Pterosaur diversity

From their humble beginnings as arboreal leapers, the pterosaurs went on to become a very diverse group of flyers, filling a wide range of different niches. The Triassic pterosaurs appear to have been primarily small piscivorous and insectivorous forms. However, the Jurassic saw the evolution of a wide variety of new forms, including the small predator Dimorphodon, the piscivorous rhamphorhynchids, the insectivorous anurognathids, the filter-feeding ctenochasmids, the opportunistic dsunganipterids, and more (Wellnhofer, 1991).

Figure 1: Anurognathus, a small insectivorous pterosaur from the late Jurassic. Artist: Maija Karala.

While pterosaurs had started out with long tails used to help with stability, more short-tailed pterosaurs showed up in the Jurassic and by the Cretaceous only short-tailed forms remained. Many Cretaceous pterosaurs became both larger and stranger than ever before. One of the strangest pterosaurs to have ever lived would have to have been the ctenochasmid Pterodaustro, from the early Cretaceous of Argentina. While ctenochasmids had been filter-feeding since the late Jurassic, Pterodaustro took it to an extreme degree. Each side of its lower jaw had almost 500 elastic, baleen-like "teeth" used to sift out small aquatic organisms, much like what flamingos do today (Wellnhofer, 1991).

Figure 2: The bizarre filter-feeding pterosaur Pterodaustro. Artist: Julio Lacerda.

Many pterosaurs filled predatory niches, such as the stork-like azdarchids and the vulture-like istiodactylids (Witton, 2012). In contrast, there were also pterosaurs that preferred a more plant-based diet, such as the tapejarines, who fed on fruits and nuts (Vullo et al, 2012).

Figure 3: A pair of Tupandactylus, a type of tapejarine from the early Cretaceous. Artist: Mark Witton.

While no pterosaur ever became flightless, some like the dsunganipterids and the azhdarchids, developed more ground dwelling lifestyles. The azdarchids had lifestyles akin to giant storks and could grow to a massive sizes, with the largest having wingspans of 10-11 metres and body masses of 200-250 kilograms, making them the largest flying animals to have ever lived. Pterosaurs could reach greater sizes than flying birds, due to their wings being stronger and providing more lift than bird wings and due to their quadrupedal launch giving more launch power than the bipedal launch of birds (Witton & Habib, 2010).

Figure 4: An azhdarchid tries to keep its prey- a dromaeosaur- away from some thieving dsunganipterids. Artist: Mark Witton.

This post merely scratches the surface of pterosaur diversity but hopefully gives a good taste of it. Next up we shall see the evolution of a new group of flyers- birds and other flying dinosaurs.

References
Vullo, R., Marugan-Lobon, J., Kellner, A. W. A., Buscalioni, A. D., Gomez, B., Fuente, M. d. l. & Moratalla, J. J. (2012). A New Crested Pterosaur from the Early Cretaceous of Spain: The First European Tapejarid (Pterodactyloidea: Azhdarchoidea): e38900. PloS One, 7 (7).
Wellnhofer, P. (1991). The Illustrated Encyclopedia of Pterosaurs. Crescent Books: New York.
Witton, M. P. & Habib, M. B. (2010). On the Size and Flight Diversity of Giant Pterosaurs, the Use of Birds as Pterosaur Analogues and Comments on Pterosaur Flightlessness. PloS One, 5 (11).
Witton, M. P. (2012). New Insights into the Skull of Istiodactylus latidens (Ornithocheiroidea, Pterodactyloidea): e33170. PloS One, 7 (3).

Image sources
Figure 1: Accessed March 31, 2015, from http://41.media.tumblr.com/e5584185d859a6e0d7b91f6e78d4698e/tumblr_mvcyy72ht01rbztl0o1_1280.jpg
Figure 2: Accessed March 31, 2015, from http://fc03.deviantart.net/fs71/f/2012/131/2/1/feeding_off_a_mirror_by_karkajou1993-d4vg3i3.png
Figure 3: Accessed March 31, 2015, from http://invivomagazin.sk/admin/obrazky/382051192tupan%20witton.jpg
Figure 4: Accessed March 31, 2015, from http://scienceblogs.com/tetrapodzoology/wp-content/blogs.dir/471/files/2012/05/i-18b8cd89e1c14b6727ca53eca47ca33c-Witton-dsungaripterids-hound-azhdarchid-Sept-2010.jpg

Pretty pycnofibres, wondrous wings and amazing air sacs

The first important feature needed to be a flying tetrapod is a warm-blooded metabolism in order to produce enough energy to sustain powered flight. Pterosaurs had this. For insulation they had a thick pelt of hair-like structures called pycnofibres. A pterosaur's pelt could be quite dense. For example, the Jurassic pterosaur Rhamphorhynchus had about 20 pycnofibres per square millimetre and each one was 2-3 millimetres long (Benton, 1998).

