Sunday, 13 June 2010

Dimorphodon; a Pterosaur of prey

Above; Dimorphodon was perhaps the largest and meanest Pterosaur of its time. Here; in a reconstruction by Pterosaur guru Mark Witton, Dimorphodon is shown trying to steal a fish from a smaller Pterosaur. It now seems that Dimorphodon was no way near agile enough to victimize other Pterosaurs in this way and that it most likely fed on small land animals and not fish.

Dimorphodon was and is a very interesting Jurassic Pterosaur that is clearly related to Peteinosaurus of the Triassic due to its distinctive deep skull. People have suggested that it was a piscivore that grabbed fish from the water surface. Others think it was a carnivore; feeding on small animals. It certainly possessed the gear to do both of these things. Let us now look at the features of Dimorphodon.

Above; Dimorphodon was rather ungainly in the air due to its heavy body, short wings and large head. Artist Dmitry Bogdanov painted this reconstruction and seems to highlight the large teeth at the front of the jaw. These were most probably used to grab small prey animals.

It was small for a Pterosaur; but by modern standards it was enormous; with a wingspan of 1.4 meters; it was something to certainly watch out for when roaming the Liassic coastline of England. Something like a giant bat 1 meter long, with claws and a toothy beak coming from the sky straight at you would be a terrifying sight. However due to its heavy structure and relatively small wings Dimorphodon would have been a terrible flier. The large claws on the fingers and the flexible outer toe suggest that this animal was a climber, only using flight to attack prey from a short distance away or to get away from larger predators. It was unable to soar over the ocean for miles as puffins do today, but rather ponderously flap and glide from tree to tree. Unlike most arboreal and areal predators; Dimorphodon did not have binocular vision; according to reconstructions. These would have made it unlikely to have performed long distance or challenging areal attacks that creatures like peregrine falcons carry out today.
The name Dimorphodon means "two types of teeth"; 9 long gripping teeth at the front of the mouth and a series of smaller pointy teeth further back. It seems that the front 9 teeth were for plucking prey either from the ground or the surface of trees. Even though the short wings made Dimorphodon ungainly as a long distance flier, they would have been advantageous for a large volant animal living in a dense environment such as a forest; where individual flights would be a maximum of a few hundred meters at a time. The short wings would have made Dimorphodon relatively agile.

Because of such short wings; as mentioned earlier; Dimorphodon was not a good long distance flier and was probably inefficient as a piscivore. Volant marine fish eaters often need long, large wings to help them fly over long distance to find their sparsely populated food with little energy. The deep puffin like head of Dimorphodon and its presence in marine deposits has, however lead many to believe that it was a fish eater, but as the wing shape suggests that is not likely.

Above; an old; yet fairly up to date skeletal reconstruction of Dimorphodon. Note the large powerful skull.

Dimorphodon can therefore be visualized waiting in trees for small Dinosaurs or other animals to pass under it; possibly jumping from tree to tree as it stalked, and then; finally leaping from the air it would sail silently and precisely towards its victim, using actinofibrills (small tendons) in its wing membrane to warp its wing and keep directional control. During the attack it is likely that Dimoprphodon would land first in its powerful arms, gripping the prey with the huge claws. Perhaps with smaller prey Dimorphodon did not even need to land; its flexible outer toe was a perfect gripping organ with which the animal could grab a victim in mid flight. With larger prey it is likely that to prevent running into any trouble on the ground; Dimorphodon would lift the food with its mouth and scurry up a tree to feed in peace.

Above; yet another work of Mark Witton; this time showing Dimorphodon in its most likely habitat. It is shown climbing up a tree with a small mammal held in its jaws.

The advantages of being able to glide and fly in an arboreal environment are huge. The ability allows the hunter to attack prey in a silent manner from a long distance away. Non volant animals however, have to walk towards their prey. This takes up much more time and effort than gliding and also produces a lot of noise, decreasing the predators chances of making a kill.

Above; two possible poses for Dimorphodon when on the ground. The top reconstruction is an illustration from Seely's 1901 book "Dragons of the Air" showing Dimorphodon in an awkward looking quadrupedal pose. The image is surprisingly up to date for a Victorian Pterosaur illustration. Certainly more so than those made in the 1960's. It even appears to show a covering of hair; a supposition most likely made by the artist that later turned out to be correct! Below this is a skeletal reconstruction showing Dimorphodon in a two legged stance. The hip joints would certainly allow this to be possible, but weather or not the thin tail was capable of balancing that enormous head when standing still is unknown, but seems unlikely. Perhaps by spreading its wings whilst running enough lift necessary to keep the body and large head up would be created. A two legged stance seems to me a reasonable assumption if Dimorphodon hunted prey on the ground; as it would need to get away quickly if a ground dwelling predator arrived. In my opinion Dimorphodon probably adopted both stances when on the ground. The former quadrupedal stance for when it was moving normally and the bipedal stance for when it needed a quick getaway or to take off.

It is evidently unlikely that Dimorphodon hunted very small tree dwelling animals like insects; the jaws were too large to fit into the cracks that these animals frequently reside in. Perhaps these Pterosaurs picked on more medium sized vertebrate critters that lived both on the ground and in the air. The strong build and unspecialized teeth of Dimorphodon do suggest that this animal was designed to take on a wide range of prey.

Above; 4 images in descending order: Top is a picture of Dimorphodon in dorsal view. Circled in pink is a membrane hypothesized by Richard Owen to have been supported by the prehensile 5th toe and the tail. Upper middle is an illustration of a microbat in ventral view with a similar membrane to the one hypothesized to have existed in Dimorphodon also circled in pink. Lower middle is an illustration of a microbat skeleton in ventral view with a similar structure to the independant and prehensile toe found in the foot of Dimorphodon circled in blue. This feature; known as the calcer is made of cartilage and is not a modified toe as in Dimorphodon. The calcer supports the membrane between the legs and the tail. Bottom is a close up of a microbat foot in dorsal view showing clearly the calcer. The fact that modern day bats possess a convergent foot structureto that seen in Dimorphodon suggests that like modern bats it had a membrane extending between its leg and tail. This is further supported by the fact that both microbats and Dimorphodon possess a divergent structure from the foot to support this membrane. In the case of Dimorphodon; a modified toe. In the case of bats; a cartilage projection. The use of this extra membrane is probably to increase the surface area of the wing and; as seen in bats; to help in apprehending prey.....

In the video above you can see the hunting methods of two microbats. It is the latter Natterer's bat that we are interested in; you may wish to skip through the bit about long eared bats. Anyway; this clip shows how the Natterer's bat uses the membrane between its legs to scoop its prey (in this case a spider) up and engulf it. Perhaps with Dimorphodon a similar thing was done with its interlegged membrane. It may have swooped down on prey on the ground and merely scooped it up with its interlegged membrane, subsequently curling its legs up in mid flight to bring the prey item to its jaws to be eaten, but considering the questionable flight abilities of Dimorphon this suggestion may not be the case.

Whatever the habits of Dimorphodon it is an interesting animal that I intend to do more research on. Dimorphodon certainly needs to be studied further.

1 comment:

  1. That "big" head you say prevented bipedal locomotion was 95% non-bone, as you can see, less mass than a toucan, pelican or Diatryma. The addition of at least two vertebrae to the pterosaur sacral series also argues for an elevation of the spine. Then there's that overly long balancing tail.