Intermedium reappears in birds

Ossa-Fuentes L, Modozis J and Vargas AO 2015
discover a detail of interest in bird osteology and ontogeny.

They report, “This work has revealed that the ascending process [of the ankle] does not develop from either the heel bone or the ankle bone, but from a third element, the intermedium. In the ancient lineage of paleognath birds (such as tinamous, ostriches and kiwis) the intermedium comes closer to the anklebone, producing a dinosaur-like pattern. However, in the other major avian branch (neognaths), which includes most species of living birds, it comes closer to the heel bone; that creates the impression it is a different structure, when it is actually the same.”

And that’s not all… They continue: “More remarkably, however, this finding reveals an unexpected evolutionary transformation in birds. In embryos of the land egg-laying animals, the amniotes (which include crocodilians, lizards, turtles, and mammals, who secondarily evolved live birth) the intermedium fuses to the anklebone shortly after it forms, disappearing as a separate element. This does not occur in the bird ankle, which develops more like their very distant relatives that still lay their eggs in water, the amphibians. Since birds clearly belong within landegg-laying animals, their ankles have somehow resurrected a long-lost developmental pathway, still retained in the amphibians of today – a surprising case of evolutionary reversal. The study also presented fossil evidence from juvenile specimens of toothed birds from the Cretaceous period. These show that, at this early stage of bird evolution, the ascending process already developed separately.”

On a similar note,
as you may recall from this earlier blog post, the pre-amniote, and almost pre-tetrapod, digit zero, the manual digit medial to the thumb, which is absent in almost all derived tetrapods, also appears on Limusaurus and caused the phase shift confusion noted earlier.

References
Ossa-Fuentes L, Modozis J and Vargas AO 2015. Bird embryos uncover homology and evolution of the dinosaur ankle. Nature Communications. DOI: 10.1038/natcomms9902
Diaz RE & Trainor PA 2015. Hand/foot splitting and the ‘re-evolution’of mesopodial skeletal elements during the evolution and radiation of chameleons. BMC evolutionary biology, 15(1), 184.
https://paleobiologia.wordpress.com (blog en español)
http://www.nature.com/ncomms/2015/151113/ncomms9902/full/ncomms9902.html

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.

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 1. Pterosaur distal tibia. Left: Dimorphodon. Right Pteranodon.

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 4. Foot and tarsus of Pteranodon, FHSM-P-2062 and restored and relabeled. From OceansofKansas.com.

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.

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 6. Click to enlarge. The four tarsals identified on the the classic BMNH 41212 specimen of Dimorphodon.

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 doesn't match.

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 350 specimen attributed to Dimorphodon. Note the tarsals fuse to one another despite the smaller size. The femora do not match, though similar in most regards. So there is some doubt that this is indeed Dimorphodon.

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.

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?

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.

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.

The myth of the pterosaur tibiotarsus

Earlier and over several posts we looked at the various misconceptions promoted by Mark Witton (2013) in his new book, “Pterosaurs.” Today we’ll look at Witton’s misconstrued view of the so-called “tibiotarsus”, a purported fusion of the tibia and proximal tarsals (astragalus + calcaneum) that never actually happened.

Figure 1. Right: Witton's view of the pterosaur foot and ankle. He does not identify the individual tarsal bones but assumes the tibia and proximal tarsals are fused. At left a corrected pes of the same specimen with ankle bones identified.

Figure 1. Right: Witton’s view of the pterosaur (Peteinosaurus) foot and ankle. He does not identify the individual tarsal bones, but assumes the tibia and proximal tarsals are fused. At left a corrected pes of the same specimen with ankle bones identified. Compare these identities to those in figure 2. The metatarsus was actually appressed, not spreading as Witton presumes in this sister to Dimorphodon and anurognathids. For clarity in both views the tibia/fibula has been slightly dislocated and would, in vivo, articulate with the dorsal/anterior faces of the astragalus and calcaneum.

Wellnhofer (1991) had it right.
There’s a tibia and a tarsus. Not a tibiotarsus. Witton did not identify the two rows of individual bones of his pterosaur tarsus, but was sure that the astragalus and calcaneum were fused to the tibia. The first demonstrates a lack of knowledge and testing. The second shows a reliance on tradition without evidence.

Witton’s mistakes were fourfold.
1. The tibiotarsus misidentification goes back at least as far as Galton 1980 and Bennett 1991, who noted Pteranodon had a tibiotarsus and just two distal tarsals. The purported “distal tarsals” in Pteranodon are actually the astragalus and calcaneum fused to the centrale and distal tarsal 4 respectively. The outgroup to Pteranodon, the SMNK 6597 specimen of Germanodactylus, retains four tightly packed tarsals of subequal size indicating none were vestigial.

