| Placoderms | |
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| Fossil ofBothriolepis panderi, anantiarch placoderm showing itscaliper-likepectoral fins | |
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| Life restoration ofCoccosteus, anarthrodire placoderm | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Subphylum: | Vertebrata |
| Infraphylum: | Gnathostomata |
| Class: | †Placodermi McCoy, 1848 |
| Orders | |
| Synonyms | |
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Placoderms (fromAncient Greek πλάξ (plax,plakos) 'plate' and δέρμα (derma) 'skin')[1] arevertebrateanimals of theclassPlacodermi, anextinct group ofprehistoric fish known fromPaleozoicfossils during theSilurian and theDevonianperiods. While theirendoskeletons are mainlycartilaginous, theirhead andthorax were covered by articulatedarmoured plates (hence the name), and the rest of the body wasscaled or naked depending on thespecies.
Placoderms were among the firstjawed fish (theirjaws likelyevolved from the first pair ofgill arches), as well as the first vertebrates to have trueteeth. They were also the first fishclade to developpelvic fins, the second set ofpaired fins and thehomologous precursor tohindlimbs intetrapods.[2] 380-million-year-old fossils of three other genera,Incisoscutum,Materpiscis andAustroptyctodus, represent the oldest known examples oflive birth.[3]
Placoderms are thought to beparaphyletic, consisting of several distinctoutgroups orsister taxa to all living jawedvertebrates, which originated among their ranks.[4] In contrast, one 2016 analysis concluded that Placodermi is likelymonophyletic.[5]
The first identifiable placoderms appear in thefossil record during the lateLlandovery epoch of the early Silurian.[6] They eventually outcompeted the previouslydominant marinearthropods (e.g.eurypterids) andcephalopodmolluscs (e.g.orthocones), producing some of the first and most infamous vertebrateapex predators such asEastmanosteus,Dinichthys and the massiveDunkleosteus. Various groups of placoderms were diverse and abundant during the Devonian, but all placoderms became extinct at the end-DevonianHangenberg event 358.9 million years ago,[7] leaving theniches open for theosteichthyan andchondrichthyan survivors who subsequentlyradiated during theCarboniferous.
Many placoderms, particularly theRhenanida,Petalichthyida,Phyllolepida, andAntiarchi, were bottom-dwellers. In particular, the antiarchs, with their highly modified, jointed bony pectoral fins, were highly successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with the Middle to Late Devoniangenus,Bothriolepis, known from over 100 valid species.[8] The vast majority of placoderms werepredators, many of which lived at or near thesubstrate. Many, primarily thearthrodires, were active,nektonic predators that dwelled in the middle to upper portions of the water column. A study of the arthrodireCompagopiscis published in 2012 concluded that placoderms (at least this particular genus) likely possessed true teeth contrary to some early studies. The teeth had well definedpulp cavities and were made of both bone anddentine. However, the tooth and jaw development were not as closely integrated as in modern gnathostomes. These teeth were likely homologous to the teeth of other gnathostomes.[2]
One of the largest known arthrodires,Dunkleosteus terrelli, was 3.5–4.1 metres (11–13 ft) long,[9] and is presumed to have had a large distribution, as its remains have been found in Europe, North America and possibly Morocco. Some paleontologists regard it as the world's firstvertebrate "superpredator", preying upon other predators. Other, smaller arthrodires, such asFallacosteus andRolfosteus, both of theGogo Formation of Western Australia, had streamlined, bullet-shaped head armor, andAmazichthys, withmorphology like that of other fast-swimmingpelagicorganisms,[10] strongly supporting the idea that many, if not most,arthrodires were active swimmers, rather than passiveambush-hunters whose armor practically anchored them to the sea floor. Some placoderms were herbivorous, such as the Middle to Late Devonian arthrodireHolonema, and some wereplanktivores, such as the gigantic arthrodireTitanichthys, various members ofHomostiidae, andHeterosteus.
Extraordinary evidence of internal fertilization in a placoderm was afforded by the discovery in the Gogo Formation, nearFitzroy Crossing,Kimberley, Western Australia,[11] of a small female placoderm, about 25 cm (10 in) in length, which died in the process of giving birth to a 6 cm (2+1⁄2 in) offspring and was fossilized with the umbilical cord intact.[12] The fossil, namedMaterpiscis attenboroughi (after scientistDavid Attenborough), had eggs which were fertilized internally, the mother providing nourishment to the embryo and giving birth to live young. With this discovery, the placoderm became the oldest vertebrate known to have given birth to live young ("viviparous"),[3] pushing the date of first viviparity back some 200 million years earlier than had been previously known. Specimens of the arthrodireIncisoscutum ritchei, also from the Gogo Formation, have been found with embryos inside them indicating this group also had live bearing ability.[13] The males reproduced by inserting a longclasper into the female. Elongated basipterygia are also found on the phyllolepid placoderms, such asAustrophyllolepis[14] andCowralepis, both from the Middle Devonian of Australia, suggesting that the basipterygia were used in copulation.