Figure 1: On this Rhamphorhynchus the hair-like pycnofibres are clearly visible. Artist: Luis Rey.

The most important feature for flight though are of course wings and the pterosaur wing was one of the most amazing structures to have ever evolved. The fourth finger had become extremely elongated, while the fifth finger had vanished. From the elongated fourth finger was attached a large membrane that served as an airfoil surface. This membrane was comprised of layers of skin, muscle tissue and air pockets. Supporting the wing were special fibres called the aktinofibrils. As well as reinforcing the flight membrane, the aktinofibrils also prevented wobbling in flight and allowed pterosaurs to change their wing shape for better maneuverability (Wellnhofer, 1991).

Pterosaurs launched into the air in a quadrupedal fashion where the hind limbs were used to provide forward momentum while the forelimbs (wings) were used to provide a vertical heft that launched the pterosaur into the air (Habib, 2008). At least some pterosaurs were even able to launch from water (Habib & Cunningham, 2010).

Figure 2: Cretaceous pterosaur Ornithocheirus launching from water. Artist: Mark Witton.

 Pterosaurs also had another amazing feature. As well as lungs they also possessed an advanced system of air sacs that extended into their skeleton leaving openings in the bones. The only other animals known to possess such a system are saurischian dinosaurs (inlcuding birds). As with the dinosaurs, the air sacs of the pterosaurs allowed air to flow in a single direction and get utilised as efficiently as possible. Pterosaur air sacs, however, also ended up evolving a feature not possessed by dinosaur air sacs. The larger pterosaurs could inflate subcutaneous air sacs in their wings to alter their mechanical properties, such as relative stiffness (Claessens et al, 2009).

Figure 3: This cutaway of Cretaceous pterosaur Anhanguera shows lungs (red), neck air sacs (green) and wing air sacs (blue). Artist: Mark Witton.

Next up will be pterosaur diversity before moving onto the next group of flyers- birds and other dinosaurs.

References
Benton, M. (1998). The Reign of the Reptiles. Eagle Editions: Hertfordshire.
Claessens, L. P. A. M., O'Connor, P. M., & Unwin, D. M. (2009). Respiratory Evolution Facilitated the Origin of Pterosaur Flight and Aerial Gigantism. PloS one, 4 (2).
Habib, M. B. (2008). Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana. B28, pp. 159-166.
Habib, M. B. & Cunningham, J. (2010). Capacity for water launch in Anhanguera and Quetzalcoatlus. Acta Geosci. Sin 31, pp. 24-25.
Wellnhofer, P. (1991). The Illustrated Encyclopedia of Pterosaurs. Crescent Books: New York.

Image sources
Figure 1: Accessed March 24, 2015, from http://pterosaur.net/species/Rey%20Rhamphorhynchus.jpg
Figure 2: Accessed March 24, 2015, from http://www.markwitton.com/communities/6/004/009/119/796/images/4618976279.jpg
Figure 3: Accessed March 24, 2015, from http://i.livescience.com/images/i/000/002/908/i02/090217-pterosaur-ballooning-02.jpg?1296073588

The origin of the pterosaurs

The first tetrapods to evolve powered flight were the pterosaurs. These were a group of archosaurs related to the dinosaurs, but not dinosaurs themselves. The earliest known pterosaur was Eudimorphodon, who lived in what is now Italy around 230-220 million years ago, in the late Triassic. However, while the earliest known pterosaur, Eudimorphodon had specialised multi-cusped teeth not found in any of the later pterosaurs, so it would not have been ancestral to them but rather part of a distinct pterosaur lineage that died out in the Triassic. Furthermore, both Eudimorphodon and other late Triassic pterosaurs are "completely" developed, having all the typical pterosaur skeletal characteristics. This suggests the origins of pterosaurs may lie even further back in the past, in the earlier Triassic or perhaps even in the Permian (Wellnhofer, 1991).

