SexyPlac: love in the time of placoderms.

At the beginning of last week, fossil fish made a rare appearance in the news. Although I secretly cling onto the futile hope that this was because the world just can’t get enough of placoderms, it was probably actually because the story was about sex – dirty, Devonian sex – with a Nature paper from Long et al. revealing the earliest evidence of vertebrate internal fertilisation in the appropriately named fish Microbrachius dicki. Microbrachius (first described in 1888 by the palaeoicthyological machine Ramsay Traquair) is a placoderm from the Devonian (about 419-359 Mya) of Scotland, and said evidence comprises pelvic claspers, similar to those used for internal fertilisation in modern sharks. While the traditional view of the evolution of internal fertilisation is that it evolved from external fertilisation, Long et al. argue that Microbrachius shows that it was in fact the other way around: the modern spawning of many bony fish is derived from an internally fertilising ancestral state. Either way Microbrachius adds an interesting chapter to the story of the placoderms.

Placoderms

Phylogeny showing the various placoderm morphophologies (note that here they’re monophyletic, and sister group to extant jawed vertebrates). (From Palaeos.com)

Placoderms were a group of heavily-armoured jawed fish from the Devonian with a diverse range of morphologies; these included flattened ray-analogues such as the rhenanids, the holocephalan-like ptyctodontids, and the more conventionally fish-shaped arthrodires, which included massive predators such as Dunkleosteus. Antiarchs, the group to which our hero Microbrachius belonged, have a particularly weird bodyplan that led to them initially being described as arthropods: a box-like, plated body with jointed, armoured pectoral fins and closely-set eyes. The phylogenetic relationships of placoderms are still poorly understood, but they lie somewhere on the gnathostome stem as the closest group to modern jawed fish. Historically they have been thought of as a monophyletic group, ie. comprising all of the descendents of a single common ancestor. More recent finds, such as Entelognathus – a placoderm with osteichthyan-like facial bones – suggest they may form a paraphyly, with some groups and taxa (eg. Entelognathus) more closely related to modern gnathostomes than others.

Phylo

Diagram illustrating (a) monophyletic placoderms, where placoderms are all descendants of a single common ancestor, and the sister group to living jawed vertebrates, and (b) paraphyletic placoderms, where some groups of placoderm are more closely related to living gnathostomes than others.

Many aspects of placoderm anatomy are of great interest to palaeontologists due to the information they provide about the ancestral state for vertebrates, and those pertaining to their sex-life are no different. In addition to live birth in the ptyctodontid placoderms (Materpiscis), structures described as pelvic claspers have been described in both ptyctodontid (Austroptyctodus) and arthrodire (Incisoscutum) placoderms. The closest analogue to these claspers in modern vertebrates is the pelvic claspers of cartilaginous fish. Male sharks and rays have an extension of the pelvic fin pointing backwards, which is used to channel sperm into the female whilst mating, while male holocephalans go one further and have two claspers in addition to their pelvic ones: one anterior pair further forward on their body, and one single clasper on the midline of their head. While both placoderm and chondrichthyan claspers are grooved structures and are found in roughly the same place, they have important differences, which suggest they aren’t actually homologous: the claspers of sharks are made of cartilage and attached to the pelvic fin, whereas those of placoderms were made of dermal bone (like their armour) and were completely separate from any other structure.

Sexyplac male2

a-c are the pelvic clasper (with reconstruction) of male Microbrachius. d-g are claspers in other placoderms, and h-i show the fossil Microbrachius with claspers in situ. From Long et al.

The claspers of male Microbrachius are like those of other placoderms in that they are grooved dermal plates separate from the pelvic fin. Like the claspers of ptyctodonts, those of Microbrachius are hook-shaped (arthrodire claspers are thin rods), and would have been immobile. Female Microbrachius have also been described with blade-like plates in the same region, with ornamentation (ie. bumps and ridges) on the surface that would have faced inwards. The authors interpret these as genital plates, which would have received and gripped the males’ claspers whilstmating. With this interpretation, the presence of claspers can be used to infer the presence of internal fertilisation, reconstructed by the authors as a form of side-by-side copulation apparently akin to square dancing.

Sexyplac female2

Female Microbrachius with genital plates labelled (from Long et al.)

This is all very well, but if we already know that claspers are widespread in both placoderms and modern sharks, why do we care about Microbrachius? One reason is that in a scenario of placoderm paraphyly (ie. where the various groups are arrayed along the gnathostome stem), antiarchs are likely to be the most rootwards group, ie. least closely related to living gnathostome. Long et al. argue that if placoderms are paraphyletic then internal fertilisation, present in chondrichthyans and in multiple placoderm groups including the most ‘primitive’, is likely to be the ancestral state of jawed vertebrates. This would in turn mean that the spawning behaviour of today’s bony fish and amphibians, where females lay eggs and the males fertilise them externally, is actually secondarily derived from internal fertilisation, completely changing how we understand the evolution of internal fertilisation in vertebrates.

Sexyplac phylo

Figure showing the presence and absence of pelvic claspers through (paraphyletic here) placoderms and other vertebrates. (From Long et al.)

