Phylogenetic miniaturization in Theropoda: two hypotheses

Lee, Cau, Naish and Dyke 2014
described the ‘sustained miniaturization…in the dinosaurian ancestors of birds’ starting with an unspecified basal tetanuran (Earliest Jurassic, 190mya) at 163kg, then evolving to a basal neotetanuran at 46kg, followed by a basal coelurosaurian at 27kg, a basal maniraptoran at 10kg, a basal parades at 3.3kg, and a basal bird at 1kg. We first looked at that theropod-bird evolution paper earlier here.

A distinctly different theropod tree topology
is found in the large reptile tree (LRT, 1315 taxa). More theropods have been added since 2014 (subset Fig. 1). Here small (goose-sized) theropods (in hot pink) arise from late (Early Cretaceous) survivors of Late Triassic radiations (based on the presence of Coelophysis as a derived taxon in an early clade, derived from a sister to Sinocalliopteryx, which was fully feathered with filaments in the fossil). So feather filaments were present in theropods from at least this point forward, with scales/placodes replacing filaments in very large taxa.

Figure 1. Subset of the LRT focusing on basal theropods. Pink area are more or less goose-sized and smaller taxa.

Figure 1. Subset of the LRT focusing on basal theropods. Pink areas are more or less goose-sized and smaller taxa leading to birds and following Tawa and Sinocalliopteryx. Smaller theropods evolve to become large and giant theropods several times by convergence here.

We’ve seen phylogenetic miniaturization before
attending the genesis of new clades in turtle origins, pterosaur origins, derived pterosaur origins, dinosaur origins, archosauriform origins, reptile origins, mammal origins, snake origins, squamate origins and I’m forgetting a few.

Given that the basalmost archosaur,
PVL 4597, is also goose-sized or smaller, it is possible that small undiscovered theropods were present throughout the Mesozoic, giving rise to larger taxa in the Herrersaurus, Tawa, Zuolong and the Coelophysis clades in the early part of the Age of Dinosaurs. 

References
Lee MSY, Cau A, Naish D and Dyke GJ 2014. Sustained miniaturization and anatomicial innovation in the dinosaurian ancestors of birds. Science 345(6196):562–566.

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