Taking a Bite out of Niche Conservatism

In efforts to quash assumptions associated with niche conservatism, scientists from the Florida Museum of Natural History and the New York State Museum have enlisted the teeth of several fossil mammals to better understand the ecology of climate change.

Recently published at PLoS One, the research holds critical the idea that niches remain constant over extended periods of time. This idea, called niche conservatism, essentially holds that niches are highly specialized, ancestrally –linked, relatively inflexible and are therefore exceedingly susceptible to disturbance and rapid degradation in the face of change - particularly climate change.

While biologic response to environmental change is assumed, often such conservatism is overemphasized and is presented in a manner inconsistent with understanding gained through modern ecology and the insights of paleontology, which have both demonstrated incredible plasticity within the Earth’s biota.

Undoubtedly, the changing climate is of great concern and poses tremendous challenges to all organisms, but at the same time life is flexible and extinctions are not entirely unavoidable. As evidence to life’s flexibility, the currently discussed paper uses the testimony of change documented in the isotopic signatures of mammalian tooth enamel to show that the obstacle of climate change has been successfully negotiated in the past. More specifically, glacial-interglacial transitions during the Pliocene and Pleistocene are marked by substantial alterations in plant communities; however the contemporaneous uptake of vegetative food-stuffs by mammals seems to have fluctuated little.

The article is worth a read:
DeSantis, L., Feranec, R., & MacFadden, B. (2009). Effects of Global Warming on Ancient Mammalian Communities and Their Environments PLoS ONE, 4 (6) DOI: 10.1371/journal.pone.0005750


Vertebrates, and in particular mammals, are highly responsive to changes in the environment. Climate fluctuation can lead to an incredible amount of variation in the diet choice, distribution, and even the morphology of vertebrates.

Natural selection is a reflection of the biological and physical environment as transcribed into the genes of flora and fauna; however, the DNA found within living cells functions only as a recipe for ecological fit, the actual ingredients – the building materials - required to construct living tissue are harvested from surrounding chemistry. Because of variability inherit to this chemistry, the isotopic constituents that compose individuals may differ with respect to climate, diet, or a number of other variables.

For example, carbonate apatite - a primary constituent of both skeletal bones and dentition in vertebrates - is incorporated into organisms through diet and water uptake. Some conservationists use the isotopic signatures recovered from this apatite to better decipher the life histories of contemporary species in expectation of aiding in their continued preservation. As a case in point, remains from the world’s largest cavefish (Milyeringa veritas) have been used by Australian conservationists to better examine and comprehend the environment and ecosystem inhabited by the fish. The cavefish resides in deep water columns that are rich in dissolved limestone. Due to relative densities and variable solubility, different depths in the water column are correlated to specific concentrations of dissolved materials. By examining the isotopic signatures of the cavefish’s skeletal apatite, conservationists were able to discover the exact depth the fish occupies.

As demonstrated by the cavefish, uptake of elements within vertebrates (O, C, Sr & others) generates a highly detailed record of the environmentally available isotopes at a given point in time; this availability can even be indicative of climate.As another illustration of application, Thure Cerling and others at the University of Utah have refined the technique of laser ablation to such quality and precision that they can use it to vaporize the apatite found in teeth. They then analyze the resulting gas via spectrometry to determine its isotopic components. The teeth can be examined to such resolution as to be able to report the dietary preferences of mammals between C3 and C4 plants. A similar isotopes-from-enamel strategy was employed by the folks from the University of Florida to expound several fallacies of niche conservatism…

Examinations of isotopic records are commonplace for many paleontologists. Uncountable volumes have been written on Foram O-18 isotopes and their relationship to periods of increased glaciation, and similar correlations of speleothems and C-13, but accurate isotopic records of both of these elements can be obtained from other sources as well - including mammals.

As a side note - skeletal tissues, which possess apatite, are very susceptible to diagenic processes that can cause recrystalization and alter results, but dental samples seem to be far more reliable when it comes to maintaining their original isotopic signatures. This coupled with the knowledge acquired in the past few decades in regarding incorporation of specific O-18 and C-13 isotopes into biological entities has brought to light a highly accurate record of climate change – and the critters that adapted to it.

Panamanian Treasure Trove

Scientists with the Smithsonian Tropical Research Institute (STRI), the Florida Museum of Natural History and the University of Oregon’s Geology Department seem to be having one heck of a good time down in Panama!