2. Witton’s error was a direct result of being unable to determine the ancestry of pterosaurs. Once that is known we can see the homologies and identify the remaining four major tarsal bones (Fig. 2) in pterosaurs.

3. His third problem was his refusal to reference the one paper in the literature that actually traced the ancestry of pterosaurs and the homologies of the tarsal bones: Peters 2000. Odd that. Witton shares a trait with Hone and Benton in that regard.

4. According to the traditional paradigm, the proximal tarsals were supposed to fuse to the tibia during ontogeny, but no one has showed that yet. No pterosaur embryo has additional unfused tarsals. Furthermore, no tiny pterosaur, formerly considered a juvenile, has additional unfused tarsals. This myth of fusion evidently comes from the false paradigm associating pterosaurs with dinosaurs (which do have a tibiotarsus) and from the finished, curved articulating surfaces of the distal tibia/fibula in pterosaurs. Such a large pulley-like surface gives pterosaurs a widely swinging ankle for walking on top of the tarsus and then for flying with feet trailing to roll/flex the foot ~90 degrees back for aerodynamic reasons.

Here we’ll rectify all of Witton’s problems with phylogeny
We can identify the individual bones of the pterosaur tarsus by comparing them to ancestral nonvolant taxa with a larger number and more readily identified tarsals first reported by Peters (2000). Here’s a nice morphological sequence if you’ll ignore the autapomorphic short digit 5 of Macrocnemus. Huehuecuetzpalli, its phylogenetic ancestor, had a much longer pedal digit 5.

Figure 2. The tarsi of Macrocnemus, Langobardisaurus and Cosesaurus demonstrating the reduction of ankle bones. None of these fuse to the tibia.

Figure 2. The tarsi of Macrocnemus, Langobardisaurus and Cosesaurus demonstrating the reduction of ankle bones (Peters 2000). None of these tarsals fuse to the tibia. The reduction of pedal digit 5 in Macrocnemus is restricted to that genus. Cosesaurus has the same number of large ankle bones as pterosaurs. Occasionally smaller distal tarsals also ossify.

Tarsal homologies in fenestrasaur tritosaurs
The homologies of pterosaur ankle bones can be traced from Macrocnemus to Langobardisaurus and Cosesaurus. Distal tarsals 1 and 3 generally diminish (but sometimes ossify in various pterosaurs). This leaves the astragalus, calcaneum, centrale and distal tarsal 4 in Cosesaurus and pterosaurs to form the ankle bones and create a mesotarsal ankle joint (Peters 2000). Pterosaurs share with Langobardisaurus and Cosesaurus that short metatarsal 5 and long m5.1. Note the continued contact of the distal fibula with the calcaneum. The apparent loss of various tarsals can come about either by way of 1.) fusion or 2.) reduction to absence or by 3.) lack of ossification.

Hopefully someday
this foolish and unfounded reliance on false traditional paradigms (a genuine ‘house of cards’ constructed by Witton (2013) as we have seen over and over again) will fall by the wayside and both the large reptile tree and the large pterosaur tree will become accepted, or at least tested under an academic banner.

References
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Galton PM 1980. Avian−like tibiotarsi of pterodactyloids (Reptilia: Pterosauria) from the Upper Jurassic of East Africa. Pal.ontologische Zeitschrift 54 (3/4): 331–342.
Peters D 2000. A redescription of four prolacertiform genera and implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 293-336.
Wellnhofer P 1991. The Encyclopedia of Pterosaurs. Salamander Books.
Witton M. 2013. Pterosaurs. Princeton University Press. 291 pages.

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.

The feet of Euarchosauriformes (above in white) and Pararchosauriformes (below in grey). No higher euarchosauriformes have a longer digit 4 than 3. Both sets of feet share more traits with each other, which removes Lagerpeton from the lineage of dinosaurs, but puts it in the line of descent from Diandongosuchus.

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.

The Redevelopment of the Calcaneal Tuber in Poposaurids and Crocs

Updated April 22, 2014 to reflect the new basal archosaur position of poposaurids.

The Traditional View of Calcaneal Tuber Distribution
Most paleontologists (recently Nesbitt 2011 and references therein) assert if the calcaneum of a derived archosauriform had a tuber, the taxon was likely a “pseudosuchian“. If no tuber the taxon was likely an ornithosuchian (= “avemetatarsalian“), and most likely a dinosaur. Seems simple enough and virtually all paleontologists buy into this paradigm.

