This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Welcome to the 300th article to be published here at Tet Zoo ver 3 (note: not at Tet Zoo as a whole). I feel that this momentous occasion should be marked in some way, so here we are. The 200th article – if you’re interested – was published in September 2013 and mostly consisted of random musings about stuff that had appeared within recent months. This time round, it’s a bit more difficult to ‘look back at the past’ because we’re not that far away from Tet Zoo’s 9th birthday, and I’m saving my omphaloskepsis until then. It’s for the same reason that I won’t be providing a full list of the Tet Zoo articles that have appeared here since the 200th article was published.
If we’re not here to look back at the past, should we be talking about the future? What – you might be asking – does the future hold for Tet Zoo, and what’s going to change now that we’ve hit the Big 300?
Actually, there aren’t – and never will be – plans to really change anything at Tet Zoo. Ok, you’ve likely heard that the whole Scientific American blog network has just (as of December 15th) had a major shakedown, but that won’t affect the content or style of Tet Zoo. Indeed, I plan to continue producing the same sort of over-long articles on tetrapod-themed issues that I always have. Subjects due to be covered in the near future include cassowaries, more obscure toads, the current state of palaeoart, African mice, corvids, wading birds, and new research pertaining to ichthyosaurs and azhdarchid pterosaurs.
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Some of you will know that an expanded TetZooniverse has emerged in recent months. John Conway and I produce the TetZoopodcats (not a typo), there’s the brilliant and increasingly useful Tet Zoo wiki, and there’s also the Tet Zoo redbubble shop and patreon site. None of these projects take anything away from the effort or time I invest in writing Tet Zoo: I have paying work that does that for me. And speaking of which...
I’m (slowly) compiling a gigantic book on the vertebrate fossil record, the working title of which is The Vertebrate Fossil Record (the published title might be different). But... waitaminute... didn’t I just say ‘vertebrate’, not ‘tetrapod’? Well, yes, yes I did. You see, the book really does review the fossil record of all vertebrate groups meaning that – to my infinite distress – I’ve had to do a lot of writing and reading about fish. In fact, I’ve spent months upon months upon months preparing the fish sections of the book. It’s been terrible.
If you’re wondering, the section on jawless fishes is finished, the placoderm section is essentially finished (updates are needed in view of the new stuff those busy palaeoichthyologists keep publishing, goddam their black hearts), the gigantic chondrichthyan section is nearly finished, and the also gigantic actinopterygian section is finished and currently in review. The acanthodian and non-tetrapod sarcopterygian sections are in a state of infancy, alas.
I have learnt a lot about fish. While those of you who know me will be surprised – even shocked, staggered – to hear me say it, much of it is interesting enough that I want to share it. Combine this with the fact that I now have a vast quantity of both text and visual material on fish, and I should confess to you that I’ve even considered changing Tetrapod Zoology to Vertebrate Zoology. That’s right: the radical idea that I might begin to blog about fish has been given some serious thought. Given that this 300th article needs to be unusual – to stand out in some way – I’ve therefore decided that (drumroll).... I’m going to talk about fish. Not all of them. But about a few of the things I’ve learnt, and specifically about those things that I found especially interesting. You can consider this article a sort of behind-the-scenes teaser as goes thoughts on the book. The idea is also to showcase some of the illustrations I’ve been preparing (more of which can be seen at my patreon site).
Of placoderms and acanthodians
To launch immediately into gnathostomes (= jawed vertebrates), those of you who keep up with things in the world of fossil fish will know that the relationship between placoderms, acanthodians, chondrichthyans and osteichthyans has been in a near-constant state of flux over the last several years.
Placoderms – a hyper-diverse, mostly Devonian group best known for big, predatory arthrodires like Dunkleosteus – seem not to be a clade but instead form a series of out-groups to crown-gnathostomes. Meanwhile, acanthodians – the so-called ‘spiny sharks’ (notable for their paired ventral spines) – are also non-monophyletic.
Views on how these groups might be related to one another have changed a bit within recent years... which is ok, but slightly frustrating when you're putting a textbook together. Davis et al. (2012) identified some acanthodians as stem-gnathostomes closer to crown-gnathostomes than placoderms are, others as stem-chondrichthyans, and yet others as stem-osteichthyans. But then Entelognathus was found: a placoderm with an osteichthyan-like arrangement of dermal skull bones (Zhu et al. 2013). It was also demonstrated at about the same time that placoderm teeth – conventionally regarded as pseudoteeth that are convergent with those of crown-gnathostomes – really are true teeth after all (Rücklin et al. 2012), apparently inherited from a common ancestor that was shared with crown-gnathostomes but not with all placoderms.
Indeed, cladograms published since 2013 tend to show acanthodians as stem-chondrichthyans (not as scattered across different parts of the cladogram), and osteichthyans as the sister-group to an acanthodian + chondrichthyan clade (Zhu et al. 2013, Dupret et al. 2014, Long et al. 2014).
One interesting takehome from this is that we crown-gnathostomes evolved from among placoderms. Placoderms weren’t, as we used to think, a weird dead-end side-branch in vertebrate evolution. Another takehome is that chondrichthyans might best be imagined as the most modified and anatomically aberrant of gnathostomes, not the ‘most primitive’ as people have been inclined to think.
The many, many stem-chimaera lineages, oh, and... rays are not sharks
Moving on, let’s talk now about chondrichthyans. Those who follow my tweets and facebook lamentations might have noticed me bemoaning the frequent lack of any sort of tidy consensus on how the many, many Palaeozoic chondrichthyan lineages are allied to the crown groups.
