Evolution of Ancient Dugong Communities

Manatees and dugongs are puzzling creatures. They are large, slow-moving aquatic mammals that inhabit tropical and subtropical waters. They graze on aquatic vegetation in freshwater and shallow marine environments. Like cetaceans, the four living species of manatees and dugongs (Order Sirenia) have horizontal tail fluke in place of hind limbs and flipper-like forelimbs (Figure 1).

Christian Haugen
Figure 1. A living dugong from the Indo-Australian region (Dugong dugon). (From Christian Haugen/Flickr)

Today sirenians have disjunct and allopatric distributions. However, careful analysis of the fossil record reveals that ancient dugong communities were more diverse. Jorge Velez-Juarbe, Daryl Domning, and Nicholas Pyenson report in the journal
PLoS ONE that multispecies communities existed in many parts of the world until relatively recently (Figure 2).

dugong2
Figure 2. An illustration by Carl Buell showing the three species in each fossil dugongid assemblage. Multispecies communities are from Florida (top left), India (top right) and Mexico (bottom right) over the past 26 million years. (From Velez-Juarbe et al., 2012)


By evaluating a suite of ecomorphological characters including the size and shape of the upper tusk, the degree of downward deflection of the snout, and overall body size, the researcher constructed a phylogenetic tree of fossil sirenians. The tree revealed that late Oligocene dugongs from Florida differed in body size and tusk size (Figure 3). Likewise the early Miocene dugong assemblage from India consisted of three taxa with different degrees of skull deflection and two body size categories. The early Pliocene dugongs from Mexico differed in body size and tusk shape (cross-sectional shape).

dugong1
Figure 3. Ecomorphological features for the fossil taxa used in this study. A)
Categories of upper incisor size and depth (gray) for late Oligocene species from Florida B) body sizes comparisons among dugongids in this study. C) Fossil dugongids from India showing differences in rostral deflection. D) Cross-sectional outline of incisors of fossil dugongids from Mexico. (From
Velez-Juarbe et al., 2012)

The differences in morphology within each multispecies community suggest that ancient dugong species partitioned food resources. Resource competition may have led to differences in size and in traits related to feeding (tusk and skull shape). The smaller species in each assemblage likely foraged in very shallow waters (less than 1 meter deep) and perhaps on different sea grass species than the large-bodied dugongs in the community. Furthermore, the researchers suggest that the larger tusked dugongs in the community may have acted as keystone species by limiting the takeover of one sea grass species. The resulting sea grass communities were more diverse and offered opportunities for interspecific competition to drive evolution of several morphologically unique dugong species in each community.

Why multispecies sirenian communities disappeared leaving a single species in each region of the world remain a mystery, but changes in the diversity and abundance of sea grasses may have played a pivotal role.


References

Velez-Juarbe, J., Domning, D., & Pyenson, N. (2012). Iterative Evolution of Sympatric Seacow (Dugongidae, Sirenia) Assemblages during the Past ∼26 Million Years PLoS ONE, 7 (2) DOI: 10.1371/journal.pone.0031294
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