Category Archives: Evolution

An extinct frog that is still living

by Piter Kehoma Boll

Hybrids, as you probably know, are organisms that arise from the mating of two individuals of different species. A mule, for example, is a well known hybrid between a horse and a donkey. Hybrids are usually sterile, although not all of them are, and some of them have a very peculiar way to continue to exist by using a process called hybridogenesis.

Hybrids that rely on hybridogenesis function in the following way: there are two original species, let’s call them A and B. When they copulate with each other, they produce a hybrid offspring, AB, which has half of the genes from one parent and half from the other. In “normal” hybrids, such creatures are completely sterile, unable to produce viable gametes, or can give rise to a new hybrid species by producing mixed gametes. However, in this peculiar kind of hybrids, called kleptons, things work differently.

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Pelophylax kl. hispanicus, the holder of a treasure. Photo by Andreas Thomsen.*

When kleptons are producing gametes, they never recombine the genomes of the two parents, but rather exclude the genome of one of them and produce gametes that contain the genome of the other parent. For example, the hybrid AB produces only A gametes, while the B genome is excluded. This means that if AB mates with a partner of the species A, the offspring will be formed by pure A individuals. If mating with B, the offspring will contain only new AB hybrids.

hybridogenesis_in_water_frogs

The edible frog Pelophylax kl. esculentus is a klepton formed by breeding P. lessonae and P. ridibundus. The klepton only produces gametes of P. ridibundus, eliminating the genome of P. lessonae during meiosis. (Photo by Wikimedia user Darekk2).**

This mode of reproduction is very common in frogs of the genus Pelophylax, as the example seen in the picture above. Another interesting point about kleptons is that they are usually unable to mate with another klepton. They rely one the parent species to reproduce, therefore “parasitizing” them.

A recently published paper on Pelophylax frogs reports a peculiar case in which one of the parent species is extinct. The klepton, known as Pelophylax kl. hispanicus, is the result of P. bergeri crossing with a now extinct species of Pelophylax. The case is that the gametes that P. kl. hispanicus produce are of the extinct species, but they can only fertilize gametes of P. bergeri. In other words, we could say that the extinct species is still alive inside the klepton, relying on P. bergeri to pass to the next generations.

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Pelophylax kl. hispanicus is a klepton that maintains the genome of an extinct species alive. Image extracted from Dubey & Dufresnes (2017).**

The authors suggest that perhaps we could find a way to bring the extinct species back, separated from P. bergeri. Although the result of crossing two P. kl. hispanicus is an sterile offspring, they think that continuous trials may end up revealing an eventual fertile offspring. Is it worth trying? Perhaps. But anyway, this is one more astonishing feature of nature, don’t you agree?

How many more extinct species may be living in a similar way, trapped in a hybrid?

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References:

Wikipedia. Hybridogenesis in water frogs. Available at <https://en.wikipedia.org/wiki/Hybridogenesis_in_water_frogs&gt;. Access on October 12, 2017.

Dubey, S.; Dufresnes, C. (2017) An extinct vertebrate preserved by its living hybridogenetic descendant. Scientific Reports 7: 12768. https://dx.doi.org/10.1038/s41598-017-12942-y

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Friday Fellow: Common Water Bear

by Piter Kehoma Boll

Tiny and tough, our newest Friday Fellow can be found hidden among the moss throughout most of the planet, and perhaps even beyond it, for if there is a species to which space is a piece of cake, that species is the common water bear Milnesium tardigradum.

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A Scaning Electron Microscope (SEM) image of a specimen of the common water bear in its active state. Photo extracted from Schokraie et al. (2012).*

You may have already heard of tardigrades or water bears, tiny chubby animals that are able to withstand the harsher conditions, such as intense desiccation, radiation and even the vacuum of outer space. Most of the data regarding the toughness of these organisms comes from the common water bear, the most widespread species of the phylum Tardigrada.

Measuring up to 0.7 mm in length, the common water bear has eight legs with claws on their end and is considered a predator, feeding on a variety of other small organisms, including algae, rotifers and nematodes. It has a worldwide distribution and is commonly found living on moss, such as the cosmopolitan silvergreen moss already presented here.

