Arctometatarsals should have been a great clade trait…

But it’s not.
Arctometatarsals describe a type of theropod metatarsals in which metatarsal 3 is pinched off proximally by flanking metatarsals 2 and 4. In addition, distally metatarsal 3 is typically just slightly anterior to its flanking metatarsals, which slightly back it up. You would think such a trait would only develop once and identify or diagnose a clade – but that is not the case, as earlier workers (like Snively et al. 2004) discovered and described.

Figure 1. Arctometatarsals on T-rex vs. normal metatarsals on Allosaurus.

Similarly
the large reptile tree (LRT, subset Fig. 1) does not find an arctometatarsal clade. The LRT finds at least six convergent instances where the third metatarsal is pinched between the flanking second and fourth metatarsals. None of these instances are related to each other. Rather, all are separated from one another by normal metatarsals.

Figure 1. Theropod dinosaurs and the arctometatarsal taxa (in blue).

Snively et al report, 
“A Bayesian phylogenetic analysis indicates that an arctometatarsus evolved in the common ancestor of the Tyrannosauridae + (Ornithomimosauria + Troodontidae) clade, but other optimizations are plausible.” That hypothesis was not supported by the LRT. Moreover, only one tested troodontid has an arctometatarsal trait. Troodon also has this trait, but it is not yet a tested taxon.

Snively et al report, 
“The most likely selective benefit of the structure was increased agility; if so, homoplasy indicates multiple exaptive and adaptive pathways towards predation and escape roles.”
Perhaps so! But these taxa have little in common distinct from other theropods, other than the pinched third metatarsal.

As we’ve seen before,
and this fact just hammers it home: convergence in RAMPANT throughout the LRT. That’s why we leave it to the software to recover the tree. No one wants to pull a Larry Martin here by creating or disputing relationships based on a single trait.

References
Snively E, Russell AP and Powell GL 2004. Evolutionary morphology of the coelurosaurian arctometatarsus: descriptive, morphometric and phylogenetic approaches. Zoological Journal of the Linnean Society 142:525–553.

Pterosaurs Tarsals – More Evidence vs Padian 1983

Some pterosaurs (like Rhamphorhynchus and the new Painten pterosaur) had 4 or 5 tarsals. Others had only two (like Pteranodon, Figs. 1-3).

Figure 1. Pteranodon tarsals (in color). Blue = astragalus. Yellow + calcaneum. YPM = Yale Peabody

The question is: 
In those pterosaurs with two tarsals is it more parsimonious that the 1) distal tarsals disappeared? or 2) the distal tarsals fused to the proximal tarsals? or 3) converging with birds, did the proximal tarsals fuse seamlessly to the tibia/fibula?

What does the evidence indicate?

There are pterosaur workers (Padian 1983, Bennett 2001, Nesbitt 2011, Witton 2013) who consider the tibia + fibula of pterosaurs a “tibiotarsus” because they say the proximal tarsals (astragalus + calcaneum) fused seamlessly to the distal tibia/fibula (Fig. 1). (We looked at this earlier here.) Birds have this sort of tibiotarsus. Padian 1983 compared bird tibiotarsi to Dimorphodon (Fig. 2) and the case looked pretty good back then.

However,
It’s important to remember that birds had a long ancestry as dinosaurs with distinct ascending processes of the astragalus that ultimately fused seamlessly to the tibia after the miniaturization that preceded and succeeded Archaeopteryx. Pterosaurs don’t have that long history, nor do they have ancestors with an ascending processes, nor did they undergo phylogenetic miniaturization prior to getting their wings. Even Archaeopteryx has a distinct ascending process — not seamless.

Under the Padian 1983 hypothesis 
the two tarsals found with Dimorphodon are distal tarsals. Likewise, Bennett (2001) proposed a tibiotarsus for Pteranodon. Eaton (1913, Fig. 1) called them podials, a general name form carpals or tarsals. We don’t see the same long ancestry progress in pterosaur ankles. In fact, there’s no ancestry for this type of ankle at all.

