crotalinae (pitvipers). To fully understand the morphological shape change possible within the Viperidae, we really need to examine all possibilities with both the Old World vipers and the pitvipers. Old World vipers are found only in the Eastern Hemisphere, whereas the pitvipers are found not only throughout North, South, and Central America but also in eastern Asia such as China, Vietnam, and islands off its coast. Typical of most cranial bones, the size and shape of the prefrontal bone can change shape as the individual gets older – we certainly see bones change in shape within humans as we get older. Because of this issue, ‘young’ and adult specimens of vipers are examined of all types – as much as possible. A total of 23 genera and 56 species of vipers are being examined for a total of at least 170 individuals. The following Asian pitvipers are examined for our project: Calloselasma (Malayan pitviper), Gloydius (Mamushi), Hypnale (hump-nosed moccasin), Ovophis (Okinawa pitviper), Protobothrops (brown spotted pitviper), Trimericerus (bamboo pitviper), and Tropidolaemus (temple pitviper). The pitvipers from the Americas include; Agkistrodon (copperhead, water moccasin, cantil), Atropoides (jumping pitviper), Bothriechis (palm-pitviper), Bothropoides (lancehead), Bothrops (fer-de-lance), Cerrophidion (montane pitviper), Crotalus (rattlesnake; 20 species in our study), Lachesis (bushmaster), Porthidium (hognosed pitviper), Rhinocerophis (Urutu), and Sistrurus (pygmy rattlesnake).
The photographs below were taken by Sandy Swift who is an integral part of this overall project. The black bar in each photograph with the prefrontal is a 1 mm scale bar for size.
The shape of the prefrontal from Old World vipers shown below is distinctly different from those in the subfamily Crotalinae. The below are two views (front and back) of the same bone from different species of vipers.
Now compare the below, the same prefrontal bone in the same views as above show that pitvipers have a distinct shape and are readily identifiable. First is the water moccasin (Agkistrodon) and the next is the western diamondback rattlesnake (Crotalus). Quite distinctly different shapes can be observed in these two New World pitvipers.
Now compare these two American pitvipers with an East Asian pitviper (Tropidolaemus) below.
These changes seen above and the many more observed in the 23 genera of vipers examined will allow paleontologists to identify these 2-4 mm long bones found in fossils sites. We are recovering these minute bones in caves in southern China (Guangxi), in the Miocene-age fossil site in eastern Tennessee (Gray Fossil Site), and in Persistence Cave right here in the Black Hills of South Dakota. AND more about our Persistence Cave project very soon!