Category Archives: Paleontology

Caught in the act: Insect sex preserved in amber

by Piter Kehoma Boll

A recently published paper describes a new species of insect of the order Zoraptera from two specimens found in mid-cretaceous amber from northern Myanmar.

The preserved couple. They did not leave descendants but were eternized in science. Credits to Chen & Su (2019).*

But the most impressive thing about this new pre-historic species, named Zorotypus pusillus, is the fact that the fossil contains a male and a female that apparently died while they were mating. This is concluded because the two individuals are very close to each other and the male has an elongate structure coming out of his abdomen, which is probably the aedeagus or intromittent organ, a penis-like organ found in most zorapterans and used to deliver sperm into the female.

A detail of the posterior end of the male showing the aedeagus or intromittent organ. An anatomical reconstruction is shown to the right. Credits to Chen & Su (2019).*

The order Zoraptera contains a very small number of species, currently 44 extant ones and 14 fossils. They are very small, live in groups and look like tiny termites, although they are not closely related to them. Most extant species mate with the male introducing its aedeagus into the female to deliver sperm, but at least one species, Zorotypus impolitus, does not copulate. In this species, the male deposits microscopic spermatophores on the abdomen of the female.

The discovery of the preserved mating behavior in this species from the cretaceous period indicates that the mating behavior seen in most extant species was already used by species living 99 million years ago. The origin of zorapterans is not well known yet, but this and other fossil species indicate that they exist at least since the beginning of the cretaceous.

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Reference:

Chen X, Su G (2019) A new species of Zorotypus (Insecta, Zoraptera, Zorotypidae) and the earliest known suspicious mating behavior of Zorapterans from the mid-cretaceous amber of northern Myanmar. Journal of Zoological Systematics and Evolutionary Research. doi: 10.1111/jzs.12283

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*Creative Commons License This work is licensed under a Creative Commons Attribution 4.0 International License.

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Hagfish: Another Phylogenetic Headache

by Piter Kehoma Boll

Years ago, I wrote a post about the problematic Acoelomorpha and their controversial position among bilaterian animals. Now I am going to talk about another headache: hagfish.

Hagfish are primitive chordates that make up the class Myxini. They are marine animals that live at the bottom of the sea and feed mainly on polychaete worms that they pull out of the substrate. However, they are also scavengers and have a peculiar behavior in which they perforate the body of dead fish and enter it, eating the dead animal from inside out.

Specimen of the Pacific hagfish Eptatretus stoutii. Photo by Jeanette Bham.*

Morphologically, hagfish are characterized by the presence of a cartilaginous skull, like vertebrates, but lack a vertebral column, keeping the notochord, the dorsal cartilage-like structure of chordates, during their whole lives. Due to this lack of vertebrae, the hagfish were classified outside of the vertebrates, but united to them due to the presence of the skull. Thus, Myxini was seen as the sister-group of Vertebrata and both together formed the clade Craniata.

Among the vertebrates, most extant groups have a jaw that evolved from modified gill arches, making up the clade Gnathostomata. The only animals with a vertebral column that lack jaws are the lampreys (Petromyzontiformes) and, although this lack of jaws is shared with hagfish, it is not usually seen as a synapomorphy uniting these groups. In hagfish, the jawless mouth have lateral keratin plates with tooth-like structures that act somewhat like the true jaws of Gnathostomata, but working from the sides and not from above and below. In lampreys, on the other hand, the mouth is circular and have keratin tooth-like structures arranged circularly.

General organization of the head of hagfish, lampreys and jawed vertebrates, with special attention to the mouths. Extracted from Oisi et al. (2012).

There are a lot of morphological features that unite lampreys to vertebrates and separate them from hagfish, the main one being the already mentioned vertebrate column. Likewise, lampreys and jawed vertebrates have dorsal fins while hagfish lack them. Lampreys also have lensed eyes in common with jawed vertebrates, while hagfish have simple eyesposts without lenses or even associated muscles.