The placoderm claspers are nothomologous with the claspers incartilaginous fishes. The similarities between the structures has been revealed to be an example ofconvergent evolution. While the claspers in cartilaginous fishes are specialized parts of their paired pelvic fins that have been modified for copulation due to changes in thehox genes hoxd13, the origin of the mating organs in placoderms most likely relied on different sets of hox genes and were structures that developed further down the body as an extra and independent pair of appendages, but which during development turned into body parts used for reproduction only. Because they were not attached to the pelvic fins, as are the claspers in fish like sharks, they were much more flexible and could probably be rotated forward.[15]
A study onKolymaspis showcases that the vertebrateshoulder girdle evolved from gill arches.[16]
It was thought for a time that placoderms became extinct due to competition from the firstbony fish and earlysharks, given a combination of the supposed inherent superiority of bony fish and the presumed sluggishness of placoderms. With more accurate summaries of prehistoric organisms, it is now thought that they systematically died out as marine and freshwater ecologies suffered from the environmental catastrophes of theLate Devonian andend-Devonian extinctions.
The earliest identifiable placoderm fossils are of Chinese origin and date to the earlySilurian. At that time, they were already differentiated intoantiarchs andarthrodires, as well as other, more primitive, groups. Earlier fossils ofbasal placoderms have not yet been discovered.
The Silurian fossil record of the placoderms is both literally and figuratively fragmented. Until the discovery ofSilurolepis (and then, the discoveries ofEntelognathus andQilinyu), Silurian-aged placoderm specimens consisted of fragments. Some of them have been tentatively identified as antiarch or arthrodire due to histological similarities; and many of them have not yet been formally described or even named. The most commonly cited example of a Silurian placoderm,Wangolepis of Silurian China and possibly Vietnam, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders. So far, only three officially described Silurian placoderms are known from more than scraps:
The first officially described Silurian placoderm is an antiarch,Shimenolepis, which is known from distinctively ornamented plates fromHunan, China. It was originally considered to be from the lateLlandovery, although later study reconsidered its age atLudfordian.[18]Shimenolepis plates are very similar to the early DevonianyunnanolepidZhanjilepis, also known from distinctively ornamented plates.[6][19] In 2022,Xiushanosteus is described from complete fossils fromTelychian, late Llandovery ofChongqing, China.[20]
Paleontologists and placoderm specialists suspect that the scarcity of placoderms in the Silurian fossil record is due to placoderms' living in environments unconducive to fossil preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly instantaneous appearance and diversity at the very beginning of theDevonian.
During the Devonian, placoderms went on to inhabit and dominate almost all known aquatic ecosystems, bothfreshwater andsaltwater.[21] But this diversity ultimately suffered many casualties during the extinction event at theFrasnian–Famennian boundary, the Late Devonian extinctions. The remaining species then died out during the end-Devonian extinction; not a single placoderm species has been confirmed to have survived into theCarboniferous.
The earliest studies of placoderms were published byLouis Agassiz, in his five volumes on fossil fishes, 1833–1843. In those days, placoderms were thought to be shelled jawless fish akin toostracoderms. Some naturalists even suggested that they were shelled invertebrates or eventurtle-like vertebrates.
In the late 1920s, Dr.Erik Stensiö, at theSwedish Museum of Natural History inStockholm, established the details of placoderm anatomy and identified them as true jawed fishes related tosharks. He took fossil specimens with well-preserved skulls and ground them away, one tenth of a millimeter at a time. After each layer had been removed, he made an imprint of the next surface inwax. Once the specimens had been completely ground away (and so destroyed), he made enlarged, three-dimensional models of the skulls to examine the anatomical details more thoroughly. Many other placoderm specialists thought that Stensiö was trying to shoehorn placoderms into a relationship withsharks; however, as more fossils were found, placoderms were accepted as a sister group ofchondrichthyans.
Much later, the exquisitely preserved placoderm fossils from Gogo reef changed the picture again. They showed that placoderms shared anatomical features not only with chondrichthyans but with othergnathostome groups as well. For example, Gogo placoderms show separate bones for the nasal capsules as in gnathostomes; in both sharks and bony fish those bones are incorporated into the braincase.[22][23]
Placoderms also share certain anatomical features only with the jawlessosteostracans; because of this, the theory that placoderms are the sister group of chondrichthyans has been replaced by the theory that placoderms are a group of basal gnathostomes.
