The average person is probably most accustomed to thinking of 'cusp' along the lines of "on the edge" or "on the verge." As in, "we're on the cusp of returning to a normal life with this vaccine." But 'cusp' actually has a different simple meaning, which is derived from the Latin word for 'point' or 'apex.' Architects use it to refer to a feature that became particularly popular in the Gothic period, a small projection between small arches. Mathematicians use it to refer to the point where a curve drastically changes directions. And biologists use it to refer to either a tissue fold that contributes to a valve to prevent blood backflow or to a pointed protrusion on the surface of the tooth. (There's also usage in astrology, but I'm not getting into that). I'm not an architect or a mathematician, nor do I work on blood vessels, so here we are talking about teeth! A decidedly non-model systemIn the old days, a lot of what we inferred about other "complex" animals was based on what we knew about humans, since we knew ourselves best (hypothetically), which is why a lot of anatomical features were first named based on humans. Of course, if you go to a nice nature area and look around, it's pretty easy to see that humans are not really a great "model" for other animals beyond primates. Ignoring things like complex societies and technology, we walk on two legs but can't fly, have reduced hair, and have opposable thumbs, features that are mostly shared (through common ancestry) only with other primates. Mammals are generally bad models for other animals, which is why when scientists refer to "model organisms" that are used for their combination of adaptability to lab settings and applicability to a wide range of biological systems, we don't use many mammals. Anyone who's been through a high school or college biology class probably knows a little bit about the humble common fruit fly (Drosophila melanogaster), or perhaps the microscopic nematode Caenorhabditis elegans (usually just called 'C elegans' to dodge the wordy genus name), or maybe even the quietly invasive African clawed frog (usually called Xenopus by scientists and not to be confused with the African dwarf frog that Petsmart sells). These are considered model organisms for many reasons, none the least of which is that they're very hardy in lab settings, but also because their anatomy/physiology makes them reasonable proxies for something that's more complex and harder (or impossible) to work with in labs. At least some readers of the blog may be surprised to know that in the 50's, the frog I mentioned was used as a living pregnancy test for women with shocking accuracy (details in article; frogs not really harmed in process). I'm pretty sure it would be illegal to do the test in reverse to test whether a frog was pregnant by experimenting on a human. Which is one reason why Xenopus continues to be a model organism despite being discontinued as a pregnancy test. Coincidentally, Xenopus is also a good model for the topic of today's blog post - the primitive tooth condition in tetrapods. Teeth come in a wide variety of shapes, none more evident than in our own mouth, in which the various tooth types are named as such because they look different. Most animals don't have this kind of differentiation though (called 'heterodonty'); we call animals without differentiated teeth 'homodont.' Look in the mouth of the fish on your cutting board, and you're likely to see a lot of teeth that all look the same. They probably look quite simple too - simple cones with a single point. We call this tooth 'monocuspid' because it has one cusp. Teeth with two cusps are 'bicuspid,' teeth with three cusps are 'tricuspid,' and so on (though usually 4+ is just called 'multicuspid'). Monocuspid teeth are found in Xenopus, as well as most early tetrapods, including most temnospondyls. But Xenopus is an outlier among its froggy friends. Most frogs actually have bicuspid teeth (although toads, which are frogs, have no teeth), and a handful are reported to have tricuspid teeth (e.g., Greven & Ritz, 2009), but none have more than three. If you want an example of a tooth with more than three cusps, just look at your own molars - there are 4-5 in humans. Bicuspid teeth are typical for salamanders and caecilians, the other two groups of living amphibians, but they are not super common in animals with homodont dentition or are only found in part of the mouth (our premolars are bicuspid, which you maybe heard your dentist say when looking at your x-rays). Amphibians usually have homodont dentition (same size and shape throughout the mouth), so naturally, this has put an emphasis on searching for bicuspid teeth in the fossil record as one means of elucidating the origins of modern amphibians. Tales of the teethLike I mentioned before, monocuspid teeth are the base model for tetrapods and temnospondyls. This isn't really surprising; adding points is complex and requires more nuanced spatial