Some of the traits shared between hagfish and lampreys, just as the lack of jaws, are usually seen as a primitive state that changed in jawed vertebrates, or have clearly evolved independently. For example, both hagfish and lampreys have only a single nostril, while jawed vertebrates have two, but this is likely a primitive character. Adult hagfish and lampreys have also a single gonad, but this appears in hagfish by an atrophy of the left gonad, so that only the right one develops, while in lampreys the left and right gonads fuse into a single organ.

Specimens of the least brook lamprey Lampetra aepyptera. Photo by Jerry Reynolds.*

Therefore, morphologically, it seems logical to consider hagfish as a sister group of vertebrates, which include lampreys and jawed vertebrates. It is also important to mention that there are more groups of jawless vertebrates that are currently extinct, such as the class Osteostraci, one of several fossil groups traditionally called ostracoderms. Although lacking a jaw as well, these vertebrates had paired fins just like jawed vertebrates. Thus, the phylogenetic organization of these major groups based on morphology would be as shown in the figure below:

The craniate hypothesis, where hagfish are a sister-group to vertebrates.

However, in the last decades, the use of molecular phylogenetics has challenged this view by grouping hagfish and lampreys into a monophyletic clade that is sister-group of jawed vertebrates. But how could this be possible? Such a relationship would imply that the primitive state of hagfish is the result of secondary loss.

The cyclostome hypothesis. Hagfish are a sister-group to lampreys.

Evidence from fossils could help clarify this issue, but most fossils that have been associated with hagfish have not good enough morphological characters preserved to assess their correct phylogenetic position. Recently, however, a well preserved hagfish fossil from the Cretaceous helped to elucidate part of the hagfish phylogeny. The divergence between lampreys and hagfish, considering previous knowledge, was usually put around the early Cambrian period, just after the beginning of the divergence of most animal phyla, but with data of the new fossil, it is pushed to a more recent point in time, around the Early Silurian, more than 130 million years after. This new fossil, named Tethymyxine tapirostrum, clearly lacks a skeleton or dorsal fins as seen in lampreys and jawed vertebrates, but has several characters shared with extant hagfish.

Fossil of Tethymyxine tapirostrum found in Lebanon. Extracted from Miyashita et al. (2019).

At least two synapomorphies can be found uniting hagfish and lampreys and separating them from jawed vertebrates. The first one are the teeth, which in these two groups are composed of keratin plates. The second one is the organization of the myomeres, the series of muscles arranged along the body of chordates in a somewhat segmented fashion, that in both hagfish and lampreys begin right around the eyes.

Considering the evidence from molecular data, the new fossil that makes it likely that hagfish and lampreys diverged more recently if they form a monophyletic group, and the likely true synapomorphies uniting these two jawless vertebrate groups, it seems that hagfish and lampreys are indeed sister-groups, forming a clade called Cyclostomata and sister-group of the jawed vertebrates Gnathostomata. If this is really the case, then the apparently more primitive features of hagfish are in fact the result of secondary losses and its ancestor likely had a more vertebrate look, with a vertebral column, dorsal fins and lensed eyes.

But let’s keep watching. Things may change again in the future as new data become available.

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References:

Miyashita T, Coates MI, Farrar R, Larson P, Manning PL, Wogelius RA, Edwards NP, Anné J, Bergmann U, Palmer AR, Currie PJ (2019) Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. PNAS 116(6): 2146–2151. doi: 10.1073/pnas.1814794116

Oisi Y, Ota KG, Kuraku S, Fujimoto S, Kuratani S (2012) Craniofacial development of hagfishes and the evolution of vertebrates. Nature 493: 175–180. doi: 10.1038/nature11794

Wikipedia. Cyclostomata. Available at <
https://en.wikipedia.org/wiki/Cyclostomata >. Access on March 25, 2019;

Wikipedia. Hagfish. Available at <
https://en.wikipedia.org/wiki/Hagfish >. Access on March 25, 2019.

