Kaye et al. 2019
illuminated feathers with laser-stimulated fluorescence (Fig. 1) in a tiny, unnamed, enantiornithine bird specimen, MPCM-LH-26189 from the Las Hoyas locality (Barreminian, Early Cretaceous) of Spain. Based on the presence of those illuminated feathers and the size of the specimen (Fig. 2) the authors judged it to be a precocial hatchling, capable of walking shortly after hatching. This is the same specimen first described and not named by Knoll et al. 2018.
A reconstruction of the new extra-tiny bird
is shown (Fig. 2) alongside that of another tiny coeval and closely related enantiornithine bird, Iberomesornis, to scale. Note the tiny fingers in the tiny MPCM specimen indicating flightlessness. The lower crus, distal tail and feet extend off the matrix block, so they remain unknown. Contra Kaye et al. 2019, the tiny MPCM specimen does not appear to have juvenile proportions, despite its reduced size.
It is always a good idea
to create a reconstruction (Fig. 2) from ‘road-kill’ taxa (Fig. 1). Such a reconstruction would have indicated the MPCM specimen did not have juvenile proportions, despite its small size… and it did not have traditional bird wings.
It is also a good idea to compare taxa
in a phylogenetic analysis to see how what you have relates to others of its kind. Here in the large reptile tree (LRT, 1423 taxa) the MPCM specimen nests close to Iberomesornis within the clade Enantiornithes.
The MPCM specimen is the first enantiornithine to have short un-birdlike fingers (a reversal due to neotony) and such short forelimbs (another reversal).
The small size of this possible adult specimen is also due to the same forces that led to tiny Iberomesornis in Early Cretaceous Spain. If the MPCM specimen had nested with much larger specimens, rather than tiny Protopteryx and Iberomesornis, then the MPCM specimen would more likely have been considered a juvenile.
Knoll et al. 2018 first studied the MPCM specimen
or its osteological correlates with other juvenile birds, not considering the possibility that phylogenetic miniaturization might make a tiny adult bird appear to be a juvenile. Perhaps that is why they concluded, “the hatchlings of these phylogenetically basal birds varied greatly in size and tempo of skeletal maturation.” Knoll et al. did not create a reconstruction nor put this specimen under phylogenetic analysis, probably on the basis of its presumed juvenile character. As your mother told you, if you assume something, you might miss out on its most intriguing aspects.
Phylogenetic analysis is so important
because it reveals so much more than just ‘eyeballing’ specimens.
In the Late Jurassic
tiny pterosaurs experienced a similar size squeeze. Traditionally considered juveniles, tiny hummingbird-sized taxa like B St 1967 I 276 (Fig. 3) and BMNH 42736 with fly-sized hatchlings, were among the few pterosaur lineages to survive the Jurassic and produce Cretaceous taxa.
Paleo bird expert, Jingmai O’Connor reports, “All enantiornithines were super-precocial, born fully-fledged and ready to fly.”
A closer examination
indicates the MPCM specimen was never going to be ‘ready to fly.’
Is the MPCM specimen the smallest dinosaur?
If it is an adult, the MPCM specimen appears to be slightly larger than the smallest known dinosaur, the bee hummingbird (Fig. 3).
Since no one else wants to name the MPCM specimen,
probably because others considered this a hatchling rather than a phylogenetically miniaturized adult, let’s call him Microcursor sanspedes (‘tiny runner without feet”) in the meantime.
Kaye TG, Pittman M, Marugán-Lobón J, Martín-Abad H, Sanz JL and Buscalioni AD 2019. Fully fledged enantiornithine hatchling revealed by Laser-Stimulated Fluorescence supports precocial nesting behavior. Nature.com/scientific reports (2019) 9:5006 https://doi.org/10.1038/s41598-019-41423-7
Knoll, F. et al. (16 co-authors) 2018. A diminutive perinate European Enantiornithes reveals an asynchronous ossification pattern in early birds. Nature Communications 9, 937 (2018).