control over the tooth's development rather than just forming one point. There is only one example of a temnospondyl with tricuspid teeth, the branchiosaurid Tungussogyrinus from Siberia (Werneburg, 2009). The broad acquisition of more complex teeth, as well as heterodont dentition, tracks the expansion of tetrapods into different ecological roles, more specifically their dietary preferences. Scraping algae off of rocks, for example, is apparently a key driver of tricuspid teeth in marine iguanas (e.g., Melstrom, 2017); this idea has been extended down to some extinct tetrapods with tricuspid teeth as well (e.g., Mann & Maddin, 2019). Temnospondyls, like amphibians today, aren't picky, but they aren't generalists either. Amphibians can't really eat plant matter, for example, whereas the capacity for herbivory was an acquisition that facilitates the diversification of amniotes on land. But temnospondyls did eat other animals, and for that, monocuspid teeth work just fine, whether for grabbing, piercing, or stabbing. There are several examples of temnospondyls with bicuspid teeth, or to be a little safer, several taxa in which bicuspid teeth are reported. Probably the most unequivocal of them is Doleserpeton annectens, which John Bolt keyed in on in the 1960s as a "proto-lissamphibian" based on his observation of this feature. With a lot of material and excellent preservation, subsequent research has confirmed the presence of bicuspid teeth in this taxon (e.g., Sigurdsen & Bolt, 2010). With the recognition of this feature in one amphibamiform, workers began looking for more evidence of bicuspid teeth in what was then called 'amphibamids.' Amphibamus, a close relative of Doleserpeton, also has bicuspid teeth (Bolt, 1979). Platyrhinops, which is somewhat debated over whether it is the same as Amphibamus, also has bicuspid teeth (Clack & Milner, 2009). Most amphibamiforms do not have bicuspid teeth however, including Gerobatrachus, the 'frogamanader' ('batrachian' is the technical term for the grouping of frogs and salamanders; Anderson et al., 2008). There is also some controversy about whether any species of Tersomius has bicuspid teeth. Bolt (1977) indicated some specimens of Tersomius texensis (but not the holotype) have bicuspid teeth, but he only figured one of these bicuspid teeth (it was hard to get clear photographs or microphotographs in the 70's). The specimen that he did figure does appear to have two cusps, but they are not nearly as well-developed as in Doleserpeton or in most living amphibians. Probably important to bear in mind here is that we're talking about teeth that are less than one-tenth of a millimeter in width, so the precision in the plane of sectioning could be a factor. Notable too is that all of the Tersomius material with bicuspid teeth is from the South Grandfield site in Oklahoma; possibly, it is not actually T. texensis (or Tersomius at all). The original assignment was provisional (cf. designation by Daly, 1973) and made no comments on the teeth. That material has not been redescribed. This is further complicated by Anderson & Bolt's (2013) description of Tersomius dolesensis. Not only does it have the peculiar combination of monocuspid marginal teeth with at least one bicuspid palatal tooth, but there is disagreement about whether this is actually properly placed in Tersomius (their phylogenetic analysis excludes other species). In Maddin et al.'s (2013) analysis with the type and one good referred specimen of T. texensis, T. dolesensis does not group with what are now termed micropholids (Pasawioops + Micropholis + Tersomius). Suffice it to say that some amphibamiforms have bicuspid teeth and most do not. Unsurprisingly, paleontologists have looked to other possible lissamphibian candidates, primarily 'microsaurs,' to see whether they too might have bicuspid teeth (diminishing the value of bicuspidity in amphibamiforms). And indeed, there are numerous reports of bicuspidity among 'microsaurs,' including Carrolla craddocki (Langston & Olson, 1986; Maddin et al., 2011), the now defunct 'Bolterpeton carrolli' (Anderson & Reisz, 2003; Bolterpeton is synonymous with the parareptile Delorhynchus), and Cardiocephalus peabodyi (Anderson & Reisz). The description of two cusps in C. craddocki still stands, whereas that of C. peabodyi seems to have been quietly abandoned. Haridy et al. (2017) suggest that in taxa with cutting edges on the teeth, like in many parareptiles, these edges may be misinterpreted as the ridges leaving to two cusps; they specifically suggest that what used to be 'B. carrolli' appeared to have two cusps only because of uneven tooth wear. It's also worth noting that the putative stem caecilian Chinlestegophis, a stereospondyl temnospondyl, also lacks bicuspid teeth (Pardo et al., 2017), as does the more highly nested amphibamiform Gerobatrachus (Anderson et al., 2008). It's just a phaseOne of the key caveats in bicuspidity in modern amphibians is that it is