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Shaking dinosaur hips and messing with their heads

by Piter Kehoma Boll

This week brought astonishing news regarding the phylogeny of dinosaurus, as you perhaps have heard or read. New anatomical evidences have completely rebuilt the basis of the dinosaur family tree and I’m here to explain a little bit of what happened.

As we all know, Dinosaurs include a great variety of beasts, from the meat-eating theropods to the long-necked sauropods and from the horned ceratopsians to the armored ankylosaurs, among many others.

largestdinosaursbysuborder_scale

Silhouette of a human compared to the largest known dinosaurs of each major group. Picture by Matt Martyniuk.*

For more than a century now, dinosaurus have been divided into two groups, called Ornithischia and Saurischia. Ornithischia (“bird-hipped”) includes dinosaurus whose pelvic bones are more similar to what is found in birds, with a pubis directed backward. Saurischia (“lizard-hipped”), on the other hand, have a pubis directed forward, as in reptiles in general. This grouped the theropods and the sauropods in the same group as Saurischia while other dinosaurus were grouped as Ornithischia. But birds are actually theropods, thus being lizard-hipped dinosaurus and not bird-hipped dinosaurus! Confusing, isn’t it? So let’s take a look at their hips:

Pelvic_bones

Comparison of the hips of a crocodile (Crocodylus), a sauropod (Diplodocus), a non-avian theropod (Tyrannosaurus), a bird (Apteryx), a thyreophoran (Stegosaurus), and an ornithopod (Iguanodon). Red = pubis; Blue = ischium; Yellow = ilium. Picture by myself, Piter K. Boll.**

As you can see, the primitive state, found in crocodiles, sauropods and early theropods, is a pubis pointing forward. A backward-pointing pubis evolved at least twice independently, both in more advanced theropods (such as birds) and the ornithischian dinosaurus. But could we be so certain that Tyrannosaurus and Diplodocus are more closely related to each other (forming a clade Saurischia) just because of their hips? Afterall, this is a primitive hip, so it is very unlikely to be a synapomorphy (a shared derived character). Nevertheless, it continued to be used as a character uniting sauropods and theropods.

A new paper published by Nature this week, however, showed new evidences that point to a different relationship of the groups. After a detailed analysis of the bone anatomy, Matthew G. Baron, David B. Norman and Paul M. Barrett have found 20 characters that unite theropods with ornithischians and not with sauropods. Among those we can mention the presence of a foramen (a hole) at the anterior region of the premaxillary bone that is inside the narial fossa (the depression of the bone that surrounds the nostril’s opening) and a sharp longitudinal ridge along the maxilla.

skulls

The skulls of both ornithischians and theropods (above) show an anterior premaxillary foramen in the narial fossa (shown in yellow) and and a sharp ridge on the maxilla (shown in green), as well as other characters that are not present in sauropodomorphs and herrerasaurids (below). Composition using original pictures by Carol Abraczinskas and Paul C. Sereno (Heterodontosaurus), Wikimedia user Ghedoghedo (Eoraptor and Herrerasaurus), and flickr user philosophygeek (Plateosaurus).**

In his blog Tetrapod Zoology, Dr. Darren Naish comments the new classification and points out some problems that arise with this new view. One of them is the fact that both theropods and sauropodomorphs have pneumatic (hollow) bones, while ornithischians do not. If the new phylogeny is closer to the truth, that means that pneumacity evolved twice independently or evolved once and was lost in ornithischians.

He also mentions that both ornithischians and theropods had hair-like or quill-like structures on their skin. In theropods this eventually led to feathers. Could this be another synapomorphy uniting these groups? Maybe… but when we think that pterosaurs also had “hairs”, one could also conclude that a “hairy” integumentary structure was already presented in the common ancestor of dinosaurus. In this case, perhaps, we only had not found it yet on sauropods. Now imagine a giant Argentinosaurus covered with feathers!

