This is pretty remarkable.
Wang et al. 2016 reported on a growth series for Limusaurus (Xu et al. 2009; Jurassic, Oxfordian; 1.7m in est. length; IVPP V 15923; Figs. 1-5,) “the only known reptile to lose its teeth and form a beak after birth.”
You might remember
Limusaurus became famous earlier for its tiny forelimbs complete with a digit 0 medial to digit 1, that made theropod workers go bonkers because they assumed the digits present were 1-4, not 0-3.
Wang et al. report,
“The available data are important for understanding the evolution of the avian beak.” Except… Limusaurus is not close to the avian line of ancestry anyway you look at it. The LRT nests Limusaurus, with or without teeth, with Khaan, a toothless, beaked oviraptorid. Wang et al. nest Limusaurus with Elaphrosaurus (Fig. 3) even though Khaan is part of their taxon list. So something is not scored right. Not sure about the discrepancy, but some of that could be due to the misidentification of manual digits 0-3.
Wang et al. report,
“The ontogenetically variable features (e.g. teeth/no teeth, etc.) have little effect on its phylogenetic position.” The LRT agrees. Wang et al. report that no matter which ontogenetic stage is tested for Limusaurus, it always nests with or near the ceratosaur, Elaphrosaurus (Fig. 3).The LRT disagrees. In other words, with or without teeth, the topology does not change. In the LRT toothed juvenile Limusaurus also nested with Khaan. Toothed Juravenator and Sinosauropteryx nest as sisters to that clade. The large Compsognathus specimen CNJ79 (Fig. 6) was a basal taxon. All of these sisters are closer to Limusaurus in size and morphology than is Elaphrosauru (Fig. 3).
The ontogenetic series of Limusaurus
is shown in figure 4. Not all the specimens are complete. None are shown to scale. All are portrayed as tiny rough tracings. I think this lack of detail is one shortcoming of the paper.
Wang et al. also provided
reconstructions of a juvenile and adult Limusaurus (Fig. 5). Unfortunately, Wang et al. filled in all the missing bones and gave both reconstructions something of a generic theropod character, lacking some of the traits unique to this genus.
That Limusaurus juveniles had teeth
and adults did not, tells us less about the avian line and more about the oviraptorid line of theropod dinosaurs.
Wang S, Stiegler J, Amiot R, Xu W, Du G-H, Clark JM, Xu X 2016. Extreme ontogenetic changes in a ceratosaurian theropod. Currently Biology 27:1-5 plus SupData.