known to change in the development of the animal. Teeth are monocuspid early in development and only later do they become bicuspid. This is one of the reasons why John Bolt proposed that some amphibamiforms might in fact be juvenile dissorophids, and he proposed the inverse scenario in which the adult stage was a monocuspid tooth that transitioned from a bicuspid tooth. Features that are transient throughout an animal's life become difficult to work with in the fossil record because it means that absence (of bicuspidity in this case) is not necessarily attributable to a legitimate absence of homologous features - instead, the material under study might be immature and not yet have achieved the derived condition that is being sought but would have at a more advanced stage of life. Bolt wasn't the first to suggest that some terrestrial amphibamiforms might actually be juveniles of larger animals (same problem with the mostly aquatic branchiosaurids), but the data collected since his work in the 70s and 80s has not supported this conjecture. A higher purpose
References
2020 has certainly been some kind of year, which makes it easy to miss the latest scientific discoveries related to everyone's favourite (usually) four-fingered tetrapods. Last year I did a wrap on the entire decade, so there was a quite a lot, but in spite of the circumstances, 2020 has still produced some very exciting temnospondyl research! As with last year's wrap-up, this focuses on temnospondyl-centric research; there are obviously plenty of papers that make cursory mention of them or that might include a picture or two in an assemblage description, but those are not summarized here (I gotta be efficient with my time). As usual, links to everything are in the reference list as the end. Hopefully looking forward to getting back on a more regular track again in 2021 - stay tuned! Leaving a markIn lieu of body fossils, we often have evidence of temnospondyls in the form of trace fossils, which usually consist of a trackway. In contrast to amniotes, which basically all have pentadactyl (5-digit) hands, temnospondyl trackways have been considered to be historically easy to ID because of the tetradactyl (4-digit) hand that typifies temnospondyls and many modern amphibians (at least the ones that still have hands). Body and trace fossils rarely mix because the optimal conditions to preserve both the hard parts and the traces of activity will differ somewhat starkly, but this is good because it means where we might lack a body fossil record, perhaps we could supplement the total record with traces. Bird et al. (J Geol Soc) reported a trackway from the early Carboniferous (Visean) of the U.K. (paper technically online in late December last year, but in print this year). This time period is a critical window of tetrapod evolution, when stem tetrapods really radiate; however, we still don't have most of the groups that directly lead to modern groups, and there is only one Visean temnospondyl, Balanerpeton woodi from Scotland (which is also debated about whether it might be a stem tetrapod and not a temnospondyl). The U.K. in general is pretty sparse on the Paleozoic tetrapods. As such, this is a good example where the trace fossil record pre-dates the main body fossil record and thus hints at a large gap in the body fossil record of the earliest stages of a group's evolution (temnospondyls in this case). The ichnotaxon (taxonomic framework for trace fossils) that they assigned the material to is Palaeosauropus, which is also known from the Blue Beach locality in Nova Scotia and Pennsylvania and which was previously interpreted to belong to an edopoid (the validity of this association is a little questionable). The oldest body fossil record of an edopoid is the Bashkirian, which begins around 323 million years ago, whereas the Visean ends at 330 million years, so at minimum, this possible edopoid record extends the group back by at least 7 million years, which is congruent with phylogenetic analyses that recover edopoids as a very early diverging group. Herron et al. (Norwegian J Geol) reported slightly younger tracks from the Late Carboniferous (Moscovian-Kasimovian), when the temnospondyl body fossil record is much better established, from Svalbard. This archipelago is usually better known for its Triassic temnospondyl body fossil, which compares favourably with the closely situated Greenland assemblage, and the Scandinavian Arctic region has long been productive for some of the most important stem tetrapod fossils like Acanthostega. The new material that Herron et al. report is a massive block (almost 800 photos to produce a composite model) that captures a rare setting: the margin of a pond. As a result, the trackway captures the transition between the animal moving in the deeper part of the pond (with more scratch-like traces made through minimal contact with the pond bottom) and then moving out onto land (with more pronounced footprints).