One concern that appeared with this new organization is whether sauropodomorphs would still be considered dinosaurs. The term “dinosaur” was coined by Richard Owen in 1842 to refer to the remains of the three genera known at the time, Iguanodon, Hylaeosaurus and Megalosaurus, the first two being ornithischians and the latter a theropod. As a consequence, the original definition of dinosaur did not include sauropods. Similarly, the modern phylogenetic definition of dinosaur was “the least inclusive clade containing Passer domesticus (the house sparrow) and Triceratops horridus“. In order to allow Brachiosaurus and his friends to continue sitting  with the dinosaurs, Baron et al. suggested to expand the definition to include Diplodocus carnegii. So, dinosaurus would be the least inclusive clade containing P. domesticusT. horridus and D. carnegii.

In this new family tree, the name Saurischia would still be used, but to refer only to the sauropodomorphs and some primitive carnivores, the herrerasaurids. The new clade formed by uniting theropods and ornithischians was proposed to be called Ornithoscelida (“bird-legged”), a name coined in 1870 to refer to the bird-like hindlimbs of both theropods and ornithopods (the subgroup of ornithischians that includes dinosaurs such as Iguanodon and the duck-billed dinosaurs).

What can we conclude with all that? Nothing will change if you are just a dinosaur enthusiast and do not care about what’s an ornithischian and a saurischian. Now if you are a phylogeny fan, as I am, you are used to sudden changes in the branches. Most fossils of basal dinosaurs are incomplete, thus increasing the problem to know how they are related to each other. Perhaps this new view will last, perhaps new evidence will change all over again the next week.

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ResearchBlogging.orgReferences and further reading:

Baron, M., Norman, D., & Barrett, P. (2017). A new hypothesis of dinosaur relationships and early dinosaur evolution Nature, 543 (7646), 501-506 DOI: 10.1038/nature21700

Naish, D. (2017). Ornithoscelida Rises: A New Family Tree for DinosaursTetrapod Zoology.

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Biological fight: kites, mites, quite bright plights

ResearchBlogging.orgby Piter Kehoma Boll

A recently described fossil from the Silurian Herefordshire Lagerstätte in the United Kingdom has called much attention.

A photo of the fossil itself. Image by Briggs et al., extracted from news.nationalgeographic.com

A photo of the fossil itself. Image by Briggs et al., extracted from news.nationalgeographic.com

The appearance of the creature was build by scanning the rock and creating a 3D reconstruction of the fossil. It revealed that the animal, obviously and arthropod, had several smaller creatures attached by long threads, like kites. The species was named Aquilonifer spinosus, meaning “spiny kite-bearer”.

A 3D reconstruction of what Aquilonifer and its kites would have looked like. Image by Briggs et al. extracted from sci-news.com

A 3D reconstruction of what Aquilonifer and its kites would have looked like. Image by Briggs et al. extracted from sci-news.com

The authors (Briggs et al., 2016) thought about three possibilities to explain the unusual “kites”. They could be parasites, phoronts (i.e., hitchhikers), or babies. The idea of parasites was discarded because such long threads separating them from the host would have made it difficult to feed properly. They also considered it unlikely to be a case of phoronts, i.e., a species that uses the host as a mean to move from one site to another, because there were too many of them and the host most likely would have removed them by using the long antennae.

Artistic impression of Aquilonifer spinosus by Andrey Atuchin.

Artistic impression of Aquilonifer spinosus by Andrey Atuchin.

The remaining option is that the kites were offspring. The mother (or father) would have attached them to itself in order do carry them around in a unique mode of brood care. The authors compare it to several other arthropod groups in which some species carry their babies around during their first days. They also consider that the animal could have delayed its molting process to avoid discarding the babies with the exoskeleton.

But can we be sure that this is the case? The entomologist Ross Piper thinks differently. He compares the kites to uropodine mites, in which the juveniles (deutonymphs) attatch themselves to beetles by long stalks in order to be transported from one food source to another. As there are marine mites, that could be the case. He also points out that the kites are scattered through the body, which would make them unlikely to be offspring, as such a distribution would only hinder the parent’s mobility.