Figure 2. Pterosaur distal tibia. Left: Dimorphodon. Right Pteranodon in anterior (above) and posterior (below) views. Padian (1983) and Bennett (2001) consider the bulbous parts to be the fused proximal tarsals. They are not. The proximal tarsals, astragalus (blue) and calcaneum (yellow) are distinct. Missing here are any distal tarsals. Padian identified this view of Dimorphodon as the anterior, because it looked so much like the anterior of the distal bird tibiotarsus (not shown here). But look again. It looks more like the posterior of the distal tibia of Pteranodon identified by Bennett.

Figure 3. Foot and tarsus of Pteranodon, FHSM-P-2062 and restored and relabeled on top, from original online mislabeled image found at OceansofKansas.com (below). Note, the distal tibia bulge is posterior in Pteranodon, but bulges both ways in Dimorphodon and other pterosaurs, like the Painten pterosaur.

Rather, when you look at basal pterosaurs like Peteinosaurus (Fig. 4), you find four distinct tarsals.

Figure 4. Peteinosaurus and Dimorphodon BMNH4212 pedes. Four tarsals are present on both. Yes the tarsals have moved in Dimorphodon with the distal tarsals rising to the level of the proximal tarsals. 

Same with the classic specimen of Dimorphodon. The engraving (Fig. 5) shows four tarsals.

Figure 5. Click to enlarge. The four tarsals identified on the the classic BMNH 41212 specimen of Dimorphodon. Non-foot bones are ghosted out. Calcaneum = yellow. Astragalus = blue. Distal tarsal 4 = pink. Centrale = magenta. Yes, they have moved during taphonomy, If you count four tarsals, that’s all I’m asking for now.

This is in contrast to Padian’s (1983) interpretation of BMNH 41212 (Fig. 6) where he adds a cylindrical joint to the distal tibia with a circumference smaller than in the other tibia at left.

Figure 6. Tarsals of Dimorphodon BMNH 41212 specimen according to Padian 1983. Figure 5 matches in most regards — except for the tarsals.

Padian 1983 removed tarsals from the matrix of two far less complete specimens attributed to Dimorphodon, YPM 350 and YPM 9182 (Figs. 7-9). Oddly, the smaller of the two specimens (YPM 9182) fused the two large tarsals to one another (the only such event I am aware of). The larger specimen (YPM 350) did not.

Figure 7. The YPM 9182 specimen attributed to Dimorphodon. Note the tarsals fuse to one another despite the smaller size. The femora do not match. The ventral maxilla is straighter. The jugal is deeper. M4.2 is shorter.  So there is some doubt that this is indeed congeneric with Dimorphodon. The question here is: did the calcaneum fuse to the fourth distal tarsal? And if so, did Padian get his tarsal backwards? With Padian’s orientation the tarsal has a posterior tuber. But no pterosaur ever developed a tuber, certainly not on any distal tarsals. And not on any calcaneum either. Let’s keep an eye out for further examples of this. 

Figure 8. About the size of the classic Dimorphodon, the YPM 350 specimen has unfused tarsals. Note the very few bones. The specimen is extremely disarticulated. The other two tarsals could have been easily scattered. This specimen appears to be closer to the classic Dimorphodon in all regards.

Figure 9. Location of the tarsals (red circles) on the YPM 350 and YPM 9182 specimens attributed to Dimorphodon by Padian 1983. Do you think some other tarsals could have escaped?

Padian 1983 noted the cylindrical shape of the distal tarsals and their convergence with the bird tibiotarsus. But there are pterosaurs, like the Painten pterosaur (Fig. 10), that have a cylindrical distal tibia AND four tarsals.

Figure 10. The Painten pterosaur with tarsals colorized. There are four of them. Note the cylindrical shape of the distal tibia/fibula.

So, the evidence for Dimorphodon having only two tarsals is fading. The evidence for cylindrical distal tarsals is strong. Pteranodon has only two tarsals. Whether they were created by fusion or reduction awaits further evidence. There is no evidence for a gradual evolution of fusion in the tarsals and tibia/fibula. Rather, there is plenty of evidence for the retention of paired distal and paired proximal tarsals. There is also evidence in YPM 9182 for the fusion of the proximal tarsals in certain pterosaurs.

