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.
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 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 Gracilisuchus, Scleromochlus 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.
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.
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.
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.