Briggs at al. responded to Piper’s critique arguing that marine mites have only recently evolved and that Aquilonifer is very different from a terrestrial beetle. It was most likely a bentonic species, crawling on the ocean’s floor, and not a swimmer, so that it would not be a very good dispersal agent.

What do you think of it? I find it difficult to choose one side. Piper’s comparison with mites is interesting, but only as a way to suggest a convergent evolution. I cannot see how the kites would have been really mites or even arachnids. Now the argument on the kites’ position on the body is a good point. No other group of animals carries their young attached to long stalks spread all over the body. Furthermore, how would the parent properly place the juveniles there? I can only see it as a plausible way if the host were the father and the mother crawled over him to stick the eggs in place. Additionally, couldn’t they be true phoronts  that were benefitial to the host? The little fellows could benefit by moving around on the big pal and reaching new food sources while giving protection or other advantage in return. And regarding the delay in molting, I cannot see any evidence that there was any delay. We don’t know how long the kites remained there and perhaps after molting they could simply leave their little houses and build new ones on the host’s new skeleton.

We may never know the truth, but we can keep exchanging ideas.

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References:

Briggs, D., Siveter, D., Siveter, D., Sutton, M., & Legg, D. (2016). Tiny individuals attached to a new Silurian arthropod suggest a unique mode of brood care Proceedings of the National Academy of Sciences, 113 (16), 4410-4415 DOI: 10.1073/pnas.1600489113

Briggs, D., Siveter, D., Siveter, D., Sutton, M., & Legg, D. (2016). Reply to Piper: Aquilonifer’s kites are not mitesProceedings of the National Academy of Sciences, 113 (24) DOI: 10.1073/pnas.1606265113

Piper, R. (2016). Offspring or phoronts? An alternative interpretation of the “kite-runner” fossil Proceedings of the National Academy of Sciences, 113 (24) DOI: 10.1073/pnas.1605909113

Switek, B. 2016. This bizarre creature flew its babies like kites. National Geographic News. Available at < http://news.nationalgeographic.com/2016/04/160404-bizarre-creature-flew-babies-kites-arthropod-fossils-science/ >. Access on July 07, 2016.

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Biological fight: Should we bring mammoths back?

by Piter Kehoma Boll

Everybody knows the amazing large animals that are found in Africa and Southeast Asia. Elephants, giraffes, rhinos, hippos, horses, lions, tigers… such large creatures, mostly mammals, are usually called megafauna, the “large fauna”.

Mammals as big as the African bush elephant once roamed the Americas. Photo by flickr user nickmandel2006*.

Mammals as big as the African bush elephant once roamed the Americas. Photo by flickr user nickmandel2006*.

The Americas once had an astonishing megafauna too, full of mastodons, mammoths, giant sloths, giant armadillos and sabertooth tigers. Nowadays it is restricted to some bears and jaguars. What happened to the rest of them? Well, most went extinct at the end of the Pleistocene, around 11,ooo years ago.

South America once had mammals as big as an African bush elephant. Picture by Dmitry Bogdanov** (dibgd.deviantart.com)

South America once had mammals as big as an African bush elephant, such as the giant sloth. Picture by Dmitry Bogdanov** (dibgd.deviantart.com)

As humans already inhabited the Americas by this time, it was always speculated if humans had something to do with their extinction. It is true that nowadays hundreds, thousands of species are endangered due to human activities, so it is easy to think that humans are the best explanation for their extinction, but 10 thousands years ago the number of humans on the planet was thousands of times smaller than today and our technology was still very primitive, so it is unlikely that we could hunt a species to extinction by that period… if we were working alone.

No, I’m not talking about humans cooperating with aliens! Our sidekick was the famous climate change. Periods of extreme warming during the pleistocene seem to have had a strong impact on the populations of many large mammals and, with the aid of humans hunting them down and spreading like an invasive species, the poor giants perished.

Le Mammouth by Paul Jamin

Le Mammouth by Paul Jamin

This happened more than 10 thousand years ago, TEN THOUSAND YEARS.