While this would be an exciting, paradigm-shifting set of circumstances,it remains far from firmly established. One issue, which the authors acknowledge, is that placoderm paraphyly is far from definite. While it has been repeatedly found to be the case in various recent analyses, the various taxa and groups tend to switch position a lot between analyses suggesting that the result isn’t very stable. If placoderms, or even just placoderm groups with claspers, were monophyletic then internal fertilisation with pelvic claspers could just be a synapomorphy (uniting character) of this group, derived independently from that of chondrichthyans. It’s also possible that the various placoderm claspers aren’t homologous at all. Structurally they appear (to my untrained eye, at least) quite different and in some antiarch taxa, such as Bothriolepis, no direct evidence of claspers has been found despite there being thousands of known specimens. If the claspers aren’t homologous, it doesn’t actually mean that the internal fertilisation isn’t, but it would reduce the argument’s weight somewhat.

Sexyplac

Microbrachius apparently loves an audience. Mandatory listening. (From Long et al.)

Whatever the answer, this new description of Microbrachius offers an intriguing glimpse into the sex life of a 400 million-year-old fish. Its significance will ultimately come down to the resolution of placoderm phylogenetic relationships, as well as a better understanding of the homology of the claspers in various placoderm taxa. At the very least we can be thankful for the serendipity of the first fish known to have sex coincidentally having the species name dicki.

 

References

Long et al. (2014) Copulation in antiarch placoderms and the origin of gnathostome internal fertilisation. Nature.

Taxon of the week: Euphanerops

Having been treated to a dinosaur last time in taxon of the week, this week I’ve decided to go for an obscure Palaeozoic fish.  This fish is pretty obscure even for a Palaeozoic fish; so obscure, in fact, that it doesn’t even have its own Wikipedia article.  I speak, as you’ll know if you bothered to follow that link, of the Devonian jawless fish Euphanerops.

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Euphanerops in all its fossilised glory (Sansom et al, 2013)

Loathe as I am to introduce even more palaeontological nudity to The Dinosirs, Euphanerops is a so-called ‘naked anaspid’, a group that look similar, but not the same, as modern lampreys.  The naked anaspids consist of a handful of taxa that are united by the fact that they look a bit like members of a larger group, the anaspids, but don’t have their distinctive bony scales.  Both groups look like they lie somewhere on the gnathostome stem-lineage, which we met when we looked at heterostracans a few weeks ago.  As we saw then, these stem-gnathostome groups are extremely important in understanding vertebrate evolution as they provide our only morphological information on how characters like jaws and paired fins evolved in vertebrates.

miguasha.ca Euphanerops

A lamprey-like reconstruction of Euphanerops. Ten points if you can spot the mistake before getting to the end of the post. (From miguahsha.ca)

The specific area onto which Euphanerops sheds light is the evolution of paired fins.  Jawed vertebrates today all possess paired fins, whether as traditional ‘fins’ or as limbs in tetrapods (although some, like snakes, have secondarily lost them).  Modern jawless vertebrates, the lampreys and the hagfish, have no paired fins, they have only single ‘median’ fins down their midline.  Unfortunately this gives us no information regarding how paired fins evolved, for all we know they may have just popped into existence.  One avenue of investigation in solving the puzzle is evolutionary developmental studies (evo-devo), where the development of modern animals is studied for clues.  This gives us intriguing hints, for example paired fins seem to develop in similar ways to both gill arches and median fins, suggesting a possible shared evolutionary origin.  They can also be induced to develop along a lateral line along the sides of the body.   However, if we want to know how the structure changed morphologically we have but one recourse.  To the fossil record!

SansomPhyloOrig

A cartoon phylogeny of vertebrates with handy colour-coded fins. Green for dorsal fins, blue for anal fins and red for paired fins. Mixes arise from combinations (from Sansom et al, 2013).

Fossils clear up the picture quite a lot.  The earliest ‘paired fins’ we see are in groups like the thelodonts and the (clothed) anaspids, but these are all paired ‘fin-folds’, fin-like structures that lie paired along the body in vaguely the same place as pectoral fins, but which have no skeleton supporting them. Some heterostracans, such as Pteraspisalso have bony paired fin-like projections from their head-shields, but it looks like these are just convergent adaptations rather than actually being fins. Osteostracans, a group with a wide array of fancy head shields, have the first paired fins as we know them today, with a skeleton on the inside (like your limb bones).  They only had pectoral paired fins (ie. the front fins of a fish, or your arms), and the skeleton was made of cartilage, but they were paired fins nonetheless, although some groups of osteostracan secondarily lost them again.  Later, in jawed groups such as the placoderms, we finally see the full complement, of pectoral and pelvic paired fins.

anaspid fin folds wikipedia

Anaspids (of the clothed variety) with fin-folds coloured in green (from Wikipedia).

Combined with the molecular evidence this progression from no paired fins to paired fin-folds to paired fins with skeleton has been used to propose an evolutionary mechanism.  The story runs that lampreys and hagfish have a single stripe of ‘fin-competency’ down their midline, where fins can develop.  This stripe was somehow double and spread onto the sides of groups such as anaspids, allowing paired fin-folds to develop on the sides as well.  These paired ‘fin-competency’ zones then began to interact with the developmental tissue that bone is formed from, leading to skeletonisation of the fin in later groups, and eventually to the complex fin skeletons we see today.