There were a few other papers that briefly show some temnospondyl traces or trackways that might be temnospondyls:
An abundance of skepticism
The second study was a broader review of the fossil record of caecilians by Santos et al. (Biol J Linn Soc); the second author (Michel Laurin) is a longstanding proponent of a single origin of lissamphibians from lepospondyls. This paper repeatedly calls out various lines of argument used by Pardo et al. and even has a discussion section titled "Chinlestegophis: a true gymnophionomorphan?" In fact, they go so far as to say that every identified synapomorphy is questionable, and other shared features are plesiomorphies found in other temnospondyls. The issue of whether the element at the front of the eye is a lacrimal or a LEP is just one of those questioned features. No new phylogenetic analysis here, but a great summary of the available information on fossil caecilians (it is very poor) like the figure below on the left. In case you're wondering why the record is so bad, burrowing animals by virtue of where they live are not nearly as likely to end up near the types of environments that most frequently preserve in the fossil record, like ponds or floodplains. Their small size, like that of other lissamphibians, is another issue that is generally less favourable for preservation. The dissorophid dynastyThe temnospondyls long considered to be closely related to lissamphibians in one form or another, dissorophoids, continued to see major research interest, even outside of the context of lissamphibian origins. 2020 mostly brought re-descriptions, though we got a few other types of studies thrown in there as well. One important redescription that we got in 2020 was of the holotype of "Dissorophus" angusta, a species named by Bob Carroll in 1964. Basically everybody has recognized for decades that it is definitely not Dissorophus, lacking the large first osteoderm and having narrow osteoderms throughout the body, but it is only now, 56 years later, that its status is finally resolved as a new genus, Diploseira, by Dilkes (J Vert Paleontol). The only known specimen is mostly the postcranial skeleton, which makes it hard to compare with many other dissorophids, but it definitely has distinctive features. David's usual impeccable attention to detail and illustrations really flush out every little nuance of the anatomy. This taxon is quite interesting in preserving a transitional series of osteoderms; there are two series at the front and only one at the back. As far as we know, this is the only dissorophid with this condition - other species either have a single continuous series or two continuous series (external and internal). Much of the osteoderm anatomy (though not the width) is shared with dissorophines (Broiliellus and Dissorophus), and Dilkes' phylogenetic analysis indeed recovered Diploseira within that group. This really mucks up how we use qualitative patterns of osteoderms to make taxonomic frameworks, continuing to highlight the issues with single-feature phenetic taxonomy (essentially arbitrary emphasis of certain observational data).
A slice of lifeIn addition to my Doleserpeton paper, there were a few other studies that sliced and diced up some temno bones. The first one was technically published online last year (in print this year), and I very briefly mentioned it in my year-end summary last year, but it's worth discussing here in greater detail. Mukherjee et al. (Papers Palaeontol) describe the histology of a few Middle Triassic capitosaurs from India, which is a follow-up to another paper led by Muhkerjee a decade ago on a more preliminary sample of Indian temnos. Despite the material of the two taxa (Cherninia and Paracyclotosaurus) coming from the same small locality, they show distinctive differences in their bone histology. Cherninia shows a lot of what temnospondyl workers term 'incipient fibrolamellar bone,' which generally meets the criteria of fibrolamellar bone in amniotes. Note that contrary to some oversimplifications, fibrolamellar bone is not an unequivocal hallmark of endothermy - it only indicates rapid growth, which can be accomplished in ectotherms living in harsh conditions that require fast growth (for example, taxa with larval forms that need to get out of water before it dries up). Comparison of histology of the humerus of Cherninia (on the left) and Paracyclotosaurus (on the right). In Cherninia, this fibrolamellar bone tissue is found in sub-adults too (it would be more likely to be found in juveniles that are still growing), but it is not found in any growth stage of Paracyclotosaurus, which also lacks the woven-fibered bone of the smallest and most rapidly growing individuals of Cherninia. So Cherninia = fast grower, Paracyclotosaurus = slow grower. The histological / microanatomical differences can be correlated with distinct differences in the proportion of limb features and the torsion of the humerus and femur, suggesting biomechanical differences in their locomotion. The authors proposed that Cherninia was a classic obligately aquatic taxon, like what we think of most stereospondyls, whereas Paracyclotosaurus was more capable of moving around on land, not like what we typically think of large stereospondyls. It's debatable in my opinion whether the latter "spent a considerable amount of time on land," but it definitely seems likely that being able to at least move between ponds would have been advantageous, and it suggests that there may be a lot of cryptic information to be derived from the histology and microanatomy that is obscured by more conserved external anatomy. Comparison of sections of the humerus of Panthasaurus (on the left) with the femur, ulna, and tibia (on the right; A-H = femur; I-J = ulna; K-M = tibia). 