During the last few months, there’s been a variety of paleontology related research coming out of the Panama Canal's spoils; I’ve seen articles on paleoclimate, paleobotany and now vertebrate paleontology.

As copied from STRI materials (PDF available HERE):

Aldo Rincón, STRI paleontology intern, unearthed a set of fossil teeth in the Panama Canal that Bruce MacFadden, curator of vertebrate paleontology at the Florida Museum of Natural History, describes as belonging to Anchitherium clarencei, a three-toed browsing horse the size of the modern donkey, living 15 to 18 million years ago.

Expanding the Panama Canal to make way for super-sized ships is providing geologists and paleontologists with rare finds. Carlos Jaramillo, STRI stratigrapher, has, in collaboration with the University of Florida and the Panama Canal Authority, organized a team of researchers and students who move in following dynamite blasts to map and collect exposed fossils.

"This is one of very few places in the tropics where we have access to fresh outcrops before they are washed away by torrential rains or overgrown by vegetation, and we expect the fossils that we have been salvaging to resolve some major scientific mysteries," said Jaramillo. "What geological forces combined to create the Panama land bridge? Were the flora and fauna of Panama before the land bridge closed similar to those of North America, or did they include other elements?"

The latest finding appears in the Journal of Paleontology, vol. 83: 489-492.

Rincón, MacFadden, Jaramillo

Good stuff…

Gregory Retallack (University of Oregon) and Michael Kirby (STRI) have also been actively publishing on materials from the Canal’s excavations. I plan to post about some of their work over the next few days - so keeping it brief for now.

Anchitherium Image Credits: FMNH

Andean to Amazon, an Anuran Account

The tropical rainforests of the Americas are a hotspot of biodiversity and boasts the largest reservoir of amphibian diversity on the planet; but how did such variety arise in our slippery skinned friends?

If I would have been asked this question yesterday, my reply would have a bit different than it would be today – this after reading an article by Juan C. Santos of the Integrative Biology and Texas Natural Science Center recently published at PLoS.



“The unstable coexistence of lineages within a large community for extended periods of time has been hypothesized as a cause of Neotropical diversity. However, our results suggest that such a model is incomplete; rather, the complex pattern of diversification is strongly intertwined with paleogeographic events. Our inferences about the past history of the poison frogs using ancestral area reconstructions and diversification analyses provide new insights on speciation and extinction patterns in the Neotropics. Three species richness patterns are potential explanations for the extant diversity differences among regions of the Neotropics:

(1) high immigration into one region after suitable geoclimatic conditions are established;

(2) gradual in situ diversification of old endemic clades, regardless of the geoclimatic conditions, promoting species accumulation; or

(3) rapid in situ diversification of endemic clades after favorable geoclimatic conditions are established. We found that all three patterns might apply to different areas depending on historical context.

All extant Amazonian species descended from 14 lineages that dispersed into the Amazon Basin, mostly after the Miocene floodbasin system receded. The recurrent immigrations that originated mostly in the adjacent Andes, combined with an increased rate of diversification, explain the high α–diversity of Amazonia. Later, from the Miocene-Pliocene boundary to the present, a rapid in situ diversification gave rise to the extant Amazonian endemic biota. Therefore, most species in Amazonia originated in the last 10 MY. Moreover, lineages immigrating into Amazonia at <8.0>

The diversity in the Chocoan-Central American super-region derived from scattered immigrations from Andes to the early Chocoan rainforest during the late Miocene. However, starting at the Miocene-Pliocene boundary, significant orogenic events gave rise to the Central American archipelago followed by sea level fluctuations, which provided the conditions for repeated dispersal and vicariance events in pre-PLB islands. Evidence of rapid in situ diversification is supported by the high genetic diversity observed among poison frogs and other lineages especially between Western and Eastern Panamá. Interestingly, our results might explain the high β–diversity of other endemic clades within the Chocó-Central America super-region as originating initially from long-distance dispersals between disconnected islands, with diversification later during isolation by high sea levels.