While things are still unresolved for many groups, several lineages – perhaps including iniopterygians, orodonts, helodonts, psephodonts, petalodonts and eugeneodonts (also known as eugeneodontiforms, edestiforms or edestoids) – might be stem-members of the chimaera lineage (e.g., Grogan & Lund 2008, Grogan et al. 2012, Tapanila et al. 2013), the whole lot belonging to a major group termed Euchondrocephali. Even Holocephali, the euchondrocephalan clade that includes crown-chimaeras and similar groups, encompasses impressive variation, what with the elongate, laterally compressed chondrenchelyiforms, ray-like Squaloraja, the spiny, dragon-like menaspoids and myriacanthoids, and so on.
Among the ray-shark clade, one story that’s been fun to follow concerns the relationships between sharks, rays, and the ray-like sawfishes, sawsharks and angelsharks. When I started the project I was familiar with the hypothesis that rays (batoids or batomorphs) are deeply nested within sharks, and that angelsharks and sawsharks are close relatives of rays within the shark clade (e.g., Shirai 1992, 1996, Carvalho & Maisey 1996). This is termed the Hypnosqualea hypothesis.
However, this picture has been seriously challenged in recent years. Molecular studies have consistently found batoids to be the sister-group to a clade that contains all other elasmobranchs (Douady et al. 2003, Winchell et al. 2004, Vélez-Zuazo & Agnarson 2011), including the ray-like angelsharks and sawsharks. According to these studies, sawfish are batoids but angelsharks and sawsharks are convergently ray-like sharks. So, rays are not highly modified sharks – rather, rays and sharks share an ancestor, and rays go back as far as sharks do. While I could talk a whole lot more about sharks (the shark section of the book is huge), it’s time to move on to the final group...
There is no escape from bony fish
That final group is Osteichthyes, the bony fish. My god, this group is vast and complex. As I said above, I haven’t yet done the (non-tetrapod) sarcopterygian section of the book, but I have done the actinopterygians. By far the majority of actinopterygians are teleosts, a group that contains over 20,000 living species (perhaps over 25,000) and which have proved an absolute nightmare as goes the resolving of their evolutionary history. Crown-teleosts fall into five primary groups (osteoglossomorphs, elopomorphs, clupeomorphs, ostariophysans, and euteleosts), the affinities of which vary between studies.
The size and diversity of certain of these groups, or of specific sub-groups within them, is staggering, and reviewing them for the purposes of a book has been something of a soul-crushing task for someone who only wanted to write about tetrapods. Within Ostariophysi, for example, catfishes alone contain over 3000 extant species (plus an estimated 1500 or so that have yet to be named), a great many of which belong to lineages that have fossil records extending back to the Eocene at least. The catfish section of the book includes the line “I hope readers will forgive me for not devoting anything more than a few hundred words to this enormous group”.
Deepsea fishes, lizardfishes and kin, and the terrible acanthomorphs
Within euteleosts – the clade that includes pikes, salmon, oceanic sunfishes, seahorses and flatfishes – it’s been fun to learn that all of those weird deepsea fishes (dragonfishes, loosejaws and lanternfishes) have a pretty good fossil record. Loosejaws, bristlemouths, lightfishes and deepsea hatchetfishes are all known from the Oligocene, for example, with extant taxa being present from the Miocene onwards.
The section of the book on Aulopiformes – the euteleost group that includes the awesome lizardfishes, lancetfishes, daggertooths, tripodfishes, pearleyes and so on – is surprisingly long since their fossil record is also far better than I ever imagined. Enchodontids and several other mostly Cretaceous and Paleogene fossil groups seem to belong here (that is, within Aulopiformes). They are best known for big, fang-toothed pelagic predators like Enchodus.
Finally, the euteleost clade Acanthomorpha is also gargantuan. You get the impression that, essentially, those fish that are not catfishes are acanthomorphs. Molecular studies indicate that trout-perches (Percopsiformes), dories (Zeiformes), threadtails (Stylephoriformes) and cod and kin (Gadiformes) are close kin and form a clade termed Paracanthopterygii or Paracanthomorphacea (Betancur-R et al. 2013). Lampriforms (moonfishes, oarfishes and kin) seem to be the paracanthopterygian sister-group.
And then there are the beardfishes, toadfishes, the enormous gobiomorpharian clade, the scombromorpharians (containing such strange bedfellows as tunas, seahorses, dragonets and flying gurnards), and percomorpharians... and that last group is so huge and so complex that I can barely bring myself to start talking about it. Note that I just discussed scombroids (tuna, mackerel and kin) as being outside of the huge group that includes perciforms. Yup, molecular phylogenies have revised so many traditional ideas.
On that note, this article has already, in fact, become far longer than I planned and I really must stop.
Given the scant, skeletal, speedy run-through of just some of fish diversity that I’ve just outlined, can you imagine how gargantuan a project that covers the histories of all of these groups might be? Pretty gargantuan indeed, I can tell you. Will I ever blog about fish again? I mean, non-tetrapod fish, anyway (tetrapods being a specialised sub-set of fish). I’m not planning to. But you get the point: this being that I’m well underway on a whole book about this stuff, that it contains a vast quantity of content and detail, and that, yes, it covers fish. At. Considerable. Length. Goddammit.
So, I said that the 300th article would be a weird one. I hope it met your expectations. Next: tetrapods. Not fish.
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