2020 was the first year that I didn't have any new metoposaurid papers published (there's stuff in the pipeline, don't worry), but the metoposaurid histology train rolls on. Teschner et al. (PeerJ) reported histology and microanatomy of the Indian metoposaurid, Panthasaurus. This taxon shows pretty normal features for obligately aquatic stereospondyls, like parallel-fibered bone with remodelling that's accompanied by lamellar bone tissue (these reflect slower rates of growth), a fairly poorly vascularized cortex, and distinct growth marks (but not LAGs). This study is really nice because it adds to the growing body of literature helping us to understand how closely related taxa (within the same family) might differ in ecology or response to local climate, things we might not be able to nuance out from the external anatomy alone. Years ago, there was a nice comparative study by Dorota Konietzko-Meier (who's on this paper) and Nicole Klein comparing the signals in Dutuitosaurus from Morocco and Metoposaurus from Poland, which showed that the Polish paleoenvironment was milder and led to slow-downs or short stagnations but not long cessations in growth. Panthasaurus shows a similar signal to Metoposaurus (growth zones and annuli but no LAGs), indicating the Indian climate was also not too harsh. The authors also sampled a bunch of different elements, which is a key step forward to expanding the utility of non-limbs for histological studies by providing a reference point that isn't a humerus or a femur; if you wonder why nobody has done any of this work on North American taxa, it's because there are very few limb bones that are basically off-limits to destructive sampling right now (a number of people other than me have looked around, and this is why I keep cutting intercentra instead). Finally, Uliakhin et al. (Paleontol J) reported what is probably the highest age estimation for any Paleozoic vertebrate and definitely the highest one for any temnospondyl - a whopping 57 years! This was reported from the late Permian dvinosaur Dvinosaurus campbelli from Russia; dvinosaurs are one of the few uncontroversially fully aquatic Paleozoic temnospondyls, and there remains fringe speculation that one of the stereospondyl clades might actually be tied to this group. We see a lot of the same features that typify stereospondyls (e.g., parallel-fibered bone, lots of secondary remodeling from the medullary cavity, calcified cartilage), which is, at minimum, a reflection of what happens when you're neotenic and end up permanently living in the water. How about the 57 years - is that out of bounds? We know that slow-growing animals tend to live longer (why grow slow if you'll die in a year), and some modern amphibians top 60 years (and it's not one of those weird one-offs that far exceeds the norm for the species). Like the authors point out, neotenic individuals tend to live longer than metamorphosing ones. Of course, this assumes that the interpretation of the LAGs is correct; could it be double LAGs (two cessations in growth per year) for example (thus halving the estimated age)? Hard to say. The giveaway for double LAGs is their spacing - two closely spaced lines with a gap smaller than that from the line on either side. That doesn't mean you couldn't get double LAGs that are spatially indistinguishable from normal LAGs if the process that formed them was timed differently. For example, the way that we define seasons usually sets the start of winter and the start of summer as exactly 6 months apart. But as most people experience them, the "peak conditions" of each season aren't on Day 1, and the peaks may not be 6 months apart. Growing up in SoCal, I would say the hottest month of the year can be as late as September, and having lived in Canada, February or March can be the coldest. That asymmetry is likely what produces the characteristic spacing of double LAGs. But of course there's variation - who really knows the finer nuances of the climate in Russia over 250 million years ago, after all. So it's possible that climate peaks really could have been nearly 6 months apart and thus produce a pattern resembling single LAGs when the animal actually stopped growing twice a year. But we can't really prove that either. The best way to interpret this would be to cautiously accept the author's interpretation (single LAGs = 57 years) but not to make too much of it. Comparing maximum ages in extinct taxa is hard because how do you know that you got one that died around its maximum age? A lot of animals (especially those lower on the food chain) frequently die well before their maximum age, often because they're eaten, so the fact that other temnospondyls rarely exceed estimates of 15 years only says something about the sample, not necessarily the species as a whole. Piling up
