Building a model requires a thorough understanding of the osteology of the animal, so fossil specimens of Kuehneosaurus and Kuehneosuchus were studied at the Natural History Museum (London). Pamela Robinson had completed a full, formal description of Kuehneosaurus and Kuehneosuchus, but this remained unpublished at her death. A thorough revision of her work is underway by S. E. Evans, and this will provide a full monographic description of the material. The published reconstructions by Robinson (1962) and Romer (1966) and the unpublished reconstructions (Robinson, 1979) are well supported by the osteological evidence (S. E. Evans, pers. comm. 2007).
Robinson (1962, 1967, 1979) reported that apart from the dorsal vertebrae and ribs, the osteological differences between Kuehneosaurus and Kuehneosuchus are minute, so a full individual description of each taxon is not necessary. Kuehneosuchus is a delicately built animal (Text-fig. 1), with slender limbs, a long tail, and a broad pair of ‘wings’ supported by elongate ribs that extend laterally and backwards at about 45 degrees to the midline. It differs from Kuehneosaurus primarily in the extent of the lateral ‘wings’ (Text-fig. 2).
The long bones (BMNH R5983, R.6112, R.6189, R.6200) have hollow, thin-walled shafts and coarsely spongy articular ends thinly covered with compact bone. The flat bones, particularly the coracoids, the larger ribs, and the transverse processes of the mid-dorsal vertebrae, are built from surfaces of compact bone ‘no thicker than tissue paper’, surrounding a coarsely spongy interior. This makes the bones extremely fragile and light. Although the lightness of the bones may be an adaptation for aerial locomotion, there is no sign that they were pneumatic; there are certainly no openings for air sac extensions from the lungs (Robinson 1979).
The third cervical vertebra (BMNH R.6009, R.6016) has two rib articulations on the centrum, a ventral parapophysis, and a dorsal diapophysis. These articulations supported a very small, double-headed rib. On the fifth cervical vertebra (BMNH R.6015) a third rib articulation develops on the neural arch dorsal to the diapophysis, termed by Robinson (1962) the dorsal diapophysis. The ribs have three heads, and this continues back to the ninth cervical vertebra (BMNH P.L.R.134). The dorsal diapophysis elongates until, at the seventh vertebra, it is the most prominent rib articulation. At the tenth vertebra (BMNH R.5998), the parapophysis disappears, and the dorsal diapophysis increases further in importance. From this region backwards, both diapophysis and dorsal diapophysis elongate laterally, one remaining vertically below the other, and the space between them is gradually filled in by bone, except at the distal ends.
The anterior dorsal vertebrae have very long, broad, transverse processes that are divided into two portions at their distal ends but, near the mid-dorsal region, the processes become a single undivided structure. By this point, the dorsal vertebrae have become extremely broad, measuring as much across the transverse processes as the width of the back of the skull. The broad, blade-like transverse processes have their edges directed almost vertically, and they are supported on the neural arch by well-developed buttresses. The dorsoventral width of the transverse processes gradually diminishes towards the sacral region until the bones become rod-like. The length of the rod then decreases until it has almost disappeared in the first sacral vertebra. Kuehneosaurus differs from Kuehneosuchus in minor details of the relative length, breadth, and buttressing of the transverse processes of some of the mid-dorsal vertebrae (Robinson 1962).
In Kuehneosaurus the ribs in the anterior dorsal region curve round to the sternal cartilage and support the pectoral girdle. The mid-dorsal vertebrae bear elongate ribs (BMNH R.8172), about two and a half to three times the length of the extensive transverse processes that bear them. These ribs hardly curve, so they extend out almost horizontally, and are nearly perpendicular to the long axis of the body. Their vertical width matches that of the transverse process that bears them, and they are very thin anteroposteriorly, being lath-like structures. Behind the mid-dorsal region the ribs become rod-like to match the transverse processes of the vertebrae, and also shorten towards the sacral region. Kuehneosaurus and Kuehneosuchus also retain small abdominal ribs (Robinson 1979). The type specimens of Kuehneosaurus (BMNH R.8172) and Kuehneosuchus (BMNH R.6111) consist of articulated partial skeletons, which illustrate that the 11 pairs of mid-dorsal ribs could be folded back along the body (Robinson 1962), as in Draco.