Ramifications
Nesbitt 2011 fell prey to the idea of a fused tibiotarsus in pterosaurs when he wrote: “a few peculiar features in the hind limb of lagerpetids merit discussion and suggest that they may be more closely related to pterosaurs than to dinosaurs. Specifically, the ankle of lagerpetids is more similar to that of basal pterosaurs (in particular, Dimorphodon) than to basal dinosauriforms and early dinosaurs. The calcaneum and astragalus are coossified, the ventral surface of the calcaneum is rounded like that of the astragalus, there is no posterior groove of the astragalus, and the calcaneum lacks any sort of calcaneal tuber in both Dimorphodon and lagerpetids. These four character states shared between lagerpetids and Dimorphodon are absent in basal dinosauriforms (e.g., Marasuchus, Asilisaurus). Basal dinosauriforms have a separate calcaneum and astragalus, the ventral surface of the calcaneum, although rounded, is different from the ventral surface of the astragalus, they have a posterior groove of the astragalus, and the calcaneum bears a small calcaneal tuber. It is possible that pterosaurs and lagerpetids share additional ankle characters or differences; however, the ankle of Dimorphodon is heavily ossified, thus concealing the distal end of the tibia and the proximal surface of the astragalus.”

The large reptile tree demonstrates that pterosaurs have no relationship with Lagerpeton and neither do basal dinosaurs, which are distinct from both.

References
Bennett SC 2001. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Padian K 1983. Osteology and Functional Morphology of Dimorphodon macronyx (Buckland) (Pterosauria: Rhamphorhynchoidesa) Based on New Material in the Yale Peabody Museum. Postilla 189 44pp.

Non-dinosaurian Dinosauromorpha (Langer et al. 2013)

Continuing to push Lagerpeton as a “dinosauromorph” (which is traditional thinking), Langer et al. (2013) continues to ignore certain basic facts starting in the feet that divide pararchosauriforms (including Lagerpeton) and euarchosauriforms (including dinosaurs) into two major clades.

Figure 1. Click to enlarge. The feet of Euarchosauriformes (above in white) and Pararchosauriformes (below in grey). No higher euarchosauriformes have a longer digit 4 than 3. Both clades share more foot traits with each other, which removes Lagerpeton from the lineage of dinosaurs in the Euarchosauriformes, and puts it in the line of descent from Diandongosuchus (with its long digit 4) and/or Proterochampsa (with its short digit 1). Also note that the ascending process of the astragalus is posterior in Lagerpeton, anterior in dinosaurs.

Euarchosauriformes
It’s unfortunate that so few euarchosauriform feet are known that include a complete digit 4, but what we do know demonstrates that digit 4 is always shorter than 3 and metatarsal 4 is always shorter than mt3.

Pararchosauriformes
In this clade pedal digit 4 can sometimes be longer than 3 and metarsal 4 is never shorter than mt3. Sometimes pedal digit 4 is reduced to a vestige, other times, even within a genus, it is not. In any case, Lagerpeton belongs in this clade, a small biped at the acme of a  large, flat-headed, quadrupedal clade. It does not belong with dinosaurs or their short pedal digit 4 kin. In Lagerpeton, the astragalus flange rises in back of the tibia, not in the front, as in dinosaurs.

The way to separate the Euarchosauriformes from the Pararchosaurifomes
is to introduce protorosaurs, Youngina, Youngoides, Choristodera, Doswellia and the traditional archosauriformes, as demonstrated by the large reptile tree.

Mistaking Early Triassic bipedal lizard tracks for dinosauromorph tracks
Earlier we discussed the mistakes of Brusatte et al. (2012) who claimed that certain ichnites related to Rotodactylus in the Early Triassic belonged to lagerpetids, when in reality they belong to cosesaurids, in the ancestry of pterosaurs.

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
Langer MC, Nesbitt SJ, Bittencourt JS and Irmis RB 2013.  Non-dinosaurian Dinosauromorpha.  Geological Society, London, Special Publications v.379, first published February 13, 2013; doi 10.1144/SP379.9 From: Nesbitt SJ, Desojo JB and Irmis RB eds) Anatomy, Phylogeny and Palaeobiology of Early Archosaurs and their Kin. Geological Society, London, Special Publications, 379, http://dx.doi.org/10.1144/SP379.9 # The Geological Society of London 2013.