Based on a slew of taphonomic analyses, there appears to be disparate pre-burial conditions between the different tetrapods found at the site that explains the differences in their representation (the complex graph on the top left). There are mixed age classes for the metoposaurids, whereas the phytosaurs and rhynchosaurs are skewed towards either juveniles or adults. Obviously some of this has to do with which animals lived in the immediate environment versus which might have been carried in and the cause of death. This is not a mass death assemblage like that of Dutuitosaurus or the Lamy quarry insofar as it appears to be time-averaged for metoposaurids (but not rhynchosaurs, which may have mostly gotten swept in by a single event), and instead seems to be accumulation of their remains through periodic flooding and a build-up of remains in low-energy spots; the model is on the top right. The authors went so far as to argue that the metoposaurid remains might have been left out on the banks, exposed for years and constantly weathering, which might account for the total lack of complete skulls. Really great prospects based on the sheer volume of material that's come out of the site - hope to see more in the future! That's gotta hurtWith advances in technology and application of modern methods to paleontological specimens, paleopathology (studying diseases in fossils) has come a long way - we're seeing more and more reports of specific diagnoses and maladies beyond "that is clearly not right." Novikov et al. (Paleontol J) describe a bone lesion in the Early Triassic trematosaur Benthosuchus. This is one of the most common Early Triassic tetrapods found around the world (but mostly in Russia in this case), and the authors report a bone lesion on the lower jaw, which would be immediately identifiable to even non-scientists as a very odd-looking round protrusion from the side. Based on features such as denser structure compared to surrounding bone, localized presence of a smooth feature, and no apparent connection to the teeth, the authors propose that this represents a non-odontogenic osteoma (non-tooth-related tumour). Because they couldn't do histology on the jaw, their results are based only on the external examination and the CT data (not the sharpest), so there are a few other possibilities (like a cyst or specifically a bone cancer). This remains the oldest example of a tumour forming in a tetrapod, although other reports like my friend Yara Haridy's 240-million-year-old turtle tumour are not that much later. Welcome to the club
A few other temnospondyl adjacent studies:
And the recent special issue on Karoo biozonation: https://pubs.geoscienceworld.org/sajg/issue/123/2 (note there are a few included taxa that are probably junior synonyms in some articles). In closingAll things considered, an excellent year for temnospondyl research! I reviewed a bunch of these papers as well, which is always neat to see what people are working on ahead of press. It's been a few months since I had anything come out (a coincidental pile-up of my 2019 productivity in the first half of this year), but I've got a few things working their way through the pipeline and will hopefully be getting back to some semblance of regular blogging in the new year! Thanks for reading, and best wishes for the new year! References
So this week, I want to highlight a few of the amphibamiforms that are not always included in phylogenetic analyses, in part because some of them have clear destabilizing effects on the phylogenetic analysis. There are at least 16 recognized non-branchiosaurid amphibamiforms (so the classic 'amphibamids'), but I would say that only half of those are always included in dissorophoid / amphibamiform analyses, such as Amphibamus. But if you think about it, not including some taxa doesn't mean they don't exist, and it's important to be mindful that excluding taxa can skew the picture, whether intended to or not. As always, good to be constructively critical of methodologies and to think about how they can bias results!
Frequent museum-goers are probably aware that most of those mounted skeletons that you see at museums aren't one individual - they're composites (like the metoposaurid above at the American Museum), sometimes of multiple individuals found together and sometimes of multiple individuals found apart. The fossil record isn't kind to most skeletons, and 99.99999% (more or less) of all fossils are just isolated bits. The good news is you can often cobble together the many bits to make a collection of bits, which maybe looks more like a skeleton, but the bits need to fit. Think about it like fashion (this is not my expertise, don't crucify me): you can usually make many different combinations of clothes that work, but they need to fit - you probably don't pair winter coats with speedos (though this would be done in California if anywhere). Not only that, but you probably can't (or wouldn't) use clothes from when you were a teenager as an adult. Of course, maybe your teenage fashion sense was super cringy, and that's why you won't do it, but one of the other reasons might be that your clothes (even your favourite hoodie from that concert in 11th grade) from back then don't fit. It's the same idea when making a composite fossil - the bits all have to fit, and that means they have to come from individuals that were about the same size.
We begin the amphibamiform showcase with the eponymous Amphibamus, one of the oldest known dissorophoids (both in terms of temporal occurrence and history of study)! A brief history of study
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