Tag Archives: phylogenetics

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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*Creative Commons License
This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.

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

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Filed under Evolution, Extinction, Paleontology, Systematics