A study recently published in the journal Nature shows that the skull of todays birds was developed through a sequence of episodes associated with a shortening of the growth patterns in carnivorous dinosaurs (theropods). This process, known as paedomorphosis, implies that the shape of the avian skull is - in general terms- an adult version of the juvenile skulls of their dinosaurian ancestors.
To reach this conclusion, a multidisciplinary team of scientists used an array of digital and statistical shape measurement techniques, known as geometric morphometrics techniques. These techniques made it possible to compare the most complete sample collected so far from juvenile and adult dinosaur embryos, both fossil and current - considering birds as modern day dinosaurs-.
Explaining Different Growth Patterns
In addition to specialists from the Palaeontology Unit at the Universidad Autónoma de Madrid (UAM), the research team authoring the study also includes embryologists from Harvard University and palaeontologists from the Universities of Texas and New York, as well as from the American Museum of Natural History (USA).
The study shows that typical physical features of birds today such as a small body size, large eyes and enlarged (encephalized) brains, are the result of at least four consecutive episodes of shortening of the normal growth pattern (from the embryonic to the adult state) in their ancestors, the theropods. This explains why the more primitive dinosaurs had longer growth patterns than those of their descendants whose shortened growth pattern is clearly evident in birds.
The Value Of Integrated Studies
Jesús Marugán, of the Palaeontology Unit at the UAM and co-author of this article, explains that the first of the four episodes in the shortening of the growth pattern in theropods indicates a change in the overall morphology of the skull: “more quadrangular in the more basal or primeval species, such as the archosauromorph Euparkeria, towards cranial morphologies involving lighter, narrower skulls, such as the tyrannosauroid Guanlong”.
“The second stage is the actual onset of the reduced growth pattern associated with a shortening of the animal’s face: The emblematic Archaeopteryx, the most primitive bird genus we know. In addition to this drastic diminution in body size, this stage gives rise to birds’ most distinctive features: beaks, cephalisation and increase in eye socket size”, adds this researcher.
As stated in the article, precisely these features that became progressively more prominent in modern birds (reduced size and cephalic development) have been decisive in the evolutionary process that led to configuring the mechanical and neuronal control necessary for flight. For this team of researchers, this finding is more than merely a further piece of paleobiological evidence to show that birds are dinosaurs. They have pointed out that it is also proof that the key to understanding the nature of the evolutionary mechanisms are integrated studies in which extinct species are compared with their descendants, the living species that inhabit the planet today.
Development and Evolution
The relationship between alterations in the growth pattern (embryonic development) and the emergence of evolutionary innovations through time is a widely acknowledged biological fact. This relationship between development and evolution is perfectly summed up thus: “Ontogeny recapitulates Phylogeny”, the phrase proposed by the German philosopher and biologist Ernst Haeckel in 1892 and known as the Theory of Recapitulation. This theory maintains that embryonic development in a given species (ontogeny) reflects the evolutionary history of that species (phylogeny). In other words, each of the stages an individual of a species undergoes throughout its embryonic development represents one of the adult forms that occurred in its evolutionary history.