As members of the supergroup Ecdysozoa (which also includes arthropods and roundworms), tardigrades undergo ecdysis, also commonly known as molting, a process through which they shed their exoskeleton. In the common water bear, females always lay eggs around the time of molting. Before leaving the old exoskeleton, the females lay the clutch of eggs, which may vary from 1 to 12 eggs, between the old and the new exoskeleton and usually remain inside the old exoskeleton several hours after laying the eggs. When they finally leave, the eggs remain inside the shed skin, which perhaps helps them to be more protected from danger.

Milnesium_tardigradum

A clutch of seven eggs is left in the empty exoskeleton while the female leaves. Photo by Carolina Biological Supply Company.**

When the habitat of the common water bear gets dry, it enters in a state called cryptobiosis, in which the body shrinks and the metabolism stops. Under this state, known as tun, it can withstand high doses of radiation and both high and zero air pressure, surviving even in the environment of outer space. It is not invincible, however. Radiation in doses above 1000 Gy may not always kill them, but always let them sterile, which is, evolutionary, basically the same thing.

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SEM image of the common water bear in the tun state. Photo extracted from Schokraie et al. (2012).*

Nevertheless, the American cockroach is just an amateur regarding survival when compared to the common water bear.

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References:

Horikawa, D. D.; Sakashita, T.; Katagiri, C.; et al. (2009) Radiation tolerance in the tardigrade Milnesium tardigradumInternatonal Journal of Radiation Biology, 86(12): 843–848. http://dx.doi.org/10.1080/09553000600972956

Schokraie E, Warnken U, Hotz-Wagenblatt A, Grohme MA, Hengherr S, et al. (2012) Comparative proteome analysis of Milnesium tardigradum in early embryonic state versus adults in active and anhydrobiotic state. PLoS ONE 7(9): e45682. https://dx.doi.org/10.1371/journal.pone.0045682

Suzuki, A. C. (2003) Life history of Milnesium tardigradum Doyère (Tardigrada) under a rearing environment. Zoological Science 20(1): 49–57. https://doi.org/10.2108/zsj.20.49

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The bat folk songs: cultural evolution in our winged relatives

by Piter Kehoma Boll

For a long time, culture was considered a human trait, but nowadays we recognize the existence of culture in many other species, such as other primates, whales and some birds too. Now there are some evidences of culture being found in bats too.

A group of researchers from China studied the calls of the Chinese rufous horseshoe bat (Rhinolophus sinicus) across different populations and compared them to genetic and environmental variables to determine whether the differences where linked to genetic differences between the populations or to different environments that would force the bats to change their calls in order to use them more successfully.

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The smile of a cult bat (Rhinolophus sinicus). Photo by Ecohealth Alliance, extracted from Eureka Alert.

The results indicate that none of those two factors were strongly linked to the acoustic differences in the calls. The most likely explanation is that the differences happen due to cultural drift. The bats are teaching a way to speak to their children that is slightly different from what their neighbors speak, even if the neighbors are genetically similar and live in a similar environment.

As an animal’s call is an important variable during mating, this may eventually lead to reproductive isolation even without genetic differences. Culture can also shape evolution!

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Reference:

Xie, L.; Sun, K.; Jiang, T.; Liu, S.; Lu, G.; Jin, L.; Feng, J. (2017) The effects of cultural drift on geographic variation in echolocation calls of the Chinese rufous horseshoe bat (Rhinolophus sinicus)Ethology 123(8): 532-541.

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How do new species form?

by Piter Kehoma Boll

A long, long time ago, I wrote two posts here about the definition of species, explaining briefly the most important horizontal and vertical species concepts. So we all agree that species exists, but how they emerge? How one species become two, or how one species become another?

The phenomenon by which it occurs is called speciation. Well, sort of… It all depends on how you define a species, actually (so be certain to have read the posts I mentioned above).

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Model of a lineage splitting into two lineages that evolve independently and eventually become separated species. Extracted from Hawlitschek et al. (2012)*

Speciation is usually defined as the evolution of reproductive isolation, therefore it deals more with the concept of biological species, but also with the ecological concept and certainly needs some insights on the vertical concepts. If two populations are reproductively isolated, it means that the individuals of one of them are unable or unwilling to breed with those of the other. This usually arrives through genetic and ecological differences that lead to differences in behavior, morphology, physiology. And considering that, we can classify reproductive isolation into two groups: pre-zygotic and post-zygotic isolation.

