Sea Level Change in the Ordovician did NOT Induce the Lilliput Effect

Body size trends in the Ordovician to earliest Silurian of the Oslo Region

Authors:

Sigurdsen et al

Abstract:

Body size is an important ecological parameter that can be understood in both evolutionary and environmental terms. We present a database of changes in overall body size in brachiopods and trilobites through the Ordovician and earliest Silurian of the Oslo Region, Norway. In contrast with global studies, the limited geographical extent of our data allows correlation with environmental parameters such as climate and water depth, and simplifies taxonomical standardization and stratigraphical correlation. In our data set there is a clear indication of increasing size both for brachiopods and trilobites during most of the Ordovician, although this could result from an unbiased random walk. The size increase is followed by a reduction in size during the latest Ordovician. Trilobites show a strong increase followed by a decrease in body size during the Middle Ordovician, while brachiopod body size peaks later, in the Late Ordovician. These changes are partly correlated with the changes in species diversity in the Oslo Region. We discuss the results in terms of two models for change in body size — Cope's rule and the Lilliput effect. Cope's rule is the theory of an increasing body size through geological time, while the Lilliput effect describes a decrease in body size in the aftermath of a mass extinction or a severe environmental disturbance. We find no clear correlations between body size and sea level.

The Liliput Effect Noted for the Devonian Mass Extinctions

When times are good, it pays to be the big fish in the sea; in the aftermath of disaster, however, smaller is better.

According to new research led by the University of Pennsylvania's Lauren Sallan, a mass extinction 359 million years ago known as the Hangenberg event triggered a drastic and lasting transformation of Earth's vertebrate community. Beforehand, large creatures were the norm, but, for at least 40 million years following the die-off, the oceans were dominated by markedly smaller fish.

"Rather than having this thriving ecosystem of large things, you may have one gigantic relict, but otherwise everything is the size of a sardine," said Sallan, an assistant professor in Penn's Department of Earth and Environmental Science in the School of Arts & Sciences.

The finding, which suggests that small, fast-reproducing fish possessed an evolutionary advantage over larger animals in the disturbed, post-extinction environment, may have implications for trends we see in modern species today, such as in fish populations, many of which are crashing due to overfishing. The research is reported in Science.

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Observing the Lilliput Effects in Therocephalians Across the Permian Triassic Mass Extinction

Bone microstructure and the evolution of growth patterns in Permo-Triassic therocephalians (Amniota, Therapsida) of South Africa

Authors:

Huttenlocker et al

Abstract:

Therocephalians were a speciose clade of nonmammalian therapsids whose ecological diversity and survivorship of the end-Permian mass extinction offer the potential to investigate the evolution of growth patterns across the clade and their underlying influences on post-extinction body size reductions, or ‘Lilliput effects’. We present a phylogenetic survey of limb bone histology and growth patterns in therocephalians from the Middle Permian through Middle Triassic of the Karoo Basin, South Africa. Histologic sections were prepared from 80 limb bones representing 11 genera of therocephalians. Histologic indicators of skeletal growth, including cortical vascularity (%CV) and mean primary osteon diameters (POD), were evaluated in a phylogenetic framework and assessed for correlations with other biologically significant variables (e.g., size and robusticity). Changes in %CV and POD correlated strongly with evolutionary changes in body size (i.e., smaller-bodied descendants tended to have lower %CV than their larger-bodied ancestors across the tree). Bone wall thickness tended to be high in early therocephalians and lower in the gracile-limbed baurioids, but showed no general correlation with cross-sectional area or degree of vascularity (and, thus, growth). Clade-level patterns, however, deviated from previously studied within-lineage patterns. For example, Moschorhinus, one of few therapsid genera to have survived the extinction boundary, demonstrated higher %CV in the Triassic than in the Permian despite its smaller size in the extinction aftermath. Results support a synergistic model of size reductions for Triassic therocephalians, influenced both by within-lineage heterochronic shifts in survivor taxa (as reported in Moschorhinus and the dicynodont Lystrosaurus) and phylogenetically inferred survival of small-bodied taxa that had evolved short growth durations (e.g., baurioids). These findings mirror the multi-causal Lilliput patterns described in marine faunas, but contrast with skeletochronologic studies that suggest slow, prolonged shell secretion over several years in marine benthos. Applications of phylogenetic comparative methods to new histologic data will continue to improve our understanding of the evolutionary dynamics of growth and body size shifts during mass extinctions and recoveries.

Not All Organisms Respond to Mass Extinctions in the Same Way: no Lilliput Effect on K-Pg Extinction Bryozoans

The sizes of organisms following mass extinction events may vary more than previously thought, which may be inconsistent with the predictions of the so-called 'Lilliput effect,' according to a study published in PLOS ONE on February 5, 2014 by Caroline Sogot from University of Cambridge and colleagues.

Scientists associate mass extinction events like the Cretaceous-Paleogene (abbreviated K-Pg) event with a reduction in organism size in the aftermath, a phenomenon termed 'the Lilliput effect.' These pronounced changes are thought to be in response to lower food availability and other alterations in the environment that can occur following a mass extinction event. Therefore, survivors of the K-Pg mass extinction should exhibit smaller body size than their pre-extinction relatives. To delve more into this effect, scientists investigated the changes in size of an aquatic invertebrate at the individual- and colony-level before and after the mass extinction.

Scientists analyzed of the 59 bryozoan species and found no significant change in body length. Additionally, the sizes of two types of bryozoan colonies, 210 Maastrichtian colonies and 163 Danian colonies, did not show consistent size decrease before and after the K-Pg extinction event, although maximum colony size did decline in three out of four surviving types of bryozoan. The authors suggest that the lack of size change in the majority of bryozoans studied here may indicate that the Lilliput effect is not universal at all levels, and that the response may vary across organisms.

Dr. Sogot added, "The absence of a clear 'Lilliput effect' in the bryozoans analysed in this study suggests that not all organisms respond in the same manner to all mass extinction events."

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