Currently, Placodermi are divided into eight recognizedorders. There are two further controversial orders: One is themonotypic Stensioellida, containing the enigmaticStensioella; the other is the equally enigmaticPseudopetalichthyida. These orders are considered to be basal or primitive groups within Placodermi, though their precise placement within the class remains unsure. Fossils of both are currently known only from theHunsrucklagerstatten.
Arthrodira ("jointed neck") were the most diverse and numerically successful of the placoderm orders, occupying roles from giantapex predators todetritus-nibblingbottom dwellers. They had a movable joint between armour surrounding the head and body. As the lower jaw moved down, the head shield moved, allowing for a larger opening. All arthrodires, save forCompagopiscis, lacked teeth, and used instead the sharpened edges of a bony plate, termed a "tooth plate", as a biting surface (Compagopiscis had true teeth in addition to tooth plates). The eye sockets are protected by a bony ring, a feature shared by birds and someichthyosaurs. Early arthrodires, such as the genusArctolepis, were well-armoured fishes with flattened bodies. The largest member of this group,Dunkleosteus, was a true "superpredator" of the latest Devonian period, reaching 3 to as much as 8 metres in length. In contrast, the long-nosedRolfosteus measured just 15 cm. Fossils ofIncisoscutum have been found containing unborn fetuses, indicating that arthrodires gave birth to live young.[24]
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Antiarchi ("opposite anus") were the second most successful order of placoderms known, after theArthrodira. The order's name was coined byEdward Drinker Cope, who, after incorrectly identifying the first fossils as being those of an armoredtunicate, mistakenly thought theeye-hole was the mouth, and the opening for the anal siphon was on the other side of the body, as opposed to having both oral and anal siphons together at one end. The front portions of their bodies were heavily armoured, to the point of literally resembling a box with eyes, with the sometimes scaled, sometimes naked rear portions often becomingsinuous, particularly with later forms. The pair ofpectoral fins were modified into a pair ofcaliper-like, orarthropod-like limbs. In primitive forms, such asYunnanolepis, the limbs were thick and short, while in advanced forms, such asBothriolepis, the limbs were long and had elbow-like joints. The function of the limbs is still not perfectly understood, but most hypothesize that they helped their owners pull themselves across the substrate, as well as allowing their owners to bury themselves into the substrate.[citation needed]
Brindabellaspis ("Brindabella's shield") was a long-snouted placoderm from theEarly Devonian. When it was first discovered in 1980, it was originally regarded as aweejasperaspidacanthothoracid due to anatomical similarities with the other species found at the same locality. According toPhilippe Janvier, anatomical similarities in the brain ofBrindabellaspis stensioi and the brain of ajawless fish suggest it is a basal placoderm closest to the ancestral placoderm. Various Early to Middle Devonian placodermincertae sedis have also been inserted in the order.
Phyllolepida ("leaf scales") were flattened placoderms found throughout the world. Like other flattened placoderms they were bottom-dwelling predators that ambushed prey. Unlike other flattened placoderms, they were freshwater fish. Their armour was made of whole plates, rather than the numerous tubercles and scales of Petalichthyida. The eyes were on the sides of the head, unlike visual bottom-dwelling predators, such asstargazers orflatfish, which have eyes on the top of their head. The orbits for the eyes were extremely small, suggesting the eyes were vestigial and that the phyllolepids may have been blind.
Ptyctodontida ("folded teeth") were lightly armoured placoderms with big heads, big eyes and long bodies. They have a strong but superficial resemblance to modern daychimaeras. Their armour was reduced to a pattern of small plates around the head and neck. Like the extinct and relatedacanthothoracids, and the living and unrelated holocephalians, most of the ptyctodontids are thought to have lived near the sea bottom and preyed onshellfish. On account of their lack of armour, some paleontologists have suggested that the Ptyctodontida were not placoderms, butholocephalians or the ancestors of holocephalians. Anatomical examinations of whole fossil specimens have shown that the similarities between these two groups are superficial. The major differences were that holocephalians haveshagreen on their skin, while ptyctodontids do not; the armoured plates and scales of holocephalians are made ofdentine, while those of ptyctodontids are made of bone; the craniums of holocephalians are similar to sharks, while those of ptyctodontids are similar to those of other placoderms; and, most importantly, that holocephalians have true teeth, while ptyctodonts have beak-like tooth plates. Ptyctodontids weresexually dimorphic, with the males having pelvicclaspers and possibly claspers on the head as well.