Kuehneosuchus differs from Kuehneosaurus in a few main characters of the 11 pairs of mid-dorsal ribs (BMNH R.6111), most prominently their much greater elongation (Text-fig. 3). The first pair of these ribs is about two and a half times the width of the skull, and the second pair more than five times this width, or nearly three times the length of ribs from the same region in Kuehneosaurus. The following pairs progressively diminish in length. The distal half of the shaft of the longer ribs is rod-shaped in the posterior pairs, whereas in Kuehneosaurus they are vertically wide at the distal ends. In all but the first pair there is a moderate ventral curvature of the more distal portion of the shaft.
Figure TEXT-FIG. 3.. A, mid-dorsal vertebra and rib in anteroposterior view of Kuehneosaurus latus, based on BNMH R.8172 and R.6017; length of rib is 48 mm. B, mid-dorsal vertebra and rib in anteroposterior view of Kuehneosuchus latissimus, based on BNMH R.6111; length of the incomplete rib is c. 130 mm. Note the difference in overall morphology and extent of the wing-ribs in both species. Redrawn from Robinson (1962).
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The first four caudal vertebrae are preserved associated in Kuehneosaurus (BMNH R.8172). Based on the large number of dissociated caudals of various intermediate types, Robinson (1979) assumed that, like Icarosaurus, the British kuehneosaurids had a long tail. However, there is no definitive evidence for this, so the aerodynamic effect of tail length was investigated.
The models and the wind tunnel
The two-dimensional skeletal reconstruction of Kuehneosuchus (Romer 1966) was used as a basis for the model-making. Key measurements (Table 1) were checked on specimens and from Robinson (1979). Modelling plasticine was added to the skeletal templates to make a three-dimensional body outline. The limbs and main body were constructed separately in order to make the limbs articulate. Hands and feet were cut from a 3-mm-thick aluminium sheet and wings were cut from 1-mm-thick aluminium. The wings of Draco show little sign of deflection under aerodynamic load, so flexible wings were not manufactured for our models. Rubber moulds were made from the plasticine reconstructions, allowing multiple cast plastic models with different wing types to be made. The plastic body parts were then assembled by drilling holes in the flanges on the limb parts (and hands and feet) and bolting them together. The bolted joints were covered with plasticine and the wings were attached to the trunk with bolts.
Table 1. Key measurements in mm (unless otherwise denoted) of the two British kuehneosaurid genera, based on specimens in the BMNH and on Robinson (1962, 1979).
|Length of skull||42||42||Reconstruction in Robinson (1962, 1979)|
|Maximum width of skull||38||38||Reconstruction in Robinson (1962, 1979)|
|Length of humerus||39||39||R.5981, R.6111, R.6189, R.6200, P.L.R.95, P.L.R.96, P.L.R.97|
|Length of radius||26||26||P.L.R.107|
|Length of ulna||26||26||P.L.R.94|
|Length of femur||57||57||R.5982, R.5983, R.6112, P.L.R.81, P.L.R.87|
|Length of tibia||51||51||R.6112|
|Snout-vent length||250||250||Reconstructions in Robinson (1979)|
|Total length with original tail||720||720||Reconstructions in Romer (1966) and Robinson (1979)|
|Total length with short tail||580||580||Measured on model|
|Wingspan||400||143||Reconstructions in Romer (1966) and Robinson (1979)|
|Chord||102||100||Reconstructions in Romer (1966) and Robinson (1979)|
|Wing surface area (cm2)||406.0||143.0||Reconstructions in Romer (1966) and Robinson (1979)|
Four life-size models were built (Text-fig. 4), each with differing aerodynamic properties (Table 2): three of Kuehneosuchus with wings with different degrees of camber (Text-fig. 5) and one of Kuehneosaurus.
Figure TEXT-FIG. 4.. Top view of a nearly finished model of the original reconstruction of Kuehneosuchus, with the limb joints exposed: these were coated with plasticine to reduce drag effects before the model was run in the wind tunnel.