In Africa, elephants and large carnivores are well known for their importance in structuring communities, especially due to their trophic interactions that shape other populations. The extinct American megafauna most likely had the same impact on the ecosystem. As a result, suggestions to restore this extinct megafauna has been proposed, either by cloning some of the extinct species or, more plausibly, by introduced extant species with a similar ecological role.

Svenning et al. (2015) review the subject and argue in favor of the reintroduction of megafauna to restore ecological roles lost in the Pleistocene, an idea called “Pleistocene rewilding” or “trophic rewilding”, as they prefer. They present some maps showing the current distribution of large mammals and their historical distribution in the Pleistocene, which they call “natural”. They also propose some species to be introduced to substitute the ones extinct, including replacements for species extinct as long as 30 thousand years ago. Now is this a good idea? They think it is and one of the examples used is the reintroduction of wolves in the Yellowstone National Park. But wolves were not extinct for millenia there, neither are they a different species that would replace the role of an extinct one.

A wolf pack in Yellowstone National Park

A wolf pack in Yellowstone National Park

Rubenstein & Rubenstein (2016) criticized the idea, arguing that we should focus on protecting the remaining ecosystems and not trying to restore those that were corrupted thousands of years ago. They also argue that using similar species may have unintended consequences. Svenning et al. answered that this is mere opinion and that a systematic research program on trophic rewilding should be developed. The reintroduction of horses in the New World and its non-catastrophic consequences is another point used to respond to the critiques.

So what’s your opinion? Should we bring mammoths, mastodonts, giant sloths and sabertooth tigers back? Should we introduce elephants and lions in the Americas to play the role that mastodonts and smilodonts had?

My opinion is no. The idea may seem beautiful, but I think it is actually fantastic, too fabulous and sensational. Horses may have come back to the Americas without bringing destruction, but we cannot be sure with anything, even with several theoretical and small-scale studies. We all know how often introducing species goes wrong, very wrong. Look at poor Australia and Hawaii, for instance. Furthermore, those giant mammals went extinct TEN THOUSAND YEARS AGO. Certainly ecosystems have adapted to their extinction. Life always finds a way. There are worse threats to those ecosystems to be addressed, such as their eminent destruction to build more cities and raise more cattle and crops.

Get over it. Mammoths are gone. Let’s try to save the elephants instead, but without bringing them to the Brazilian cerrado. They don’t belong there. They belong in the African savannah.

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References:

Rubenstein, D. R.; Rubenstein, D. I. From Pleistocene to trophic rewilding: A wolf in sheep’s clothing. PNAS, 113(1): E1. DOI: 10.1073/pnas.1521757113

Svenning, J-C.; Pedersen, P. B. M.; Donlan, C. J.; Ejrnæs, R.; Faurby, S.; Galetti, M.; Hansen, D. M.; Sandel, B.; Sandom, C. J.; Terborgh, J. W.; Vera, F. W. M. 2016. Science for a wilder Anthropocene: Synthesis and future directions for trophic rewilding research. PNAS, 113(4): 898-906. DOI: 10.1073/pnas.150255611

Svenning, J-C.; Pedersen, P. B. M.; Donlan, C. J.; Ejrnæs, R.; Faurby, S.; Galetti, M.; Hansen, D. M.; Sandel, B.; Sandom, C. J.; Terborgh, J. W.; Vera, F. W. M. 2016. Time to move on from ideological debates on rewilding. PNAS, 113(1): E2-E3. DOI: 10.1073/pnas.1521891113

Wade, L. 2016. Giant jaguars, colossal bears done in by deadly combo of humans and heat. Science News. DOI: 10.1126/science.aag0623

Wade, L. 2016. Humans spread through South America like an invasive species. Science News. DOI: 10.1126/science.aaf9881

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The tegu lizard and the origin of warm-blooded animals

ResearchBlogging.org by Piter Kehoma Boll

Warm blood is the popular way to refer to endothermy, the ability that certain animals have to maintain a high body temperature by the use of heat generated via metabolism, especially in internal organs. Mammals and birds are the only extant groups in which all representatives are endothermic, but some fish also have this feature.