Figure 1: Triassic pterosaurs Eudimorphodon (right) and Peteinosaurus (left). In the background a Ticinosuchus wanders about. Artist: John Sibbick.

No fossils of the pterosaurs' immediate ancestors are known. The most likely theory on their origins is that they evolved from arboreal creatures that would leap from branch to branch, flapping their forelimbs to stay airborne longer. Pterosaur hips had great freedom of movement, their knees and ankles were hinge-like and their feet were plantigrade. The knees and ankles did not permit the necessary rotation for them to move bipedally, so pterosaurs were obligate quadrupeds (though they may have had bipedal ancestors). A possible explanation for these features is that the early pterosaurs or proto-pterosaurs were arboreal creatures that evolved powerful leaping from branch to branch as an active mode of transport not dissimilar to that of arboreal leaping primates (Christopher, 1997). These arboreal leapers would not have been gliders, who merely fall slowly downwards and forwards with the help of special flaps, but rather creatures utilising a quite different form of locomotion, one that led them to eventually having their forelimbs evolve into more and more sophisticated flapping airfoils.

Figure 2: A hypothetical series of pterosaur ancestors. Artist: Maija Karala.

The next blog post shall delve into more detail on the anatomical and physiological features that made the pterosaurs such great fliers.

References
Christopher, B. S. (1997). The arboreal leaping theory of the origin of pterosaur flight. Historical Biology. 12 (3).
Wellnhofer, P. (1991). The Illustrated Encyclopedia of Pterosaurs. Crescent Books: New York.

Image sources
Figure 1: http://www.moensklint.dk/media/37420/eudimorphodon_large.jpg
Figure 2: https://gwawinapterus.files.wordpress.com/2014/02/tumblr_mrseawh0qk1sx3tcvo1_1280.jpg

When tetrapods learned to fly

The earliest animals to fly were insects and for a long period these were the only flyers on Earth. Later on, however, flight also evolved independently in more than one group of tetrapod.

If one includes gliding as a form of flight, then the earliest flying tetrapods would have been the weigeltisaurs of the Permian (Benton, 1998). Since then, gliding has independantly evolved in numerous different groups of tetrapod. However, if one is referring to true powered flight then the first flying tetrapods were the pterosaurs, who first appeared in the Triassic. Later, powered flight evolved in certain maniraptoran dinosaurs, including the birds. Later still it evolved in a group of mammals, the bats.

Coelurasauravus, a gliding creature from the Permian.

In all of these groups, the forelimbs evolved into a new type of structure- wings. A wing is an aerofoil structure, with a curved top and a flat bottom. This causes air flowing across the wing to "stretch" over the top, resulting in lower air pressure above the wing than below it and resulting in the higher air pressure below pushing the wing upwards. Each of the groups of flying tetrapod had forelimb-derived wings utilising this basic principle. However, each group had a very different type of wing design. The pterosaurs had "finger wings" in which an elongated fourth finger supported a membranous wing reinforced by stiffening fibres. Birds and other flying dinosaurs had "arm wings" comprised primarily of feathers. Bats had "hand wings" in which a leathery elastic membrane stretched between adjacent fingers. (Raymer, 1988 & Shipman, 1998). These different designs of structures used for the same purpose show how evolution can produce differing solutions to a single problem.

Bird, bat and pterosaur wings all derive from the forelimbs but each in a different way.

 Future blog posts will go into further detail on the origins and evolution of the pterosaurs, the birds and other flying dinosaurs, and the bats, and on the wide diversity that evolved within all these groups of flyers.

References
Benton, M. (1998). The Reign of the Reptiles. Eagle Editions:Hertfordshire.
Raymer, J. M. W. (1988). The evolution of vertebrate flight. Biological Journal of the Linnaean Society, 34 (3), pp. 269-287.
Shipman, P. (1998). Taking Wing. Touchstone: Rockefeller Center.

Image sources
http://f.tqn.com/y/dinosaurs/1/S/j/a/-/-/ABcoelurasauravus.jpg
http://ncse.com/files/images/Wing_morphology.img_assist_custom.jpg