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Table 2. The aerodynamic parameters used in the four models, three of Kuehneosuchus with wings set at different cambers and one of Kuehneosaurus latus.
| ||Long tail||Short Tail||Stout legs||Thin legs||Limbs tucked||Limbs outstretched||Additional patagia||Ribs|
|Low camber||X||X||X||X||X||X|| || |
|Medium camber||X||X||X||X|| ||X|| ||X|
|High camber||X||X||X||X|| ||X||X|| |
|K. latus||X||X|| ||X|| ||X|| || |
Figure TEXT-FIG. 5.. Side view of the different degrees of camber for the Kuehneosuchus models: high camber (top), medium camber (middle), and low camber (bottom).
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The wind tunnel tests were performed in the Department of Engineering at Bristol University. Testing was done with the model inverted in a low turbulence wind tunnel using a three-degrees-of-freedom balance (Text-fig. 6). The angle of attack was varied in steps of 2 degrees between 12 and 25 degrees. Each test was performed at speeds between 10 and 20.5 m/s.
Figure TEXT-FIG. 6.. Wind-tunnel set-up of the high-camber model of Kuehneosuchus. Note that the model is suspended upside-down and that there is a vertical offset of the attachment structure from the centre of the body.
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Mass and centre of mass estimates
Accurate mass and centre of mass estimates are essential in order to assess an animal’s flight capabilities, and it is essential to get these estimates right. Pough (1973) plotted snout to vent lengths against mass for extant lizards, and these data fall on a smooth curve. Evans (1982) used this snout-vent/weight relationship to extrapolate the weight of Kuehneosuchus. She estimated the wing loading to be in the range of 157 to 216 N/m2, well outside the range of wing loadings for extant non-anatid birds, and so casting doubt on the gliding capability of Kuehneosuchus. We obtained similar results when we repeated the calculations. Kuehneosuchus has a snout-vent length of 250 mm, which implies a mass of 0.56 kg in Pough’s (1973) snout-vent/weight relationship, resulting in a wing loading of 135 N/m2.
We think it is unlikely that the kuehneosaurids, with extensive anatomical adaptations to gliding, were too heavy to glide. It is most likely that full account has not been taken of the ways in which flying animals save weight. This can be illustrated with Draco. Measurements of mass and snout-vent length of Draco do not fall on the curve provided by Pough (1973). Draco melanopogon, for example, measures 77 mm in average snout-vent length (Shine et al. 1998), and has an average mass of 3.8 g (McGuire and Dudley 2005). A prediction of the weight of this animal from Pough’s (1973) snout-vent/weight relation suggests its mass would be about 15 g, a factor of four higher. If the four times factor is applied to Kuehneosuchus, its mass is predicted to be 0.14 kg, not 0.56 kg. Even if the maximum weight of D. melanopogon is considered (5.9 g), the factor is 2.66, predicting a mass for Kuehneosuchus of 0.21 kg.
We cross-checked the calculated body mass and the weight-saving factors for the kuehneosaurids by Henderson’s (1999) mathematical slicing technique. Henderson (pers. comm. 2006) did the calculations using a standard lung volume fraction of 8.75, and determined a mass estimate of 0.40 kg for Kuehneosuchus and a centre of mass at 242 mm from the tip of the snout of the animal. It should be noted that the model used for these calculations had a rather plump tail, causing the centre of mass to lie well towards the rear end of the animal. In addition, it needs to be repeated that the exact length of the tail of the kuehneosaurs remains equivocal. Therefore, calculations were made for two new models, one with a very slender tail, and one with a shorter tail (total length of the animal, 580 mm). The centre of mass lies respectively at 196 and 195 mm from the tip of the snout, while the mass is estimated to be 0.38 kg for both models.
In summary, we used five body mass estimates for the British kuehneosaurids, namely 0.14, 0.21, 0.38, 0.40, and 0.56 kg, and the location of the centre of mass ranges from 242 to 195 mm from the tip of the snout of the animals.