Yonei-Tamura

This figure shows the proposed spread of a line of ‘fin-competency’ first from the midline onto the flanks to create ‘fin-flaps’ in various places, and then interacting with other developmental tissues to create the familiar skeletonised fins we see today at the bottom (from Yonei-Tamura et al, 2008)

This seems like a good explanation, and fits the evidence well.  Euphanerops makes the picture a bit more complicated however.  The genus has been known for a long time, and is also interesting in that some fossils appear to preserve a lamprey-like gill skeleton, which would imply that this is ancestral for gnathostomes.  Our interests lie however in the realm of paired fins, and in 2013, a fossil was described as having paired fins.  In itself this is not unusual, as the similar anaspids sometimes have paired fins in a position roughly where pectoral fins are in modern fish.  Euphanerops however has paired fins where the anal fin is in a modern fish, ie. between the anus and the tail.  This state of affairs is not known in any modern fish, or in any other extinct ones we know of.

Paired fin comparison

The paired anal fins of Euphanerops fossilised above and outline below (from Sansom et al, 2013)

This has interesting implications for the evolution of paired fins.  It seems that rather than just following a progression from no fins to paired fins, evolution had a period of ‘experimentation’ with paired fins in different places before settling on the paired fins we see today.  This fits in with our overall picture of evolution: while it can be tempting to think that it ‘progresses’ towards an aim, in fact it occurs piecemeal, leaving behind a mosaic of different ‘experiments’ with morphologies.  It’s also worth noting that lines of fin competency idea of fin evolution mentioned before isn’t overturned by Euphanerops and its paired fins, but our picture of paired fin evolution is broadened.

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Euphanerops restored with paired anal fins! (Sansom et al 2013)

So, far from being merely an unclothed anaspid and a Twitter handle that rolls off the tongue, Euphanerops and its paired fins have much to tell us about both paired fin evolution and wider evolutionary principles.  Like Limusaurus last week it also illustrates how palaeontological and molecular methods can be used in a synthesis to better understand evolution.   Pretty impressive all round for a fish that not even Wikipedia loves*.

*But I love you Euphanerops!.

 

References

Sansom et al (2013) Unusual anal fin in a Devonian jawless vertebrate reveals complex origins of paired appendages.

Janvier et al (2006) Lamprey-like gills in a gnathostome-related Devonian jawless vertebrate.

Yonei-Tamura et al (2008) Competent stripes for diverse positions of limbs/fins in gnathostome embryos.

Johansen (2010) Evolution of paired fins and the lateral somatic frontier.

 

What’s new/s 03/03/14

What’s news has been neglected a bit over the last few weeks, but fear not! Here are five exciting palaeo news stories from the last month or so:

Placoderm faces

For no particularly good reason, placoderms are a group close to my heart.  This group of fish existed during the Devonian, and are the earliest example of vertebrate with jaws in the fossil record,  As well as being generally awesome, they are also crucial to understanding how gnathostomes, the jawed vertebrates, evolved from their jawless ancestors.  Dupret et al have CT imaged a primitive placoderm, Romundina, and have shown that it has a mixture of jawless fish (‘agnathan’) and gnathostome  cranial  characters.  As with so much of evolution, the transition to attaining jaws seems to have been piecemeal, part by part, an example of mosaic evolution, with cerebral proportions remaining largely unchanged with the evolution of jaws.

Romundina

Romundina in all its glory. This is the head with front end to the left, the large, round holes are the orbits. (from phys.org)

Even more Burgess Shale

The Cambrian is famous for the weird and wonderful fossil taxa that existed during it, and the most famous Cambrian fossil site is the Burgess Shale, in the Canadian Rockies.  Since the early 20th century the incredible preservation of this site has given us an invaluable window into life 500 million years ago.  While the Burgess Shale animals were famously argued by Stephen Jay Gould to represent experimentations in body plans alien to anything alive today, more recent work has shown that actually these animals are early relatives of groups such as arthropods (ie. insects, crustaceans etc) and vertebrates (ie. you).  The other week another site was described from the Burgess Shale; hopefully this will mean we’ll see lots of new Cambrian beasties over the next few years, as well as more information on known ones,  continuing to fill in our picture of early animal evolution.

Burgessnew

A range of weird and wonderful Cambrian fossils from the new assemblage. (from Caron et al)

Aquatic Acanthostega

We already met Acanthostega, the earliest known tetrapod, a few weeks ago, when we were looking at Tiktaalik and the transition of vertebrates onto land.   Acanthostega possesses a mixture of aquatic and terrestrial qualities: like a fish, it has gills and a tail fin, but like a modern tetrapod it had digits and a ‘neck’ between the head and body.  This has been taken to demonstrate that tetrapods actually evolve many ‘adaptations to land’, such as digits, while the still lived in water, in contrast to the historical view that a (presumably rather optimistic) fish crawled onto land and only then adapted to it. Recent work by Neenan et al suggests that Acanthostega’s jaw was adapted to aquatic, rather than terrestrial, feeding, which they argue refutes recent suggestions that it might have fed on land, or at least above water.

Optimistic fish

An optimistic fish (from Pixar.wikia.com)

Playing around with flight

Imagine, if you will, a bird.  Chances are that you’re imagining something that’s probably feathered and beaked*.  The fossil record, however, shows us that many traits that we associate with birds, such as feathers and beaks, evolved before birds came into existence as a group.  Feathers are perhaps the best example, found in many dinosaurs not particularly closely related to birds, but it is also true of various skeletal characters.  Work by Mark Puttick and others shows that this is the case with the comparatively long forelimbs of birds, which originated before the origins of the group, amongst earlier dinosaurs.  Gliding dinosaurs like Microraptor suggest that many groups were independently evolutionarily ‘playing around’ with feathered gliding, it was just the birds that happened to evolve powered flight and make it through to today.