Leverage for Limb Extension
A calcaneal tuber extends more or less posteriorly to provide more leverage for foot extension. By convergence (remember that phrase!) chiniquodontid therapsids developed a calcaneal tuber that is retained by most mammals including humans. The “Achilles” tendon is attached to this posterior extension.

“Extreme Convergence”
Nesbitt and Norell (2006) and Nesbitt (2007) nested the poposaurid Effigia okeeffeae among the rauisuchians based largely on the ankle, but they noted “extreme convergence in the body plans” with ornithomimid dinosaurs. They reported that the ankle of Effigia articulated in a crocodile-normal configuration, with a morphology similar to Alligator (Figure 1). The broken and missing calcaneal “heel”would have been oriented proximally, like that of a sister taxon, Shuvosaurus (Fig. 2).

The pedes of Alligator and Effigia

Figure 1. The pedes of Alligator (left) and Effigia (right) demonstrating the convergence of the structure of the ankle bones (astragalus and calcaneum). In basal archosaurs the ankle is a simple hinge, but in Alligator the hinge takes a sharp turn between the astragalus and calcaneum with a peg from the astragalu inserting into a socket in the calcaneum. The astragalus and calcaneum of Effigia articulate in a crocodile-normal configuration and their morphologies are similar to  those of Alligator, including the peg and socket. 

The Calcaneal Tuber and its Actual Heretical Distribution
Here, according to the large reptile study, both assessments are false. Basal (and often bipedal) taxa in the croc lineage (like GracilisuchusScleromochlus and Terrestrisuchus) had little to no tuber. Similarly, basal dinosaurs (also often bipedal), like Herrerasaurus and Silesaurus, had little to no tuber. Derived crocs (typically quadrupedal) had a calcaneal tuber. Similarly, and by convergence, Lotosaurus and poposaurs  (Fig. 3) had a calcaneal tuber and sometimes a very large one. Some were bipeds. Others were not. So the pattern of development of the tuber is not one-on-one, but needs more study.

Here (Fig. 3) Effigia nested with other poposaurs as basal archosaurs based on more parsimoniously shared traits from head to toe.

A selection of basal archosaur and poposaurid pedes

Figure 1. A selection of basal archosaur and poposaurid pedes with the calcaneum highlighted in blue. PILs in red. Since Plateosaurus had four distal carpals, it appears likely that at least some of the taxa in the lower row also had distal carpals. but that data was not published.

The Dual Convergent Enlargement of the Calcaneal Tuber
There’s no controversy to the fact that in derived crocs the calcaneal tuber was and is enlarged. There’s no controversy to the fact that in most dinosaurs the calcaneum remained small. In poposaurs some had a large calcaneal tuber. Others did not.

In crocs, as the calcaneal tuber developed, the astragalus and calcaneum stayed similar to each other in size within a size ratio remaining within 40/60 to 60/40. By contrast, in basal dinos and Silesaurus the calcaneum was less than a third of the astragalus. However, in popsaurids, the calcaneum re-enlarged as the astragalus shrank, ultmately matching the ratios seen in crocs.

Shape Variation
Note the shape of the calcaneum tuber varies greatly in the poposaurids. It was small in Lotosaurus and very large in Poposaurus.

The calcaneum as a whole was wider than long in Lotosaurus. Lotosaurus was a graviportal quadruped in which metatarsal 5 was broader proximally, creating a hook shape. This lateral expansion of the metatarsus affected the size of the calcaneum which grew laterally and larger to match.  Silesaurus, despite its narrow pes and vestigial digits 1 and 5 nested basal to Lotosaurus, suggesting that a more primitive sister to Silesaurus retained five unreduced digits.

The calcaneum was longer than wide in Poposaurus. The other poposaurs remained bipedal with a narrow pes and a narrow metatarsal 5. Thus they developed different and distinct sorts of calcaneal tubers.

Too Much Emphasis on the Ankle?
According to the tree recovered by the large reptile study the traditional view placed too much emphasis on the ankle, not accepting the possibility of convergence. The heretical view is more broadly based, employing several times more taxa without emphasizing ankle traits and embracing the possibility of convergence.

And the Convergence Does Not Stop There
Traditional trees mix phytosaurs (= parasuchians) and chanaresuchids in with other archosauriforms. Here, according to the large reptile study, these two pararchosauriforms evolved separately from the euarchosauriforms. Phytosaurs had a calcaneal tuber, but their sisters the chanaresuchids, do not. Thus the calcaneal tuber of phytosaurs developed on its own and (once again) by convergence.

References
Nesbitt SJ and Norell MA 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B 273:1045–1048. online
Nesbitt S 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History, 302: 84 pp. online pdf
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.

AMNH Effigia webpage
wiki/Effigia