The Andes have undergone extended in situ diversification since the late Eocene. However, our analyses also provided evidence of decline in the diversification rate since the middle Oligocene, which has important implications for history and conservation of the endemic Andean fauna. First, the Andes uplift at the Miocene–Pliocene boundary caused significant changes in the rate of diversification in the lowland transition zone. We found that several poison frog lineages distributed on one or both sides of the Andes had dispersed repeatedly before the Miocene uplift (i.e., five cross-Andean and five Northern to Central Andes migrations). Paleogeological evidence supports introgression of shallow seas across the northern Andes during the Miocene, suggesting a historical connection between the Amazon Basin and the Chocó. Second, the Pliocene Andean uplift (>2,000 m above sea level) formed a significant barrier to dispersal, because no other cross-Andean dispersals were found. The uplift also was associated with dramatic ecological changes and a decrease in diversification rates. These results suggest a role for niche conservatism, in that some lineages may have gone extinct because of failure to adapt. Alternatively, despite greater sampling effort in the Andes region than in other areas, we failed to find some previously common Andean species (e.g., Hyloxalus jacobuspetersi and the Ecuadorian H. lehmanni). Consequently, it is difficult to separate a natural decrease in diversification rates from the current trend of amphibian species extinctions at high altitudes due to anthropogenic habitat alteration, increased UV radiation, climate change, or pandemic infection. In contrast, the montane transition zones of the Andes and adjacent lowlands (Chocó and Amazonia) have become centers of rapid cladogenesis, and species richness in these transition zones might be underestimated because many Neotropical lineages have been shown to contain several cryptic species. Therefore, dispersals within or across the Andes diminished during the Pliocene, but diversification has intensified in the Andes-lowlands interface.

Although some of the oldest lineages of poison frogs originated in the Guiana Shield and the Venezuelan Highlands (>30 species), our results suggest extended in situ diversification followed by a decline in the rate of diversification of endemic clades in both areas since the early Miocene. Along the same lines, the Guiana Shield has high poison frog endemism, which is mostly restricted to the summits of the sandstone tepuis, while recent Amazonian poison frog immigrants occupy lowlands adjacent to the tepuis. Our results suggest that the decline of endemic Guianan diversity might be associated with ecological changes in habitat due to the collapse of the ancient tepuis and repeated dispersals from Amazonian lineages since the Pliocene. However, the diversity of poison frogs in the Guiana Shield is only beginning to be revealed. In contrast, diversification in the Venezuelan region most likely reflects the oldest vicariant event in Dendrobatidae, at 40.9 MYA. The costal ranges of Mérida, Cordillera de la Costa, and Paria peninsula are species rich but their total area is less than 5% of that of the Amazon Basin. No lineage of this endemic fauna has dispersed out to other regions since the early radiation of the poison frog family in the late Eocene. However, Eocene floristic paleoecological reconstruction of the Venezuelan Highlands area showed that it was more diverse than at present, suggesting that the ancestral habitat of the first poison frogs might have been lowland. The depauperate dendrobatid fauna of the Venezuelan llanos and Brazilian Shield plateau is puzzling, but might be related to Holocene aridity.

The recurring dispersals to Amazonia suggests that a large part of dendrobatid diversity results from repeated immigration waves at <10.0>

Santos, J., Coloma, L., Summers, K., Caldwell, J., Ree, R., & Cannatella, D. (2009). Amazonian Amphibian Diversity Is Primarily Derived from Late Miocene Andean Lineages PLoS Biology, 7 (3) DOI: 10.1371/journal.pbio.1000056

Trilobites, Paleoecology and Anomalocaris

The “Explosion,” as the Cambrian Diversification is commonly referred, occurred approximately a half-billion years ago and is represented by the rapid appearance of about fifty phyla of animals. Present day scientists through the study of numerous fossils, as well as, genetic research have identified this uprising from the Kingdom Animalia and have come to the astonishing conclusion that the entire event lasted, at the very most, a mere ten million years. Most would agree that this seems an extremely minute quantity of time to permit such an all-encompassing radiation, which is one reason why this event is not only a topic of current research, but also one of heated debate.

A linear representation of animal evolution would certainly have a spike near the geologic time of the Cambrian Explosion. All present day phyla (i.e. body-plans, or animal “designs”) arose during that time (with the exception of Bryozoa) including that of the most abundant animal, and second most abundant organism (following only bacteria) on earth today, the arthropods.

One of the groups to rise from the Cambrian is the Phylum Arthropoda. Today the most common arthropods are the insects, but in pre-history, deep in geologic time, the title of most abundant arthropod may quite possibly have gone to the class Trilobita.