*If not, you may want to double check that you know what a bird is.

Microraptor

Microraptor: Evolutionary experimentation with flight? (from nhm.co.uk)

Primitive live birth in ichthyosaurs

As we’ve seen before on The Dinosirs, ichthyosaurs were a very successful group of Mesozoic reptiles, highly adapted to an aquatic life.  One oft quoted adaptation viviparity, or live birth, as a fully aquatic amniote can’t get back onto land to lay eggs.  This is also seen in other groups of marine reptile, such as plesiosaurs and mosasaurs.  However, a recent paper by Motani and others has suggested that actually live birth was present in ichthyosaurs’ terrestrial ancestors before they became aquatic.  They argue this based on a fossil of a very early ichthyosaur, Chaohusaurus, which appears to have been fossilised giving birth.  While this is fairly common in ichthyosaur fossils, all previous fossils have been found giving birth to babies tail first as in modern day whales, possibly as an adaptation to prevent suffocation.  This fossil demonstrates that Chaohusaurus  gave birth head first, as in most terrestrial vertebrates.  Motani et al argue that this demonstrates that ichthyosaurs evolved from terrestrial viviparous ‘head first’ ancestors, only later switching round as an adaptation to marine life as in whales today.  This is contrary to the traditional view, but does fit with our picture of modern reptiles: many groups of lizard are viviparous.

Viviparity icthyo

Chaohusaurus.: green, red and blue below are the ribs, paddle and tail of the mother respectively. Small ichthyosaur can be seen com in out headfirst in yellow. (from Motani et al)

 

References

Dupret et al (2014) A primitive placoderms sheds light on the origin of the jawed vertebrate face, Nature

Caron et al (2014) A new phyllopod bed-like assemblage from the Burgess Shale of the Canadian Rockies, Nature communications

Neenan et al (2014) Feeding biomechanics in Acanthostega and across the fish-tetrapod transition, Proc. Roy. Soc. B

Puttick et al (2014) High rates of evolution preceded the origin of birds, Evolution

Motani et al (2014) Terrestrial origin of viviparity in Mesozoic marine reptiles indicated by Early Triassic embryonic fossils, PLOS One

TOTW: Pteraspis

In the spirit of Ryan’s last TOTW, this week we will be looking at a member of the group of animals that my current work focuses on: Pteraspis, a heterostracan from the Early Devonian period.

The heterostracans were a group of armoured jawless fish, or ‘ostracoderm’, which lived in both saltwater and freshwater environments from the Ordovician to the Devonian period.  They were notable for their characteristic armour, with which they evolved a wide range of forms during their existence: some encased in box-like plates, others in smaller tesserae, some with flattened bodies, others with bizarre pointy nasal structures.  I’ll probably be subjecting you to a post on heterostracan morphological diversity later on, so don’t worry too much about taking notes.

Heterostraci1

Various types of heterostracan. Pteraspis is in the bottom right. (from Wikipedia)

Heterostracans are important to palaeobiology because, despite being jawless themselves, they lie somewhere on the ‘stem’ of the phylogenetic tree of the gnathostomes, or jawed vertebrates.  In the phylogeny below, the only two living clades are the gnathostomes (jawed fish, including us), which have jaws, paired fins and a bony skeleton, and the cyclostomes (hagfish and lampreys), which have neither jaws, paired fins nor bone.  This means that fossils are the only way we have of learning about how these characters evolved.  Heterostracans themselves are particularly interesting due to their bone, one of the first occurrences of this tissue amongst vertebrates.

Gnath phylo

A cladogram of vertebrate relationships. Note how all stem gnathostomes are jawless . Extinct groups marked with a cross. (from Purnell, 2002)

Enter our hero, Pteraspis.  This 20cm long animal is pretty famous as heterostracans go, and can usually be found swimming his way through any marine Devonian diorama (there’s a particularly good one in the National Museum of Scotland, Edinburgh).  The most striking thing about Pteraspis initially is its large bony head shield.  This is characteristic of all heterostracans, although the types of plate forming it and the shape vary dependent on the species.  Characters peculiar to Pteraspis and related genera are the long rostral (ie. nose) section and the large dorsal spine.  Also worth noting are the characters it lacked.  It had no paired fins, although it’s been suggested that the two wing-like protrusions coming out of its head shield served a similar function.  It also had no jaws or teeth.

Pteraspis_NT

A reconstruction of Pteraspis (from Wikipedia)

As the vast majority of living vertebrates have jaws and teeth, it might be quite hard to imagine how Pteraspis would have fed. The two groups of living jawless fish- hagfish and lampreys- have highly specialised mouthparts: the former for deep sea scavenging and the other as a ‘sucker’. Heterostracan mouthparts aren’t similar to either of these however, and they also have a more generalised swimming body plan; one thing they do have though is a series of oral plates.  These were parts of the head shield (not teeth!) that lay over the oral cavity.  Many functions have been suggested: scooping sediment, grabbing onto prey and suspension feeding.  Patterns of wear seem to exclude scooping and tiny little outwards pointing denticles on the plates would have prevented big prey from entering the mouth.  As such it seems that heterostracans like Pteraspis would have suspension fed on small animals, like many groups of animal today.

