Patterns in the Reduction of Pedal Digit 5 in Pterosaurs

Updated April 5, 2016 with a new image of Nemicolopterus.

That Very Weird Pedal Digit 5
Basal pterosaurs had an elongate pedal digit 5, emanating from a short metatarsal 5 and composed of two hyperflexed metatarsal-length phalanges tipped by an often overlooked ungual for a total of three phalanges.  Peters (2000) reported that pedal digit 5 impressed the hyperflexed dorsal surface of pedal 5.2/3 (= fused phalanges) into the matrix, as demonstrated by Rotodactylus tracks. Peters (2000) attributed matched Rotodactylus to a pterosaur outgroup sister, Cosesaurus. Peters (2011) attributed similar tracks with pedal digit 5 impressing far behind the other four digitigrade impressions to anurognathids and a specific Rhamphorhynchus.

Basal Tritosaur, Fenestrasaur and Pterosaur feet with digit 5 bones color coded.

Figure 1. Click to enlarge. Basal tritosaur, fenestrasaur and pterosaur feet with digit 5 bones color coded. Note the fusion of pedal 5.2 and 5.3 in pterosaurs. Find any single archosaur or set of archosaurs with a similar pedal digit 5 and you’ll have found the archosaur sister to pterosaurs. Good luck! Question for you: does the brevity of p5.3 in Cosesaurus and Longisquama mean it ultimately disappeared? Or did it fuse with p5.2? I think the latter, but the data is not quite there yet. 

Where Did It Come From?
Pterosaurs inherited that elongated toe 5 from their fenestrasaur and tritosaur ancestors, all of which (e.g. Tanystropheus) had an extra phalanx (Fig. 1). Pedal 5.2 fused to p5.3 in the most primitive known pterosaurs, so the ungual is p5.4, as in the basal lizards, Lacertulus and Huehuecuetzpalli (Fig. 1). In basal pterosaurs and Sharovipteryx, pedal 5.1 often extended to or beyond the distal end of metatarsal 4, but it does not do so in more primitive taxa. In several derived clades pedal digit 5 became reduced, sometimes to a vestige. We’ll look at those patterns of reduction today.

Huehuecuetzpalli
Distinct from Lacertulus, metatarsal 5 was shorter and pedal digit 5 was longer in Huehuecuetzpalli (Fig. 1). This set up the classic pattern seen in the most members of the Tritosauria (langobardisaurs, drepanosaurs, tanystropheids and fenestrasaurs). Exceptions and reductions follow.

Note: absolutely no archosaurs or archosauriformes have an elongated pedal digit 5. This trait alone should have been enough to steer workers away from the conventional and traditional “pterosaurs are archosaurs” hypothesis.

Macrocnemus. Note the reduction of pedal digit 5.

Figure 2. Macrocnemus. Note the reduction of pedal digit 5. The resemblance of this skull to that of Campylognathoides (Fig. 3) is both somewhat homologous and by convergence. Several transitional taxa differ more. 

Macrocnemus
The first instance of pedal digit 5 reduction occurred in Macrocnemus. The entire digit was shorter than metatarsal 4 and the phalanges of digit 5 were shorter distally.  A derived macrocnemid, Dinocephalosaurus, likewise had a small pedal digit 5, but it extended as far at the base of p4.2 as this reptile developed paddle-like, swimming feet. Find the missing Macrocnemus without this toe reduction and you’ll have the transitional taxon linking this genus closer to tanystropheids, langobardisaurs, drepanosaurs and fenestrasaurs.

Campylognathoides (CM 11424), the earliest pterosaur with a reduced pedal digit 5.

Figure 3. Campylognathoides, the earliest pterosaur genus with a reduced pedal digit 5. All known specimens had a reduced pedal digit 5. 

Campylognathoides 
The first pterosaur with a stunted pedal digit 5 was Campylognathoides (Fig. 3)All known specimens had a stunted pedal digit 5 in which pedal 5.1 extended no further than half the length of metatarsal 4. Not sure why this is so. The current predecessor taxon, Eudimorphodon cromptonellus, did not have a reduced pedal digit 5, but overall E. cromptonellus was much smaller. Perhaps pedal digit 5 is the one body part that did not enlarge, phylogenetically, as the rest of the body grew larger.

A taxon derived from a sister to the most derived CampylognathoidesCM 11424, Rhamphorhynchus intermedius (St/Ei 8209, No. 28 in the Wellnhofer 1975 catalog), had a longer pedal digit 5 that extended at least 3/4 of the length of metatarsal 4. So the reduction was reversed to become an elongation in Rhamphorhynchus. Pedal digit 5 varied in length in other Rhamphorhynchus specimens, with some extending no further than half the length of metatarsal 4 and others extending to the full length of metatarsal 4.

Two sister taxa bridging the basal/derived pterosaur divide.

Figure 4. Two sister taxa bridging the basal/derived pterosaur divide. TM 10341 was much smaller than the SMNS 50164 specimen of Dorygnathus (enlarged to the right). Despite the short metacarpal and long pedal digit 5, the tiny dorygnathid was considered a Pterodactylus by Wellnhofer, who listed it as No. 1 in his 1970 catalog. Pedal digit 5 no smaller relative to metatarsal 4 in the tiny sister. 

