The Underwhelming Fossil Record of Baleen Whales
Gigantism is a ubiquitous feature across all lineages of extant Baleen Whales (Mysticeti). However, despite the exhaustive fossil record and comprehensive phylogenetic framework, the evolution of such extreme body sizes remain poorly understood (Vermeij, 2016). Slater et al. (2017) conducts the first proper quantitative test focused to understand how and when gigantism originated. Phylogenetic inference and species’ mean body size are utilized to estimate the mode and tempo of the gigantism origins in mysticetes.
An Overview of the Study
The study attempted to estimate the origin of gigantism. A right shifted body size distribution was observed in extant species relative to their fossilized counterparts. This is particularly emphasized as both the largest and smallest extant species- blue whale (Balaenoptera musculus) and (Caperea marginata) respectively – are significantly larger than their fossil counterparts. Furthermore, the results suggest that gigantism evolved independently across multiple lineages. These lineages includes the Balaenoptera, Balaena and Eubalaena genuses.
The authors were able to estimate size distributions in the first 30 Ma of mysticetes evolution accurately in accordance with the constant rate process. However, the body sizes of extant species do not always conform with these predictions. In fact, contrary to the homogenous evolutionary process, there was an elaborate pulse in intra-clade variation around five million years ago. Furthermore, there was strong evidence of a shift in evolutionary model, around 4.5 MYA. This has immense implications on the origins of evolution. This can be depicted in Figure 2
What did the study infer?
Based on the results, the authors hypothesized that a mode shift model is responsible for the evolution of gigantism in mysticetes (Slater et al., 2017). The shift in evolutionary mode refutes the constant rate process as a possible mechanism for the evolution of large sizes. According to Cope’s Law, there is a generic increase in body size during the evolutionary history of population lineages. However, due to the evolutionary mode shift between during the Pleistocene, between approximately 4.5 and 0.13 Ma, the evolution of mysticete body size of mysticetes has not been consistent with the constant rate process. These inferences were also observed in both prior (Tsai and Kohno, 2016) and subsequent studies (Bianucci et al., 2019).
The Poor Fossil Record could potentially hamper Interpretations
Slater et al. (2017) mentions that the lack of concrete mysticete fossil records from the Pleistocene undermines the validity of the mode shift model. Due to this reason, it is uncertain whether the observed trends in mysticete body size was genuine or through sampling bias (Bianucci et al., 2019). However, Slater et al. (2017) refutes the possibility of taphonomical bias, based on tests utilizing stimulated data. No evidence of size-biased sampling on false detection rate for the proposed model was observed during these tests.
What was the largest baleen whale fossil?
Nonetheless, the model proposed by Slater et al. (2017) found some support following the discovery of the largest mysticete fossil. The blue whale specimen, dated between 1.5 and 1.25 Ma, had an estimated body length of 26m (Bianucci et al., 2019). A study (Bianucci et al., 2019) attempted to integrate this fossil discovery into the macroevolutionary models used by Slater et al. (2017). As such, the study suggests that the mode shift model is probably the most compelling theory (Bianucci et al., 2019). However, the analysis estimates that the mode shift occurred 3.62 Ma rather than the original estimates of approximately 0.31 Ma. Furthermore, there was less evidence for the model than the original study itself (Bianucci et al., 2019).
Did altered food distribution lead to evolution of gigantism in whales?
As such, the original paper, hypothesizes that gigantism evolved to capitalize on the redistribution of primary production, resulting from the commencement of wind induced upwelling regimes during the Late Pliocene (Slater et al., 2017). As a result, dense accumulations of mysticete prey would occur seasonally and ephemerally. Slater et al. (2017) argues that there is a strong selection for large body sizes in such environments. Lower mass specific metabolic rates would provide mysticete with a buffer against periods of low resource availability (Goldbogen et al., 2017). Furthermore, a study found that large body size is associated with faster swimming speeds in air breathing diving animals (Watanabe et al., 2010). This allows them to exploit greater ranges during a single dive, which proves beneficial during the long migration between foraging sites (Watanabe et al., 2010).
The origin of gigantism occurs almost coincidently with the decline in mysticete diversity, particularly small bodied species, approximately 3 Ma (Marx and Fordyce, 2015). Large bodied species most likely outcompeted their smaller counterparts in these oligotrophic ocean ecosystems. (Slater et al., 2017)
Does Filter Feeding have a role in the evolution of gigantism in whales?
It has also been suggested that filter feeding ensured the gigantism was maintained in the mysticete lineage following its origin. As such, all extant baleen whales display active bulk filter feeding (Slater et al., 2017). Moreover, filter feeding has been associated with large sizes across marine vertebrate taxa, including cartiligeous and bony fish lineages (Friedman, 2011). Computational modelling using kinematic data on Megaptera noveaangliae, Balaenoptera physalus, Balaenoptera musculus and Balaenoptera acutorostrata whales have proved that filter feeding provides both individual and trophic advantages in large bodied species (Potvin et al., 2012).
Slater et al. (2017) accurately refutes the initial evolution of filter feeding as the evolutionary mechanism of gigantism. Support for this claim lies in the second oldest fossil of a baleen whale Llanocetus denticrenatus, aged around 34 Ma. The species is interpreted to be a raptorial feeder rather than a filter feeder (Fordyce and Marx, 2018). Filter feeding, on the other hand, is estimated to only have evolved in the Mid-Oligocene, around 25 Ma (Tsai and Fordyce, 2015). Moreover, L. denticrenatus is thought to be an anomaly. This species, with a body length of 8m was a relative giant, given that most species were not larger than 6m throughout the Oligocene and Miocene. True gigantism, which is defined as an individual with a body length of greater than 10 m, is thought to have evolved substantially later (Fordyce and Marx, 2018).
Conclusion
Slater et al. (2017) also predicts multiple individual origins of gigantism across different mysticete lineages. This has been consistent with numerous other studies (Fordyce and Marx, 2018)and(Tsai and Kohno, 2016). Nonetheless, despite a general acceptance in the findings and inferences published by Slater et al. (2017), numerous aspects of the origins of mysticete gigantism remains unresolved. There does not seem to be a consensus with the timing of the origin of evolution. For example, Vermeji (2016) suggests that gigantism began appearing around the Neogene, approximately 20 Ma. This ambiguity can be resolved once a more robust mysticetes fossil record from the Pleistocene is discovered.
Nonetheless, the study emphasis that climate change does have a significant impact on a variety of species. It emphasizes the need to prevent a climate emergency from occurring!
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