Het smile

A heterostracan similar to Pteraspis giving us an oral platy grin. (from Purnell, 2002)

Pteraspis and the heterostracans have a lot to tell us about how the jawed vertebrates evolved.  They possess bone, but no jaws or paired fins, immediately telling us that these characters of the gnathostome body plan didn’t evolve together.  Although we’ve not really discussed it here, the structure of their bone itself can also teach us about the evolution of bone, and how it came to prominence as a tissue. These animals, completely different to anything living in the oceans today, also present their own mysteries, such as how they fed without jaws.  We’ve only briefly talked about heterostracans here, but fear not! There’s plenty more to talk about in the future.

References

What’s New(s) 31/01/14

So, only four days late putting this up.  I’ll save the things happening this week for Fridays post, but fear not! Plenty occurred in the paleontological world last week.  Some of it’s even non-Mesozoic (gasp!).

journal.pone.0084709.g020

Look at those ichthyosaurs go! Diversity remains roughly constant with the Jurassic throughout their Cretaceous range. From Fischer et al.

The last hurrah of the ichthyosaurs.  This group of iconic Mesozoic marine reptiles, who recently starred in TOTW, didn’t quite make it all the way to the K/T party, and went extinct during the late Cretaceous.  Traditionally the group has been seen as going out on a bit of a whimper after a lengthy decline from the Jurassic.  New work on European ichthyosaurs by Fischer et al has shown that this picture isn’t necessarily correct, with ichthyosaurs showing a pretty stable diversity throughout their existence, at least in Europe.

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Polypterus having a breathe. From Graham et al.

Stem tetrapods can breathe easy with the news that Polypterus, a basal ray-finned fish, breathes air through its spiracles.   These large paired openings on its had have previously been argued to have a use in air breathing, but Graham et al have for the first time demonstrated this to actually be the case.  Stem tetrapods (the lineage of fish leading up to terrestrial vertebrates, including our friend Tiktaalik) possessed spiracles which have been linked to the transition to air breathing, and this work supports this argument.

Silesaur

Reconstruction of the size of NHMUK R16303 (grey) in comparison to a more normally sized silesaurid. From Barrett et al.

Giant Silesaurids!  Silesaurids are a group of Triassic archosaurs thought to be somewhere near the base of the dinosaur lineage, and so are important to understanding early dinosaur evolution.  Previously silesaurids have all been fairly small animals, particularly when compared to their later dinosaurian brethren.  Barrett et al have, however, described the femur of an unprecedentedly large silesaurid from Tanzania.  This specimen, with the catchy name NHMUK R16303, shows that silesaurids gained bigger sizes than previously thought, bigger than some early dinosaurs, with implications for ideas of why and how dinosaurs were so successful.

Penguinterrogation

The paper just has pictures of bits of broken femur in, so instead here’s a picture of a penguin undergoing what appears to be some kind of interview. From Wikipedia.

An ancient seabird has been described from the Palaeocene of New Zealand by Mayr and Scofield, and unlike everything else described from this locality it isn’t a penguin!  While I love penguins as much as the next man, this fleshes out the picture of the avian fauna of this area shortly after the K/T extinction.  It also continues to expand the picture of the diversity of birds in the Palaeocene, showing that the dinosaurs were still doing pretty well.  Extinct indeed.

800px-Fishfinger1

Apologies for the decline in picture usefulness through this post, but it was a either this or a complicated schematic of the action of Hox genes which I’d have done a poor job of explaining.   From Wikipedia.

Fish fingers. For a long time, scientists have been trying to reconcile the digits of tetrapods with fishes’ fins.  These two structures that are superficially similar, but frustratingly different in layout.  While fossil data has been building up a picture of this transition, ‘evo-devo’ studies have also been providing valuable information.  One such recent study, by Woltering et al, suggests that the digits of tetrapods aren’t in fact homologous (ie. evolutionarily the same) to the fin radials in a fishes’fin.  This is based upon the expression of Hox genes (genes that dictate the layout of a developing organ) in zebrafish and mice.  When Hox genes from the fish were expressed in developing mice, they only affected development in the proximal parts (ie. arm) of the limb, suggesting fish don’t use the ‘digit-causing’ part of their genetic toolkit.  This in turn suggests the two structures are not homologous.

Taxon of the Week: Tiktaalik

Following all of the giant-rauisuchian based excitement of last week’s TOTW, I’ve decided to calm it down a bit by looking at an unassuming fish.  This particular unassuming fish has been in the news recently, and illustrates a transition without which there would be no Postosuchus, no mammals and (perhaps worst of all) no ‘Dinosirs’.  The fish of which I speak is, of course, the Devonian tetrapodomorph Tiktaalik roseae, but before we look at it closely, let’s have a look at the bigger evolutionary picture into which it fits.

Vertebrates that live on land are known as tetrapods, and all possess (or at least their ancestors possessed) four limbs with digits: amphibians, lizards, crocodiles, birds and mammals are all modern tetrapods. Despite their many obvious differences from what you’d typically think of as a ‘fish’, tetrapods are actually the largest group of lobe-finned fish, or sarcopterygians, named for their fins’ fleshy bases.  The only other modern groups of sarcopterygian are lungfish and the famous ‘living fossil’ coelacanth.

