74 - Differential Growth of the Adductor Chamber, Eye, and Brain in the Chick Gallus Gallus, with Comments on the Avialan Fossil Record

Poster Board Number: 74
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Abstract Body : The avian head is characterized by relatively large brain and eyes, as well as a relatively reduced adductor jaw muscles. These derived proportions evolved along an approximately 120my stem lineage that connects crown-group birds to the total archosaur group including crocodylians and all non-avian dinosaurs. Fossil evidence shows that the jaw adductor chamber of birds has been apomorphically reduced, relative to the condition in non-avian dinosaurs, with larger endocranium and orbits. Constructional demands may be assessed by tracking development of those systems, but we cannot easily observe such developmental dynamics in extinct species. Here, we describe jaw adductor anatomy of developing chickens, Gallus gallus, and compare volumetric measurements of these muscles with size of the brain and eye in these same specimens. We reasoned that if these systems constrain each other during development, then there should be allometric signal of that constraint. We hypothesized that Gallus jaw muscles would grow with negative allometry relative to the large brain and eyeball of this crown bird, and that specimens with large muscles (relative to head size) would have relatively small neurosensory organs, and vice versa. Micro-computed tomography (CT) scans were acquired of 22 specimens representing 11 ontogenetic stages ranging from embryonic day five to adult. Additional CT scans were obtained of each specimen after immersing them in an aqueous solution of Lugol’s iodine, which enhances the contrast of soft tissues in CT data and allows for micron-scale resolution of anatomical details. Jaw adductor muscles, brains, and skull bones from each specimen were digitally registered to each other. Linear dimensions of the skull and eyeballs were measured, as were volumetric measurements of jaw muscles and brains. Linear allometric regressions were calculated, with brain or eyeball as the predictor and jaw adductor muscles as the response. Contrary to expectations, we found that jaw muscles grew with positive allometry relative to eyeball (slope = 2.17) and brain (slope = 1.54). Linear allometric relationships between the relative sizes of these organs were either weakly positive or absent, also contrary to our expectations. Observations showed that the embayment for the jaw muscles on the skull extended most dorsally and was most deeply excavated in more mature specimens. Jaw adductors were nonetheless relatively small regardless of eye size just before hatching but relatively large regardless of eye size at specific time points: on the day of hatching and for about a week afterward. We propose three explanations, which are not mutually exclusive, for the findings: 1) these organ systems do not compete for space in Gallus; 2) these systems competed for space in the evolutionary past of birds, and growth of the jaw muscles was truncated early in development (paedomorphosis), which is congruent with the limited growth of the adductor embayment in this taxon compared to non-avian dinosaurs and stem birds such as Ichthyornis, in which jaw muscles grew to occupy relatively more space on the skull; and 3) trade-offs in developmental investment in these systems are limited temporally to the perinatal period. If there is a partitioning of resources, including space, during temporally restricted periods of development, as our data suggest, then a natural next line of inquiry is how developmental dynamics of the jaw apparatus and cranial sense organs differ across other avian species.