Tiny Dorygnathids Leading Toward Azhdarchids
Typical Dorygnathus specimens, like SMNS 50164, had a very large pedal digit 5, with p5.2 bent, even so, in this taxon pedal 5.1 did not extend beyond metatarsal 4, but aligned with the line extending along mt3 and mt4. Despite the large size reduction in TM 10341 (No. 1 in the Wellnhofer 1970 catalog), pedal 5.1 extended just as far. In the larger taxon, Beipiaopterus, pedal 5.1 did not extend beyond half of metatarsal 4. It was similar in No. 44 and No. 42. Pedal 5.1 did not become smaller than half of metatarsal 4 until the base of the Azhdarchidae, represented by Jidapterus. Peters (2000a, 2011) found a correlation between the reduction of pedal digit 5 and plantigrady in these taxa.

The MB.R. 3530.1 specimen

Figure 5. The MB.R. 3530.1 specimen wrongly attributed to Pterodactylus (No. 40 in the Wellnhofer 1970 catalog).

Tiny Dorygnathids Leading Toward Ctenochasmatids
Similar in fashion to the above scenario, MB.R.3530.1 (No. 40 in the Wellnhofer 1970 catalog, Fig. 5) was smaller overall and had a shorter pedal digit 5 and it became progressively shorter in more derived ctenochasmatids like Ctenochasma. In these taxa pedal digit 5 is not typically found hyperflexed, but extends its full (even though abbreviated) length alongside metatarsal 4.

No Reduction in Cycnorhamphids + Ornithocheirds
Relative to metatarsal 4, pedal digit 5 does not become reduced in the monophyletic clade of cycnorhamphids + ornithocheirids. However, because metatarsal 4 becomes so short in derived ornithocheirids like Anhanguera, pedal digit 5 likewise becomes small — but not relatively small.

Pedal digit 5 in a basal Pterodactylus, AMNH 1945.

Figure 6. Pedal digit 5 in a basal Pterodactylus, AMNH 1945.

Pterodactylus
In all specimens for Pterodactylus pedal 5.1 extends no further than half of metatarsal 4 (Fig. 6).

The smallest pterosaur. No. 6

Figure 7. The smallest of all adult pterosaurs, B St 1967 I 276 or No. 6 in the Wellnhofer (1970) catalog. At left is the foot plantigrade and with metatarsals slightly raised, which simplifies and aligns the PILs (parallel interphalangeal lines). The gray oval is a hypothetical egg based on the pelvic opening. The sternal complex is also shown separated from the lateral view reconstruction.

No. 6
The smallest of all known adult pterosaurs, B St 1967 I 276 (No. 6 in the Wellnhofer 1970 catalog, Fig. 7) extended pedal 5.1 no longer than 2/3 the length of metatarsal 4 but in larger taxa, like No. 12 and No. 23, pedal 5.1 extends to a point in line with metatarsals 3 and 4, still shorter than metatarsal 4. This pattern is also found in several other Germanodactylus specimens.

Figure 8. A larger view of Nemicolopterus. Pedal digit 5 is relatively reduced here.

Figure 8. A larger view of Nemicolopterus. Pedal digit 5 is relatively reduced here.

Nemicolopterus to Tupuxuara
The tiny pterosaur Nemicolopterus (IVPP-V-14377, Fig. 8) had a pedal 5.1 no longer than half the length of metatarsal 4. The larger sister, Shenzhoupterus, had a longer metatarsus and a shorter pedal digit 5. This pattern continues in Huaxiapterus with a reduction to a nub in Tupuxuara.

Right pes, dorsal view of Pteranodon UALVP 24238. Note pedal digit 5 is a vestige.

Figure 9. Right pes, dorsal view of Pteranodon UALVP 24238. Note pedal digit 5 is a vestige on this plantigrade foot.

No. 13 to Pteranodon
Despite the large number of Nyctosaurus and Pteranodon specimens, few preserve toe bones (Fig. 9).  Fewer still preserve digit 5 toe bones. No digit 5 toe bones are known in Nyctosaurus. A basal Pteranodon FHSM VP 2183 preserves pedal 5.1 extending no further than 1/3 the length of metatarsal 4, which is rather elongate. In derived Pteranodon specimens pedal digit 5 becomes much shorter, but the foot remains digitigrade. In another clade with a vestige pedal digit 5, the foot becomes plantigrade, according to PILs analysis.

Problems
Pedal digit 5 is sometimes hard to find. Often it is found beneath the metatarsus.  Other times it may become disarticulated and ignored or unsought by preparators, who “know” that all “pterodactyloids” lose pedal digit 5 or have vestiges at best. As you can see above and in more detail at ReptileEvolution.com, the pattern of reduction and enlargement of pedal digit 5 in pterosaurs is a little more complicated than the conventional thinking suggests.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.
Wellnhofer P 1975a-c. Teil I. Die Rhamphorhynchoidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Allgemeine Skelettmorphologie. Paleontographica A 148: 1-33.Teil II. Systematische Beschreibung. Paleontographica A 148: 132-186. Teil III. Paläokolgie und Stammesgeschichte. Palaeontographica 149: 1-30.

wiki/Pterodactylus
wiki/Rhamphorhynchus

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