Earlier the large reptile tree (LRT, 1663+ taxa) nested megapodes (like Megapodius) at a more primitive node than any other living bird, except the kiwi (Apteryx) and ratites, like (like Struthio). You might remember, a toothed bird clade restricted to the Early and Late Cretaceous was derived from toothless Crypturus (Fig. 1) in the LRT.
With that in mind
and hoping to understand the reemergence of previously lost teeth in Early Cretaceous birds, I checked out Clark 1960, who reported on megapode embryology.
To set the stage, Clark wrote,
“Young birds are exceedingly precocious, being able to fly on the day of hatching and feeding actively only a few days after hatching.” He then referenced Portmann (1938, 1951, 1955) who listed several reptile-like characters of megapodes:
- no egg tooth (megapodes hatch by kicking their way out of the shell. The ‘egg tooth’ of chickens temporarily appears on the top of the beak, not the rim);
- lack of down feathers in embryos or nestlings;
- lack of parental care;
- primitive method of incubation (by solar heat, fermentation, vulcanism);
- long incubation period (8 weeks for Leipoa);
- large number of eggs laid;
- slow growth to adult size (especially for Alectura);
- primitive structure of the brain;
- eggs usually not turned and yet hatch relatively successfully;
Clark added to Portmann’s #9
a general lack of movement of the embryo until just before hatching. This may be related to the use of fermentation as a heat source for incubation. Clark notes, “the presence of aerobic bacteria should presumably greatly deplete the available oxygen supply.” Moving embryos might have suffocated for lack of oxygen. Clark also noted: relatively large yolks, as in reptiles.
I never found a tooth thread
connecting Late Jurassic teeth in stem birds to the reemergence of teeth in Early Cretaceous crown birds (Fig. 2) following Apteryx, ratites and megapodes. Even so, every other trait indicated a transition. The above authors further support the extreme primitive nature of megapodes. Ratites no longer bury their eggs. Kiwis dig burrows.
In the post-cladistic era
Dekker and Brom 1992 wrote, “Among megapodes, four different incubation-strategies may be distinguished:
- burrow-nesting between decaying roots of trees,
- burrow-nesting at volcanically heated soils, and
- burrow-nesting at sun-exposed beaches.”
Dekker and Brom employed a cladogram
originally published by Cracraft and Mindell (1989), which mistakenly nested megapodes with galliforms (chickens and kin) due to taxon exclusion. Dekker and Brom wrote, “We conclude that similarities shared with reptiles and kiwis are due to convergence.” That traditional nesting is not confirmed by the LRT due to taxon exclusion. Burying and burrowing are primitive, but give no clue as to how Early Cretaceous birds redeveloped small teeth at first, large teeth later. Neither does megapode embryology. Perhaps that’s why this novel hypothesis of interrelationships has never appeared elsewhere.
Clark GA Jr. 1960. Notes on the embryology and evolution of the megapodes (Aves: Galliformes). Postilla 45:1–7.
Cracraft J and Mindell DP 1989. The early history of modern birds: a comparison of molecular and morphological evidence.— In: B. Fernholm, K. Bremer & H . Jörnvall, eds. The Hierarchy of Life: Molecules and Morphology in Phylogenetic Analysis: 389-403. Amsterdam, New York, Oxford.
Dekker RWRJ and Brom TG 1992. Megapode phylogeny and the interpretation of the incubation strategies. xxx 19–31. Zoologische Verhandelingen 278(2): 19–31.
Portmann A 1938. Beitrage zur Kenntnis der postembryonalen Entwick- lung der Vogel. Rev. Suisse Zool., 45: 273-348.
Portmann A 1951. Ontogenesetypus und Cerebralisation in der Evolution der Vogel und Sauger. Rev. Suisse Zool., 58: 427-434.
Portmann A 1955. Die postembryonale Entwicklung der Vogel als Evolu- tionsproblem. Acta XI Congr. Int. Orn., 1954. Pp. 138-151.