In pre-zygotic isolation, the two species are reproductively isolated because they do not want or cannot mate and produce an zygote. This may happen simply because of different behaviors in which the two species occupy different places in the environment, mate at different times of the year or even because they are not sexually attracted to each other. There are several experiments using fruitflies that demonstrate how this may evolve pretty fast.

In the late 1980s, William R. Rice and George W. Salt separated individuals of Drosophila melanogaster depending on their preference for dark × light and wet × dry environments, allowing them to mate only with other specimens showing the same preferences. After several generations, the individuals of one group were unable to mate with those of other groups because of their strong habitat preferences, making them unlikely to interact. A similar experiment was performed by Diane Dodd using the species Drosophila pseudoobscura, in which one population was raised with starch as food and other with maltose as food. In this case, after several generations the flies showed a strong preference to mate with individuals of the same group and to reject those of the other group.

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Evolution of reproductive isolation in fruit flies of the species Drosophila pseudoobscura after several generations fed with different sugars.

Such speciation events are called ecological speciation and are also well-documented in the widl, especially regarding fish preferring different habitats, such as shallow × deep water or still × running water. Eventually the individuals will diverge into two groups that are ecologically isolated in the same environment and consequently become reproductively isolated as well.

Post-zygotic isolation is generally a more advanced form of isolation that indicates deep genetic divergences. This is more commonly associated with the notion of biological species and is based on the inability of the individuals of the two species to produce viable offspring. They may mate with each other and even produce a zygote, but this will be unable to developed into an embryo or the offspring will be sterile or otherwise unable to survive enough to breed. A classical example is the mule, the hybrid of a mare and a donkey that is usully sterile.

Equus

A mare, Equus ferus caballus (left), a donkey, Equus africanus asinus (right) and a mule (center). Photos by ‘Little Miss Muffit’ (flickr.com/people/42562654@N00)(mare), Adrian Pingstone (donkey) and Dario Urruty (mule).

In both forms of speciation mentioned above, reproductive isolation usually arises from the accumulation of small differences due to natural selection. This may be enhanced by two phenomena, pleiotropy and genetic hitchhiking.

Pleiotropy is the phenomen by which a single gene have influence over more than one phenotypic trait. For example, a gene that influences the shape of a bird’s bill may also make it change its diet and its song. Several human genetic diseases, such as phenylketonuria (PKU), are examples of pleiotropy.

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The frizzled trait in chickens, which makes the feather curl outward, also leads to delayed sexual maturity and decreased metabolism rate. Photo by flickr user Just chaos.*

Genetic hitchhiking, on the other hand, is the phenomenon by which a gene that is naturally selected carries neighbours genes that are in the same DNA chain with it. In fruitflies, for example, a gene that is linked to courtship behavior may be drawn with the gene linked to a digestive enzyme, so that flies that specialize in one kind of sugar have a different courtship behavior than others specialized in another sugar.

That’s all for now. In a future post, I’ll talk about the geographic and genetic variables in species formation.

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References and further reading:

Bolnick, D. I., Snowberg, L. K., Patenia, C., Stutz, W. E., Ingram, T. & Lau, O. L. 2009. Phenotype-dependent native habitat preference
facilitates divergence between parapatric lake and stream stickleback. Evolution, 63(8): 2004-2016.

Hendry, A. P.2009. Ecological speciation! Or the lack thereof? Canadian Journal of Fisheries and Aquatic Sciences, 66: 1383-1398.

Hoskin, C. J. & Higgie, M. 2010. Speciation via species interactions: the divergence of mating traits within species. Ecology Letters, 13: 409-420.

Maan, M. E., Hofker, K. D., van Alphen, J. J. M. & Seehausen, O. 2006. Sensory drive in cichlid speciation. The American Naturalist, 167(6):
947-954.

Nosil, P. 2008. A century of evolution: Ernst Mayr (1904-2005). Ernst Mayr and the integration of geographic and ecological factors in
speciation. Biological Journal of the Linnean Society, 95: 26-46.

Turelli, M., Barton, N. H. & Coyne, J. A. 2001. Theory and speciation. TRENDS in Ecology and Evolution, 16(7): 330-343.

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Going a long way with your mouth open to new tastes

by Piter Kehoma Boll

Everybody knows that human activities have driven our environment toward an unfortunate situation. The most popular forms of human impact include pollution, deforestation and overexploitation of natural resources, but certainly an important factor in remodeling ecosystems is the invasion of species.