Tunna fish are truly endoothermic fish, similar to mammals and birds.

Tunna fish are truly endothermic fish, similar to mammals and birds. Photo by opencage.info**

In order to maintain a high body temperature, endothermic animals need a much higher amount of daily food than ectothermic animals (the ones that rely on environmental sources to adjust their body heat). There must be, therefore, a considerable advantage in endothermy to explain such a increased consumption of resources. The advantages include the ability to remain active in areas of low temperature and an increase in efficienty of enzimatic reactions, muscle contractions and molecular transmission across synapses.

The origin of endothermy is still a matter of debate and several hypothesis have been erected. The main ones are:

1. A migration from ectothermy to inertial homeothermy and finally endothermy.

According to this hypothesis, animals that were initially ectothermic grew in size, becoming inertially homeothermic, i.e., they retained a considerable constant internal body temperature due to the reduced surface area in relation to the their volume. Lately, selective pressures forced those animals to reduce in size, which made them unable to sustain a constant internal temperature and therefore their enzimatic, muscular and synaptic efficiency became threatened. As a result, they were forced to develop an alternative way to maintain a high body temperature and acquired it through endothermy.

Initially considered a plausible explanation due to the body size of the ancestors of mammals in fossil record, new phylogenetic interpretations caused a complete mix of large-bodied and small-bodied animals, so that currently fossils don’t support this idea anymore.

2. A large brain heating the body

The brain in endothermic species produces much more heat than any other organs. This led to the assumption that maybe a large brain generating heat was the responsible for the later development of full endothermy. However, evidence from both exant and extinct species point to the opposite. It seems more reasonable that a large brain evolved after endothermy and not the opposite.

3. A nocturnal life needs more heat

This idea states that the development of endothermy happened as a way to allow animals to be active during the night. The fact that most primitive mammals appear to have been nocturnal seems to support this hypothesis, but in fact many extant nocturnal mammals actually have a lower body temperature than diurnal mammals. Other aspect that counts against this hypothesis is that the ancestors of mammals already showed evidences of an increase in body temperature despite the fact that they most likely were not nocturnal.

4. Heat to help the embryos to develop

As you may know, in many ectothermic vertebrates, such as reptiles, eggs need to be incubated at a constant temperature in order to develop adequately. Endothermy, therefore, could have evolved as a way to allow parents to incubate the eggs themselves and have a higher control on temperature stability. One fact that support this theory is the dual role of thyroid hormones in reproduction and in the control of metabolic rate.

Endothermy may have evolved to incubate eggs at a constant temperature.

Endothermy may have evolved to incubate eggs at a constant temperature. Photo by Bruce Tuten**

5. Aerobic capicity leading to the heating of internal organs

According to this hypothesis, endothermy evolved after the increase of aerobic capacity, i.e., the first thing to happen was to increase the ability of muscles to consume oxygen in order to release energy, which helped the animal to move faster, among other things. This increased aerobic capicity was attained by increasing the number of mitochondria in muscle cells, which led to higher body temperature in the muscules and consequently a higher visceral temperature. Despite fossils indicating that mammal ancestors developed morphological adaptations indicating increased aerobic capacity, it is not possible to afirm that endothermy was not already present in those species.

Very recently, it has been found that the tegu lizards (Salvator merianae) from South America increase their body temperature during the reproductive season, achieving as much as 10°C above the environment temperature at night. Thus, it seems that they are able to increase heat production and heat conservation in ways similar to the ones used by fully endothermic animals.

The tegu lizard Salvator merianae is a facultative endotherm.

The tegu lizard Salvator merianae is a facultative endotherm. Photo by Jami Dwyer.

As such an increase in body temperature happens during the reproductive cycle, it supports the hypothesis of endothermy evolving to assist the development of embryos, as explained above. Also, it indicates that ectotherms may engage in temporary endothermy and perhaps permanent endothermy may have evolved by using this path.