Rhenanida ("Rhine fish") were flattened,ray-like, bottom-dwellingpredators with large, upturned mouths that lived in marine environments. The rhenanids were once presumed to be the most primitive, or at least the closest to the ancestral placoderm, as their armour was made of unfused components—a mosaic of tubercles—as opposed to the solidified plates of "advanced" placoderms, such asantiarchs andarthrodires. However, through comparisons of skull anatomies, rhenanids are now considered to be the sister group of the antiarchs. When rhenanids die, their "mosaics" come apart, and it has been suggested that the rarity of rhenanids in the fossil record reflects postmortem disassociation, and is not an actual rarity of the species.
Acanthothoraci ("spine chests") were a group ofchimaera-like placoderms closely related to the rhenanid placoderms. Superficially, acanthoracids resembled scalychimaeras or small, scaly arthrodires with bluntrostrums. They were distinguished from chimaeras by a pair of large spines that emanate from their chests, the presence of large scales and plates, tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms due to differences in the anatomy of their skulls, and due to patterns on the skull plates and thoracic plates that are unique to this order. From what can be inferred from the mouthplates of fossil specimens, acanthothoracids were shellfish hunters ecologically similar to modern-day chimaeras. Competition with their relatives, the ptyctodont placoderms, may have been one of the main reasons for the acanthothoracids' extinction prior to the mid-Devonian extinction event.
Petalichthyida ("thin-plated fish") were small, flattened placoderms, typified by their splayed fins and numerous tubercles that decorated all of the plates and scales of their armour. They reached a peak in diversity during theEarly Devonian and were found throughout the world. The petalichthidsLunaspis andWijdeaspis are among the best known. There was an independent diversification event that occurred in what is now Southern China, producing a handful of unique genera that were once placed in their own order, "Quasipetalichthyida", named after the first discovered species there,Quasipetalichthys haikouensis. Soon after the petalichthids' diversification, they went into decline. Because they had compressed body forms, it is supposed they were bottom-dwellers that pursued or ambushed smaller fish. Their diet is not clear, as none of the fossil specimens found have preserved mouth parts.
Pseudopetalichthyida ("false petalichthyids") is a group of elongated, possibly flattened fishes comprising three, poorly preserved and poorly studied genera. It is known only from rare fossils in Lower Devonian strata inHunsrück, Germany. LikeStensioella heintzi, and theRhenanida, the pseudopetalichthids had armour made up of a mosaic of tubercles. LikeStensioella heintzi, the pseudopetalichthids' placement within Placodermi is suspect. The matter is not easy to resolve because there are no complete, undamaged and articulated specimens. The anatomical studies done on the crushed specimens that have been found indicate that if they are placoderms, they may be a group more advanced than theptyctodonts. As such, placoderm experts considerPseudopetalichthyida to be the sister group of theArthrodires +Phyllolepida +Antiarchitrichotomy and theAcanthothoraci +Rhenanidadichotomy.
Stensioellida ("[Heintz's] littleStensio") contains another problematic placoderm of uncertain affinity, known only from theLower DevonianHunsrück slates of Germany.Stensioella was a thin fish that, when alive, looked vaguely like an elongatedratfish, or a skinnyGemuendina with thin, strap-like pectoral fins. Similar to those of the Rhenanida, its armour was a complex mosaic of small, scale-like tubercles. The shoulder joints of its armour are similar to other placoderms, and there are superficial similarities in skull plates, and even more superficial similarities between its tubercles and the tubercles of therhenanids. It is tentatively placed within Placodermi as a primitive placoderm, though some paleontologists believe the rationale for the placement is inadequate. The paleontologistPhilippe Janvier, as well as other paleontologists, has suggested thatStensioella is not a placoderm, but instead is aholocephalian.[25][26] If this is true, then the holocephalians diverged from sharks before theChondrichthyanDevonian radiation. Critics of Janvier's position say that aside from abodyplan superficially similar to primitiveholocephalians, the two groups have little else in common anatomically.
The followingcladogram shows the interrelationships of placoderms according to Carret al. (2009):[27]
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However, the cladogram had changed significantly over the years, and the placoderms are now thought to beparaphyletic,[28] with some being more closer to theEugnathostomata than others. The updated cladogram (Zhu et al., 2016):[29]
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Giles, Sam; Friedman, Matt; Brazeau, Martin D. (2015-01-12)."Osteichthyan-like cranial conditions in an Early Devonian stem gnathostome".Nature.520 (7545):82–85.Bibcode:2015Natur.520...82G.doi:10.1038/nature14065.ISSN 1476-4687.PMC 5536226.PMID 25581798.
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| Jawless fish |
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