While humans move around the world, they carry many species with them, either intentionally or not, an some of them establish successfully in the new environment, while others do not. But what makes some species become successful invaders while other are unable to do so?

It is clear for some time that having a broad niche, i.e., a broad tolerance in environmental conditions and a broad use of resources is very important to succeed in invading a new habitat. Food niche breadth, i.e., the amount of different food types one can ingest, is among the most important dimensions of the niche influencing the spread of a species.

I myself studied the food niche breadth of six Neotropical land planarians in my master’s thesis (see references below) and it was clear that the species with the broader niche are more likely to become invasive. Actually, the one with the broadest food niche, Obama nungara, is already an invader in Europe, as I already discussed here.

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A specimen of Obama nungara from Southern Brazil that I used in my research. Photo by myself, Piter Kehoma Boll.*

But O. nungara has a broad food niche in its native range, which includes southern Brazil, and likely reflected this breadth in Europe. But could a species that has a narrow food niche in its native range broaden it in a new environment?

A recent study by Courant et al. (see references) investigated the diet of the African clawed frog, Xenopus laevis, that is an invasive species in many parts of the world. They compared its diet in its native range in South Africa whith that in several populations in other countries (United States, Wales, Chile, Portugal and France).

Xenopus_laevis

The African clawed frog Xenopus laevis. Photo by Brian Gratwicke.**

The results indicated that X. laevis has a considerable broad niche in both its native and non-native ranges, but the diet in Portugal showed a greater shift compared to that in other areas, which indicates a great ability to adapt to new situations. In fact, the population from Portugal lives in running water, while in all other places this species prefers still water.

We can conclude that part of the success of the African clawed frog when invading new habitats is linked to its ability to try new tastes, broadening its food niche beyond that from its original populations. The situation in Portugal, including a different environment and a different diet, may also be the result of an increased selective pressure and perhaps the chances are that this population will change into a new species sooner than the others.

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References:
Boll PK & Leal-Zanchet AM (2016). Preference for different prey allows the coexistence of several land planarians in areas of the Atlantic Forest. Zoology 119: 162–168.

Courant J, Vogt S, Marques R, Measey J, Secondi J, Rebelo R, Villiers AD, Ihlow F, Busschere CD, Backeljau T, Rödder D, & Herrel A (2017). Are invasive populations characterized by a broader diet than native populations? PeerJ 5: e3250.

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Who came first? The comb or the sponge?

by Piter Kehoma Boll

The endless question is here again, but this time it appears to be settled. What animal group is the earliest of all? Who came first?

It is clear that there are five animal lineages that are usually regarded as monophyletic: sponges, placozoans, comb jellies, cnidarians and bilaterians. Let’s take a brief look at each of them:

Sponges (phylum Porifera) are always sessile, i.e., they do not move and are fixed to the substrate. They have a very simple anatomical structure. Their body is consisted of a kind of tube, having a large internal cavity and two layers of cells, an outer one and an inner one around the cavity. There are several small openings connecting the cavity to the outside, called pores, and one or more large cavities, called oscula (singular: osculum). Between the two cell layers there is a jelly-like mesohyl containing unspecialized cells, as well as the skeleton structures, including fibers of spongine and spicules of calcium carbonate or silica. Some species also secrete an outer calcium carbonate skeleton over which the organic part grows. Sponges lack muscles, nervous system, excretory system or any other kind of system. They simply live by beating the flagella of the choanocytes (the cells of the inner layer), creating a water flow entering through the pores and exiting through the osculum. The choanocytes capture organic particles in the water and ingest them by phagocytosis. All sponge cells can change from one type to another and migrate from one layer to another, so there are no true tissues.

porifera_body_structures_01

Body structures found in sponges. Picture by Philip Chalmers.*

Placozoans (phylum Placozoa) are even simpler than sponges, but they actually have true tissues. They are flat amoeboid organisms with two layers of epithelium, one dorsal and one ventral, and a thin layer of stellate cells. The ventral cell layer is slightly concave and appears to be homologous to the endoderm (the “gut” layer) of other animals, while the upper layer is homologous to the ectoderm (the “skin” layer).