An African Lungfish: note fleshy-based lobe fins (from bbc.co.uk)

An African Lungfish: note fleshy-based lobe fins, as compared to the rayed fins of your goldfish. (from bbc.co.uk)

The most obvious difference between tetrapods and other fish is that the latter are aquatic and the former mainly terrestrial (with the exception of groups that went back into water, like whales and icthyosaurs).  At some point during the evolution of tetrapods they switched from an aquatic lifestyle to a terrestrial one, and this pretty drastic transition is of great interest to palaeontologists.   Originally pictured as being a case of fish struggling onto land and then evolving limbs, in recent(ish) years very early tetrapods from the Late Devonian (~365 Mya), such as Acanthostega and Ichthyostega, have changed this view.  These fossils have four limbs with digits, but appear to be fully aquatic, suggesting that the tetrapod body plan evolved in water first, only later proving handy (pun obviously intended) on land.

Acanth tol.web

Acanthostega having a swim with its digit-y limbs (from tol.web).

Various fish-like animals have been described that have body plans somewhere between these early aquatic tetrapods and the ancestral fishy form, and which are more closely related to tetrapods than the next closest group of sarcopterygians, the lungfish.  These animals are known as ‘tetrapodomorphs’, and show an evolutionary trajectory towards air breathing and a four-finned body plan, while still retaining fish-like characters such as gills and fin rays.  It has been suggested that this was as a result of them living in shallow rivers during the oxygen-poor Late Devonian, where being able to breathe air as well as oxygen dissolved in water would have been advantageous, as would being able to navigate shallow, relatively predator free, water.

Tikphylo

A phylogeny of selected tetrapodomorphs and tetrapods, showing the change in body plan from ‘fish-like’ to ‘tetrapod-like’. (adapted from Daeschler et al, 2006)

Enter TiktaalikTiktaalik is something of a celebrity amongst tetrapodomorph fish. Described in 2006 from fossils found on Ellesmere Island in Nunavut, Canada, its name means ‘Burbot’ in Inuktitut.  It was subsequently popularised by one of its discoverers, Neil Shubin, in his book ‘Your Inner Fish’ (recommended), and is even the star (or possibly victim) of its own song on Youtube.  This stardom is well deserved, as Tiktaalik gives us a good deal of interesting information on the water-land transition.

A burbot reacts with indifference to the news that a tetrapodomorph has been named after it.

A burbot reacts with indifference to news of its namesake.

The original fossil material of Tiktaalik consisted of a pretty spectacularly articulated front half of the animal, possessing a number of interesting features.  The animal was pretty ‘fish-like’, but with some tetrapod-like characters, such as large shoulders bones and pectoral fins with wrist-like joints, a neck, and a robust ribcage.  These qualities, together with the shallow river environment in which it appears to have lived, led to the suggestion that it might have used its robust fore-fins to prop it up in ‘press-ups’, lifting its head (with the help of its neck) above water to breathe.  While there was no knowledge of the back half of Tiktaalik itself,  the small pelves (plural of pelvis) of other tetrapodomorph fossils like Panderichthys, led to a ‘front-wheel drive’ picture of tetrapodomorphs moving largely with the fore-fins, with  ‘four-wheel drive’ locomotion, with all four fins (or limbs), being a tetrapod innovation.

Tiktaalik wiki

Front-wheel drive Tiktaalik (from wikipedia)

Earlier this week however, the back half of Tiktaalik (or at least some of it) was described, and it transpires that this picture of exclusively ‘front wheel drive’ tetrapodomorphs is incorrect.  Tiktaaliks pelvis and parts of the hind limb were recovered, and it turns out that its hind-fins were as large as its fore-fins, with a fairly good range of movement.  While still compatible with their function as ‘props’ mentioned above, this also suggests that it would have been able to do more in terms of fin-based movement, with the possibility of underwater gaits, like the ‘walking’ seen in African lungfish.  It also changes our picture of the evolution of limbs, suggesting that tetrapodomorphs became ‘four-wheel drive’ before the evolution of tetrapods.

Tiktaalik comparison

The change in beefiness of hind-quarters/rear fins in Tiktaalik from the original description, above, to the recent paper, below (images from Daeschler at al 2006 and 2014 respectively)

While this recent information has changed our picture of Tiktaalik, it just adds to its importance as a source of information on the water land-transition.  This unassuming Devonian fish, along with various other fishy friends, helps illustrate that many of the changes that we associate with living on land, such as breathing air and having limbs, actually evolved as adaptations to aquatic life.  This transition also acts as a prime example of the mosaic nature of evolution: tetrapods didn’t evolve gradually from a fish-like form to a tetrapod-like form, but instead evolved tetrapod-like characters piecemeal while retaining ‘primitive’ ones. This mosaic theme is one that comes up time and time again in evolution, and is one that we’ll be discussing in future blog posts.

Some more Tiktaalik revelling in all their newly reconstructed glory.

Some more Tiktaalik revelling in all their newly reconstructed glory (image from bbc.co.uk)

Further reading

Taxon of the Week: Necrolestes

I’m afraid that this week’s taxon of the week isn’t quite as topical as last week’s, but it’s something that I heard about in a talk recently and thought would be interesting to write about.  The taxon in question is Necrolestes, a mysterious fossil mammal from South America.

The fossil material of Necrolestes consists of various fragmentary specimens from Patagonia dating to the early Miocene, about 16 million years ago.  From these remains, Necrolestes has been reconstructed as a small, digging, insectivorous animal, perhaps similar to modern golden moles in ecology.  While it’s obviously some kind of mammal it’s been assigned to various mammalian groups.

