Dataset Information

Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water.

ABSTRACT: Eocene archaeocete whales gave rise to all modern toothed and baleen whales (Odontoceti and Mysticeti) during or near the Eocene-Oligocene transition. Odontocetes have asymmetrical skulls, with asymmetry linked to high-frequency sound production and echolocation. Mysticetes are generally assumed to have symmetrical skulls and lack high-frequency hearing. Here we show that protocetid and basilosaurid archaeocete skulls are distinctly and directionally asymmetrical. Archaeocete asymmetry involves curvature and axial torsion of the cranium, but no telescoping. Cranial asymmetry evolved in Eocene archaeocetes as part of a complex of traits linked to directional hearing (such as pan-bone thinning of the lower jaws, mandibular fat pads, and isolation of the ear region), probably enabling them to hear the higher sonic frequencies of sound-producing fish on which they preyed. Ultrasonic echolocation evolved in Oligocene odontocetes, enabling them to find silent prey. Asymmetry and much of the sonic-frequency range of directional hearing were lost in Oligocene mysticetes during the shift to low-frequency hearing and bulk-straining predation.

SUBMITTER: Fahlke JM

PROVIDER: S-EPMC3167538 | biostudies-other | 2011 Aug

REPOSITORIES: biostudies-other

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Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water.

Fahlke Julia M JM Gingerich Philip D PD Welsh Robert C RC Wood Aaron R AR

Proceedings of the National Academy of Sciences of the United States of America 20110822 35

Eocene archaeocete whales gave rise to all modern toothed and baleen whales (Odontoceti and Mysticeti) during or near the Eocene-Oligocene transition. Odontocetes have asymmetrical skulls, with asymmetry linked to high-frequency sound production and echolocation. Mysticetes are generally assumed to have symmetrical skulls and lack high-frequency hearing. Here we show that protocetid and basilosaurid archaeocete skulls are distinctly and directionally asymmetrical. Archaeocete asymmetry involves ...[more]

PMID: 21873217

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Project description:ObjectivesPalate morphology is constantly changing throughout an individual's lifespan, yet its asymmetry during growth is still little understood. In this research, we focus on the study of palate morphology by using 3D geometric morphometric approaches to observe changes at different stages of life, and to quantify the impact of directional and fluctuating asymmetry on different areas at different growth stages.Materials and methodsThe sample consists of 183 individuals (1-72 years) from two identified human skeletal collections of 19th and early 20th Century Italian contexts. A 3D-template of 41 (semi)landmarks was applied on digital palate models to observe morphological variation during growth.ResultsAsymmetrical components of the morphological structure appears multidirectional on the entire palate surface in individuals <2 years old and become oriented (opposite bilateral direction) between 2 and 6 years of age. Specifically, directional asymmetry differentially impacts palate morphology at different stages of growth. Both the anterior and posterior palate are affected by mild alterations in the first year of life, while between 2 and 6 years asymmetry is segregated in the anterior area, and moderate asymmetry affects the entire palatal surface up to 12 years of age. Our results show that stability of the masticatory system seems to be reached around 13-35 years first by females and then males. From 36 years on both sexes show similar asymmetry on the anterior area. Regarding fluctuating asymmetry, inter-individual variability is mostly visible up to 12 years of age, after which only directional trends can be clearly observed at a group level.DiscussionMorphological structure appears instable during the first year of life and acquires an opposite asymmetric bilateral direction between 2 and 6 years of age. This condition has been also documented in adults; when paired with vertical alteration, anterior/posterior asymmetry seems to characterize palate morphology, which is probably due to mechanical factors during the lifespan. Fluctuating asymmetry is predominant in the first period of life due to a plausible relationship with the strength of morphological instability of the masticatory system. Directional asymmetry, on the other hand, shows that the patterning of group-level morphological change might be explained as a functional response to differential inputs (physiological forces, nutritive and non-nutritive habits, para-masticatory activity as well as the development of speech) in different growth stages. This research has implications with respect to medical and evolutionary fields. In medicine, palate morphology should be considered when planning orthodontic and surgical procedures as it could affect the outcome. As far as an evolutionary perspective is concerned the dominance of directional asymmetries in the masticatory system could provide information on dietary and cultural habits as well as pathological conditions in our ancestors.

Project description:Abstract Loss and reduction in paired appendages are common in vertebrate evolution. How often does such convergent evolution depend on similar developmental and genetic pathways? For example, many populations of the threespine stickleback and ninespine stickleback (Gasterosteidae) have independently evolved pelvic reduction, usually based on independent mutations that caused reduced Pitx1 expression. Reduced Pitx1 expression has also been implicated in pelvic reduction in manatees. Thus, hindlimb reduction stemming from reduced Pitx1 expression has arisen independently in groups that diverged tens to hundreds of millions of years ago, suggesting a potential for repeated use of Pitx1 across vertebrates. Notably, hindlimb reduction based on the reduction in Pitx1 expression produces left‐larger directional asymmetry in the vestiges. We used this phenotypic signature as a genetic proxy, testing for hindlimb directional asymmetry in six genera of squamate reptiles that independently evolved hindlimb reduction and for which genetic and developmental tools are not yet developed: Agamodon anguliceps, Bachia intermedia, Chalcides sepsoides, Indotyphlops braminus, Ophisaurus attenuatuas and O. ventralis, and Teius teyou. Significant asymmetry occurred in one taxon, Chalcides sepsoides, whose left‐side pelvis and femur vestiges were 18% and 64% larger than right‐side vestiges, respectively, suggesting modification in Pitx1 expression in that species. However, there was either right‐larger asymmetry or no directional asymmetry in the other five taxa, suggesting multiple developmental genetic pathways to hindlimb reduction in squamates and the vertebrates more generally. We quantified directional asymmetry in the vestigial hindlimbs of six squamate lineages to test for parallel loss of gene expression of Pitx1 during convergent limb reduction. We found left‐larger asymmetry in one of six taxa, suggesting that Pitx1 may be involved in that lineage. However, it is likely that non‐parallel genetic mechanisms underlie pelvic reduction in the squamates.

Dataset Information

Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water.

Publications

Cranial asymmetry in Eocene archaeocete whales and the evolution of directional hearing in water.

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