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Trichoplax adhaerens, the only species currently in the phylum Placozoa. Photo by Bernd Schierwater.**

Comb jellies (phylum Ctenophora) resemble jellyfishes, but a closer look reveals many differences. Externally they have an epidermis composed by two layers, an outer one that contains sensory cells, mucus-secreting cells and some specialized cells, like colloblasts that help capturing prey and cells containing multiple cilia used in locomotion, and an inner layer with a nerve net and muscle-like cells. They have a true mouth that leads to a pharynx and a stomach. From the stomach, a system os channels distribute the nutrients along the body. Opposite to the mouth there is a small anal pore that may excrete small unwanted particles, although most of the rejected material is expelled through the mouth. There is a layer of jelly-like material (mesoglea) between the gut and the epidermis.

bathocyroe_fosteri

The comb jelly Bathocyroe fosteri.

Cnidarians (phylum Cnidaria) have a structure similar to comb jellies, but not as complex. They also have an outer epidermis, but this is composed by a single layer of cells, and a sac-like gut surrounded by epthelial cells (gastrodermis), as well as a mesoglea between the two. Around the mouth there is one or two sets of tentacles. The most distinguishing feature of cnidarians is the presence of harpoon-like nettle cells, the cnidocytes, which are used as a defense mechanism and to help subdue prey.

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Body structure of a cnidarian (jellyfish). Picture by Mariana Ruiz Villarreal.

Bilaterians (clade Bilateria) includes all other animals. They are far more complex and are characterized by a bilateral body, cephalization (they have heads) and three main cell layers, the ectoderm, which originates the epidermis and the nervous system, the mesoderm, which give rise to muscles and blood cells, and the endoderm, which develops into the digestive and endocrine systems.

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Basic bilaterian structure.

Traditionally, sponges were always seen as the most primitive animals due to their lack of true tissues, muscular cells, nervous cells and all that stuff. However, some recent molecular studies have put the comb jellies as the most primitive animals. This was highly unexpected, as comb jellies are far more complex than sponges and placozoans, which would suggest that muscles and a nervous system evolved twice in the animal kingdom, or that sponges are some weird simplification of a more complex ancestor, which would be very hard to explain. The nervous system of comb jellies is indeed quite unusual, but not so much that it needs an independent origin.

However, now things appear to be settled. A study published this month on Current Biology by Simion et al. reconstructed a phylogenetic tree using 1719 genes of 97 animal species, and applying new and more congruent methods. With this more refined dataset, they recovered the classical reconstruction that puts sponges at the base of the animal tree, a more plausible scenario after all.

But why other studies have found comb jellies as the most basal group? Well, it seeems that comb jellies have unusually high substitution rates, meaning that their genes evolve faster. This leads to a problem called “long branch attraction” in phylogenetic reconstructions. As DNA has only four different nucleobases, namely adenine, guanine, cytosine and thymine, each one can only mutate into one of the other three. When mutations occur very often, they may go back to what they were in long lost ancestor, leading to misinterpretations in the evolutionary relationships. That seems to be what happens with comb jellies.

So, it seems that after all the sponge indeed came first.

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References and further reading:

Borowiec ML, Lee EK, Chiu JC, & Plachetzki DC 2015. Extracting phylogenetic signal and accounting for bias in whole-genome data sets supports the Ctenophora as sister to remaining Metazoa. BMC Genomics 16: 987. DOI: 10.1186/s12864-015-2146-4

Littlewood DTJ 2017. Animal Evolution: Last Word on Sponges-First? Current Biology 27: R259–R261. DOI: 10.1016/j.cub.2017.02.042

Simion P, Philippe H, Baurain D, Jager M, Richter DJ, Di Franco A, Roure B, Satoh N, Quéinnec É, Ereskovsky A, Lapébie P, Corre E, Delsuc F, King N, Wörheide G, & Manuel M 2017. A Large and Consistent Phylogenomic Dataset Supports Sponges as the Sister Group to All Other Animals. Current Biology 27: 958–967. DOI: 10.1016/j.cub.2017.02.031

Wallberg A, Thollesson M, Farris JS, & Jondelius U 2004. The phylogenetic position of the comb jellies (Ctenophora) and the importance of taxonomic sampling. Cladistics 20: 558–578. DOI: 10.1111/j.1096-0031.2004.00041.x
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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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Filed under Evolution, Extinction, Paleontology, Systematics