Golden mole

A modern golden mole. Actually more closely related to elephants than ‘true’ moles

Before digging too deep into where Necrolestes fits into the picture of mammalian evolution, it’s worth having a look at the picture itself (in fact, see the picture below).  Living mammals are divided into three groups: the placental mammals (most mammals you can think of, including you), the marsupials (kangaroos etc.) and the monotremes (echidnas and platypuses).  Of these the marsupials and placentals are more closely related to one another than either is to monotremes, and together marsupials and placentals are known as therians.  Monotremes, meanwhile, are found in a larger clade: australosphenidans.  Various extinct groups of mammals also existed, which don’t fit into these groups.  One such example is meridiolestids, a group of mammals that mainly died out in and shortly after the end-Cretaceous extinction event.

Necrolestes

Modern mammal groups in bold. Placentals and Marsupials together form theria.

Over the years various phylogenetic analyses have been carried to work out where Necrolestes fits in this picture.  Some of these have suggested it’s a member of the theria: a marsupial or a placental mammal.  Others have put it on the stem of therians, and some specifically in the group mentioned before, the meridiolestids.  Various recent studies have suggested this grouping, and while the placement of Necrolestes within the meridiolestids is variable in these studies, it does seem fairly likely that this is where it fits.

There are a number of interesting implications if, as seems likely, Necrolestes is a meridiolestid.  One is that meridiolestids were around in South America for 46 million years longer than anyone thought.   This is interesting in itself, as it leaves them with a massive ‘ghost range’ through the Cenozoic.  Another, if you recall the small, fossorial ecology of Necrolestes, is that these meridiolestids were inhabiting pretty specialised niches.  Parallels could in fact be drawn with modern monotremes, which occupy very specialised niches in Australasia, and are the remnants of a larger, more widely distributed group.

Platypus bills and echidna quills: specialisation in a modern 'reli

Platypus bills and echidna quills: specialisation in remaining australsphenidans.

Necrolestes is thus an interesting example of how a small, fragmented fossil can have big and interesting implications.  It looks like this small, South American creature was a (now dead) ‘living fossil’, a remnant of an otherwise ancient mammalian group, and while not currently particularly newsworthy, an interesting taxon to look at this week.

Animals or antediluvian monstrosities?

The famous painting Duria Antiquior, by the Victorian geologist Henry De la Beche, is acknowledged as being the first piece of palaeoart, ie. depiction of prehistoric life based upon fossil evidence.  Because of this it’s palaeontologically important, but it’s also pretty awesome in itself as a picture, with various marine creatures eating one another as pterosaurs swoop overhead, and even a rare depiction of a pooing plesiosaur.  There is in fact so much awesomeness going on that you’d struggle to find room to swing a cat (or whatever the Mesozoic equivalent is-perhaps Pakasuchus?) anywhere in the crowded landscape.  While a great picture, it doesn’t actually do a very good job of illustrating what a Mesozoic seascape would have looked like, instead depicting various monsters doing battle.

Everything looks so happy

The smiley ichthyosaurs make it all look so jolly.

This brings us to the theme of this blog post: the temptation to ‘mythologise’ prehistoric animals and the world in which they lived.  Duria Antiquior was painted in 1830, and obviously palaeontological understanding has come a long way since then.  Equally, the depiction of overcrowded, overdramatised scenes in palaeoart is fair enough.  No-one would be interested in a Mesozoic seascape if it depicted an empty ocean with something that might or might not be the silhouette of an ichthyosaur in the murky distance.  But this popular view of the prehistoric world as a planet populated by antediluvian monstrosities does still sometimes colour the way that people try to understand it.

One of the fundamental tools available to palaeontologists to help them understand extinct animals is information from animals that are alive today.  To understand how a dinosaur’s moved they would look at the principles that govern movement in modern animals, rather than making up special rules for dinosaurs.  Sometimes, however, palaeontologists give in to the temptation to treat prehistoric life specially.

Azhdarchid pterosaurs were a group of large, long-necked pterosaurs from the Cretaceous, including the famous (for a pterosaur anyway) Quetzalcoatlus.  Their shape has led some to suggest that they fed like modern ground hornbills, hunting on the ground with their enormous beaks (see picture).  One argument (among a number) put forward against this hypothesis is that any azhdarchid that landed on the ground to feed during the Cretaceous would be immediately torn apart by voracious theropods.

Quetz

Admittedly hornbills don’t eat sauropods.

But would this actually be the case? Darren Naish (a proponent of the hornbill-esque feeding idea) points out in a recent blog post that it probably wouldn’t be.  Notwithstanding that the size of these pterosaurs offered protection in itself, there’s no reason to think that every inch of the Cretaceous landscape was being constantly monitored by hungry tyrannosaurs.  Taking the modern African savannah as an example; it’s not like every animal that summons up the courage to peek around the side of a baobab tree is instantly ripped to shreds by lions.  To suggest that azhdarchids could never have been safe seems a bit like mythologising the Cretaceous environment and its predators.

It’s not just predators that have been ascribed ‘special rules’.  Amongst ornithodirans (pterosaurs and dinosaurs) are found a amazing array of crests and weird head ornaments (eg. hadrosaurs in picture below), and a number of suggestions have been put forward for why these evolved.  One of the most prominent has been that of ‘interspecific recognition’, where they helped animals to identify mates of the same species.  This hasn’t been conclusively demonstrated to be the reason for ornaments in any animals alive today, but proponents of this idea claim that dinosaurs represent a special case.

hadrosaur_heads_small

I like to think that this is what the album cover for a hadrosaur boy band would look like.