Trilobites are without a doubt one of the most easily recognized fossils in modern times, their abundance and variety have played a key role in paleontology as they act as wonderful index fossils. Variation in trilobites covers a wide range of morphological deviation, but most hold a few key characteristics in common. These common morphological characteristics include the division of the trilobite exoskeleton into three distinct regions, those being the head, (cephalon), the main body (thorax) and the tail (pygidium). These regions, especially the cephalon, have distinct sub-features that aid in the identification of individual trilobites, some of which are diagramed below in Figure 1.

FIGURE 1

Trilobites were structurally similar to many modern day arthropods; they possessed jointed appendages and hard exoskeletons, which fortunately - in conjunction with the process of molting - provided us with numerous high quality fossils today. The trilobites ranged in size from mere millimeters to over two-feet in length. They occupied primarily calm, deep waters were there was an abundance of fine silts which they plowed through with their flattened cephalons in order to search out rich debris to be used as a food source. Some trilobites however were most certainly predators, and many may have occupied other niches as well. They appeared (or they apparently appeared - some evidence suggests that they may have earlier origins) early in the Cambrian, reached their zenith in the late Cambrian and then began diversifying up till the Permian during which time they became extinct. There have been estimates of greater than 20,000 species of these incredibly successful Paleozoic marine arthropods - making classification rather tedious at times with new finds occurring on a regular basis. Luckily, some fossils retain sufficient detail as to render their classification relatively certain; however fossils don’t always readily describe the ways in which organisms interacted within their ecosystems.

This is one reason why Jennifer Dunne, et al, conducted research focused towards delineating the food-webs and niche interactions of species identified from the Chengjiang and Burgess Shales. From the Author’s Summary of Compilation and Network Analyses of Cambrian Food Webs,

“Our analyses show that for most aspects of network structure, the Early Cambrian Chengjiang Shale and Middle Cambrian Burgess Shale food webs are very similar to modern webs. This suggests that there are strong and enduring constraints on the organization of feeding interactions in ecosystems. However, a few differences, particularly in the Chengjiang Shale web, suggest that some aspects of network structure were still in flux during early phases of de novo ecosystem construction.”

In another paleoecology related story, Mariel Schotenfeld from the University of Massachusetts Amherst has challenged the widely held idea that Anomalocaris preyed on trilobites.

Anomalocaris

From upcomming G.S.A. agenda: “The Cambrian animal Anomalocaris is hypothesized to have eaten trilobites and other biomineralized prey. The lack of broken or abraded teeth on the plates comprising examination of the mouth apparatus of Anomalocaris suggests that it may not have had the ability to break the exoskeletons of any hard-shelled animal. SEM – EDS of the mouth apparatus from Burgess Shale specimens, indicate that the 32 plates are composed of organic carbon, suggesting they were originally unmineralized cuticle.

Mechanical properties of these plates were analyzed using CAD modeling and Finite Element Analysis. Poisson's ratio and Young's modulus of potential Anomalocaris plates, as well as density and fracture strength used for the FEA analyses, were estimated using a range of modern-day arthropods. Two end-member values were used both to approximate the range of strengths exhibited by Anomalocaris' cuticle, and also to encompass the range of exoskeleton strength likely exhibited by trilobites. The hardest skeletal values are from wet lobster (Homarus americanus) crusher claw cuticle; these are most likely to deform in a brittle manner. The softest are from adult dung beetle (Copris ochus) cuticles. In order to bite and successfully break the calcified cuticle of a trilobite, Anomalocaris' mouth plates would have needed to withstand forces that are greater than those required to fracture a trilobite exoskeleton. Results demonstrate that the teeth-like structures of the mouth plates should have deformed or broken when less than 90 N of force was applied perpendicular to the plates.

Additionally, documented trilobite malformations were compared to modern and extinct arthropod malformations. Abnormal trilobites previously attributed to predation of Anomalocaris might also be interpreted as molting failures or genetic mutations; such malformations occur with similar frequency in modern marine clawed lobsters, brachyuran decapods, and limulids. Furthermore, there is no direct evidence for Anomalocaris' feeding habits such as gut contents.”



Dunne, J., Williams, R., Martinez, N., Wood, R., & Erwin, D. (2008). Compilation and Network Analyses of Cambrian Food Webs PLoS Biology, 6 (4) DOI: 10.1371/journal.pbio.0060102