Further studies on the tegu lizards are needed to clarify this interesting phenomenon and expand our knowledge on endothermy evolution in mammals and birds.

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References:

Kemp, T. (2006). The origin of mammalian endothermy: a paradigm for the evolution of complex biological structure Zoological Journal of the Linnean Society, 147 (4), 473-488 DOI: 10.1111/j.1096-3642.2006.00226.x

Tattersall, G., Leite, C., Sanders, C., Cadena, V., Andrade, D., Abe, A., & Milsom, W. (2016). Seasonal reproductive endothermy in tegu lizards Science Advances, 2 (1) DOI: 10.1126/sciadv.1500951

Wikipedia. Endotherm. Available at: <https://en.wikipedia.org/wiki/Endotherm&gt;. Access on February 1, 2016.

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Pooping to evolve: how feces allowed us to exist

by Piter Kehoma Boll

ResearchBlogging.orgBillions of years ago, when the first lifeforms appeared on Earth, our planet was very different from what it is today. Oxygen, so essential for our survival, was not present in the atmosphere.

Thanks to the appearance of the first photosynthetic bacteria, the so-called Cyanobacteria or blue-green algae, our atmosphere started to accumulate oxygen. As you may know, photosynthesis is a process by which plants and other photosynthetic organisms convert water and carbon dioxide into oxygen and organic compounds.

Oxygen is a very reactive element, so it can easily interact with other compounds and is great to burn organic matter to release energy. Without oxygen, heterotrophic life, such as animals, would not be able to use large quantities of energy and therefore would have never been able to achieve large size.

As you may also know, animals most likely appeared in the oceans and only much later conquered the land. However, oxygen produced by photosynthesis accumulates mainly in the atmosphere and not in the oceans. Today, only 1% of the global oxygen is found in the oceans, and it was even worse during the first million years of multicellular life. Do you know why?

The most primitive animals alive today are sponges, which are quite different from other animals. They usually have a hollow body with several pores, which function as tiny mouths through which water carrying small planktonic organisms and other organic matter is pulled inwards and later released by a large opening on the top of the body. So the main thing sponges do is mixing water and extracting a small amount of organic matter from the water column. Their feces, when returning to the water, are not very different in size from the organic matter they initially ingested.

Sponges ingest organic particles and release organic particles. They are not very efficient in removing organic matter from water.

Sponges ingest organic particles and release organic particles. They are not very efficient in removing organic matter from water.

Thus, in a sponge-only world, the water column was possibly always crowded with dissolved organic matter. This was a feast for bacteria, which are always eager to decompose organic matter and, while doing so, they consume large amounts of oxygen. Therefore, water with high amounts of organic matter increases bacterial activity and turns the environment anoxic, i.e., without oxygen. As a result, there was no oxygen available to allow animals to become large.

Despite not growing very much, animals were still evolving, of course, and eventually the bilaterian animals appeared. Bilaterian animals have a bilateral symmetry and, the most important feature in this story, a gut. It means they ingest food, digest it, process it and later eliminate the rests as… poop! In the gut, feces become compact as fecal pellets and sink much quickly to the bottom of the ocean, cleaning the water column from organic matter and drastically reducing bacterial activity. With no bacteria decomposing in the water column, the oxygen levels rapidly started to increase, allowing animals to grow and things like fish to evolve.

Bilaterian animals produce compact fecal pellets which sink to the bottom, cleaning the water column.

Bilaterian animals produce compact fecal pellets which sink to the bottom, cleaning the water column.

If animals had never started to poop, we most likely would have never been able to arise in this world. Long live the poop!

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References:

Holland, H. (2006). The oxygenation of the atmosphere and oceans. Philosophical Transactions of the Royal Society B: Biological Sciences, 361 (1470), 903-915 DOI: 10.1098/rstb.2006.1838

Turner, J. T. (2002). Zooplankton fecal pellets, marine snow and
sinking phytoplankton blooms. Aquatic Microbial Ecology, 27, 57-102

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Filed under Ecology, Evolution, Paleontology