A counter-explanation put forward has been that of mutual sexual selection, where the crests have been selected for (in both genders) to aid attracting a mate (a more in depth discussion of which is found here).  In modern taxa this often seems to be the explanation for such ornaments, and so seems to me to be the more likely hypothesis for those in dinosaurs:  there is no need to invoke ‘special rules’ for extinct animals.  To do so is just another example of (inadvertently) mythologising them and their ecology.

It is true that there are cases where we can’t treat extinct taxa by the same rules as living ones because we have no living analogues to tell us what the rules are.  Enormous bipedal carnivores and giant fully aquatic reptiles are examples.  However, this doesn’t mean we ought to believe that widely applicable principles that we know from modern ecology wouldn’t apply for no reason other than that the animals in question were extinct.  If palaeontologists were studying the function of these animals’ bones they would prefer modern analogues to ‘special rules’, there’s no reason why the same approach shouldn’t be taken to inferring their ecology.

In the two examples I’ve given here, accusing the palaeontologists in question of viewing extinct animals as ‘antediluvian monstrosities’ is an exaggeration.  I do think however that they serve as examples of people applying ‘special rules’ to the ecology of extinct groups just because they’re, well, extinct.  In depictions such as Duria Antiquior such mythologising is both harmless and useful, and sometimes aspects of prehistoric life appear to have no direct modern analogues.  But to view them as anything more than animals in a world governed by the same ‘natural laws’ as those today just gets in the way of understanding these fascinating creatures.

FAQ: Richard.

To give people an idea of who we actually are before we start dinosauring at you, we thought we’d introduce ourselves via a series of ‘FAQs’.  Here’s mine!

Richard

First and foremost, what’s your favourite dinosaur?

At the age of 6 I’d immediately have answered Deinonychus, but the naked kind (eg. picture below) without any feathers.  I would then have proceeded to bore you with my standard soliloquy on how the raptors in Jurassic Park were actually more like Deinonychus, thus justifying my obscure dinosaur choice.

bakker deino

The awkward, naked sprint from shower to bedroom was a problem even in the Cretaceous.

Since then my dinosaur tastes have progressed a bit, but I think I’ll still pick Deinonychus.  As well as being nicely symbolic of the paradigm shift towards viewing dinosaurs as active animals, it has also become feathered fairly recently, representing another change in dino-views.  It also had HUGE CLAWS.

Secondly, what’s your favourite (preferably extinct) animal?

While lots of things are awesome I think I probably ought to choose the Devonian placoderm, Dunkleosteus.  While (obviously) all Palaeozoic fish are exciting, a 10m long one with shearing jaw bones is particularly so.  Also comes highly recommended as a fancy dress costume.

What’s your area of ‘expertise’?

I think ‘expertise’, as opposed to actual expertise, is definitely the right word to use.  I enjoy systematics and evolution-based themes, in pretty much any group.  My project this year is on a group of armoured, jawless fish called heterostracans, so I’m looking forward to learning about them as the year progresses.   My undergrad degree is in Zoology, so I like to flatter myself that I bring a critical zoological eye to palaeobiology.  This is probably not actually the case.

How did you get into palaeontology?

Playground conversations about Jurassic Park and the fact that Walking With Dinosaurs came out when I was small and impressionable both contributed to a love of palaeontology from a young age.  My grandfather is a zoologist who has done work on dinosaurs, and so he fanned the flames by doing things like introducing me to a robotic Iguanodon (see picture).  I then wanted to be a military historian for a bit, before doing a degree in natural sciences, which eventually became zoology as I tried to get as far away from cellular biology as possible.  This zoology degree heavily featured palaeo, which reignited my love of it and led me to this master’s degree.

iguano robot2

The model T-8Ig Terminator was swiftly scrapped by Skynet, after proving to be even less successful at blending into human society than Arnold Schwarzenegger.

What do you do in your spare time?

Mainly musical things.  I play the ukulele and the clarinet, and dabble in a number of other instruments.  I also enjoy singing; previously this has been in Chapel Choirs and things, but has more recently been barbershop.  I also enjoy reading and baking bread.

Favourite palaeontological paper?

I really like this paper describing paired anal fins (weird!) in the jawless fish Euphanerops because the fossil is quite pretty and it has a really nicely structured, clear diagram portraying the evolution of paired fins in vertebrates.  It also provides a tantalising glimpse into the evolution of key characters in gnathostomes (jawed fish), which (as with so much in evolution) seems to form an evolutionary mosaic rather than a straightforward progression from one character state to another.

SansomPhyloOrig

Bask in the clarity of this figure! Green is for dorsal fins, red is for paired fins and blue is for anal fins. Adapted from Sansom et al, 2013

You’re a palaeontologist, so you’re like Ross from ‘F.R.I.E.N.D.S’?

Ross never actually seemed like a very good palaeontologist, so I hope not.  I’ve also only been married twice.

Any tips for any budding palaeontologists out there?

I suspect that I still count as a ‘budding palaeontologist’, but disregarding that my tips would probably centre around a general theme of ‘get keen’.  There’s an enormous number of blogs and things on palaeobiology on the internet, and through the medium of Twitter you can get information on opportunities and palaeo news directly from palaeontological luminaries (or at least those luminaries who have Twitter).