When they looked at the pterosaur embryos, they realized that the reptile's wing bone, manus digit IV, hardened much faster than in other flying reptiles.
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Along the way, they noticed something about one bone, the manus digit IV. Equivalent to our ring finger, this is a pterosaur's "wing finger," the long, flexible appendage that is attached to its wing membrane and allowed the animal to fly.
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Twenty one autapomorphic characters have been identified in L. chanarensis, these include: the anterior inclination of the posterior dorsal neural spines, the hook-shaped femoral head and the length of digit IV and metatarsal IV being greater than digit III and metatarsal III. L. chanarensis lacks many dinosaurian characters, such as the anterior trochanter, placing it basal within Dinosauromorpha.
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The narrow and functionally didactyl pes are a further similarity to modern saltators. By condensing into a single unit, the metatarsus gains strength without the drawback of increased weight. It also appears likely that, consequently to the reduction of digit II, digit IV may have elongated to balance the pes. The hypothesis of saltatorial locomotion is debated, however.
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Two lower phalanges belong to digit IV. These phalanges are even more flattened than phalanx II-2. Their lower and upper surfaces are similar to those of phalanx II-2, but can be differentiated in being more concave. Pedal unguals are mostly flattened and convex with elliptical upper surfaces. These elements are more arrow-shaped and more sharply developed than in hadrosaurids.
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This articulation creates a virtually immobile joint between the two. Petrolacosaurus has a phalangeal formula for the manus of 2-3-4-5-3. For the pes, the formula is 2-3-4-5-4. The digits increase in length from digit I-digit V. Metatarsal IV is 3.5 times as long as metatarsal I. Many lines of post-Paleozoic reptiles have a reduction in digit IV in the manus and pes, indicating that Petrolacosaurus is more primitive than those reptiles.
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This preservation provides insights into the delicate structures of skull bones, including pneumatic features such as foramina/fossae and internal chambers. Caelestiventus is known from a single individual (BYU 20707, in the Museum of Paleontology at Brigham Young University) that preserves much of the skull (skull cap, sides of the face, and a complete lower jaw (mandible) along with a single non-skull bone – the last finger bone at the end of the elongated fourth finger (manual digit IV 4) that supported the tip of the wing.
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SVL to maximum 108.5 mm, dorsal pattern of five to seven white vertebral blotches between nape and sacrum and six to seven pairs of short white bars on flanks between limb insertions, 1–4 internasals, 30–32 ventral scale rows between weak ventrolateral folds, 14–18 precloacal pores in males, 10–14 longitudinal rows of smooth dorsal tubercles, 14–16 broad lamellae beneath digit I of pes, 17–19 broad lamellae beneath digit IV of pes, and a single transverse row of enlarged tubercles along the posterior portion of dorsum of each tail segment.
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Opisthoeoclicaudia shows even more reduction of the hand than other titanosaurs, with both carpals and phalanges completely absent. However, Diamantinasaurus, while lacking carpals, preserves a manual formula of , including a thumb claw and phalanges on all other digits. This, coupled with the preservation of a single phalanx on digit IV of Epachthosaurus and potentially Opisthocoelicaudia (further study is necessary), show that preservation biases may be responsible for the lack of hand phalanges in these taxa. This suggests that Alamosaurus, Neuquensaurus, Saltasaurus and Rapetosaurus - all known from imperfect or disarticulated remains previously associated with a lack of phalanges - may have had phalanges but lost them after death.
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Although during preparation, a cast had been made of the fossil beforehand to document the original relative positions of the bones. Carpal bones were not known from any specimen, leading some authors to suggest that they were lost in the genus. In a 2016 paper, Matthew Carrano and Jonah Choiniere suggested that one or more cartilaginous (not bony) carpals were probably present, as indicated by a gap present between the forearm bones and the metacarpals, as well as by the surface texture within this gap seen in the cast. In contrast to most more-derived theropods, which showed only three digits on each manus (digits I–III), Ceratosaurus retained four digits, with digit IV reduced in size.
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There is a debate between embryologists and paleontologists whether the hands of theropod dinosaurs and birds are essentially different, based on phalangeal counts, a count of the number of phalanges (fingers) in the hand. This is an important and fiercely debated area of research because its results may challenge the consensus that birds are (descendants of) dinosaurs. Embryologists and some paleontologists who oppose the bird-dinosaur link have long numbered the digits of birds II- III-IV on the basis of multiple studies of the development in the egg. This is based on the fact that in most amniotes, the first digit to form in a 5-fingered hand is digit IV, which develops a primary axis.
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Therefore, embryologists have identified the primary axis in birds as digit IV, and the surviving digits as II-III-IV. The fossils of advanced theropod (Tetanurae) hands appear to have the digits I-II-III (some genera within Avetheropoda also have a reduced digit IVUniversity of Maryland department of geology home page, "Theropoda I" on Avetheropoda, 14 July 2006.). If this is true, then the II- III-IV development of digits in birds is an indication against theropod (dinosaur) ancestry. However, with no ontogenical (developmental) basis to definitively state which digits are which on a theropod hand (because no non- avian theropods can be observed growing and developing today), the labelling of the theropod hand is not absolutely conclusive.
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