| Qilinyu | |
|---|---|
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| Artist's restoration | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | †Placodermi |
| Order: | †incertae sedis |
| Genus: | †Qilinyu Zhuet al., 2016 |
| Species: | †Q. rostrata |
| Binomial name | |
| †Qilinyu rostrata Zhuet al., 2016 | |
Qilinyu is a genus of earlyplacoderm from the lateSilurian (lateLudfordian stage, ~423 Ma) of China. It contains a single species,Qilinyu rostrata, from the Xiaoxiang fauna of theKuanti Formation. Along with its contemporaryEntelognathus,Qilinyu is an unusual placoderm showing some traits more similar tobony fish, such as dermal jaw bones and lobe-like fins. It can be characterized by adaptations for abenthic lifestyle, with the mouth and nostrils on the underside of the head, similar to the unrelatedantiarch placoderms. The shape of the skull has been described as "dolphin-like", with a domed cranium and a short projectingrostrum.[1]
Qilinyu rostrata is based on several well-preserved specimens found in theQilin district ofYunnan, China. Theholotype specimen is the most complete, representing a set of fully articulated head and trunk armor missing only the dentaries (lower jaws). All specimens were collected from the Xiaoxiang fauna, alagerstätte in theKuanti Formation. Fossils of the Xiaxiang fauna were excavated over several trips from 1999 to 2016, and are now stored at theInstitute of Vertebrate Paleontology and Paleoanthropology (IVPP). They include a number of ancient fish, includingEntelognathus, a relative ofQilinyu described in 2013.[1]
Conodont biostratigraphy has indicated that the fauna was deposited in the later part of the Silurian, specifically the late Ludfordian stage near the end of theLudlow Epoch. In numerical terms,Qilinyu rostrata is about 423 million years old.[1]
Qilinyu is named after the Qilin district, which in turn is named after theQilin, a legendary beast inChinese mythology. Thechimeric nature of the mythological Qilin is referenced by the combination of placoderm-like and osteichthyan-like traits apparent inQilinyu. The species namerostrata refers to the distinctive rostrum.[1]
The preserved armored head and trunk region ofQilinyu measures 12.6 cm (5.0 inches) in length. Like other placoderms, the armor is separated into head armor (i.e., the bony component of the skull) and trunk armor (a complete ring of plates behind the skull). The armor is ornamented with oval-shaped tubercles.[1]
The skull has a dolphin-like profile, with a domed forehead tapering to a short triangular rostrum (snout). The rostrum projects forwards while the jaws and nostrils are completely recessed under the head, akin to askate orsturgeon. Theorbits (eye sockets) are small, widely spaced, and shifted forwards to the base of the rostrum. Ascleral ring of three bones surrounds each orbit.[1]
LikeEntelognathus,Qilinyu can be described as a “maxillate placoderm”. This means that it has three slender bones on either side of the jaw: thepremaxilla at the front of the upper jaw, themaxilla at the side, and thedentary along the rim of the lower jaw. This contrasts with most other placoderms, which have gnathal plates (internal blade-like bones attaching to jaw cartilages) rather than external jaw bones. As a result,Entelognathus andQilinyu more closely resembleosteichthyans (bony fish andtetrapods) in the structure of their jaw. There is no evidence for additional internal jaw bones such ascoronoids ordermopalatines, which supplement the marginal jaw bones of bony fish.[1]
Another difference from bony fish is that the three external jaw bones ofQilinyu are completely toothless and have an L-shaped cross-section. They have both a narrow vertical surface (facial lamina) visible externally, and a broader horizontal surface (palatal lamina) which is bent inwards at the rim of the mouth. All three bones have this condition inQilinyu. Conversely, the dentary ofEntelognathus is blade-like, and only the premaxilla and maxilla have a palatal lamina.Qilinyu also lacksinfradentaries (bones at the lower edge of the jaw) or gular plates (throat armor), in contrast toEntelognathus and bony fish. This means that the dentary is the only component of the lower jaw inQilinyu, apart from a small cartilaginous gap at the chin.[1]
The upper side of the rostrum is shielded by arostral plate, succeeded along the midline of the skull by a broadly slopingpineal plate (between the eyes), then pairedcentral plates, and finally a narrownuchal plate with a pointed rear tip. The underside of the rostrum is encased by thepremedian plate (a large, flat, diamond-shaped bone) and smallpostnasal plates at the side of the nostrils. A broad groove extends from each nostril to the tip of the rostrum.[1]
Behind the eye, the lateral surface of the skull is dominated by two bones: theanterior postorbital plate (at the front) and thepostmarginal plate (at the back). The anterior postorbital plate is smaller and sends forward a slender strip of bone at the upper rim of the orbit, while the much larger postmarginal plate makes up most of the cheek region. These two plates are divided by a deep notch which presumably hosted aspiracle. The largest bones in the skulls are theposterior paranuchal plates, each of which forms the rear edge of the head armor, between the nuchal and postmarginal plates. Three fairly small elliptical bones (thepostorbital,marginal, andanterior paranuchal plates) extend back in a row above the postmarginal plate. Apart from the general shape of the head,Qilinyu’s head armor differs fromEntelognathus in the presence of paired central plates (rather than singular), two postorbital plates on each side of the skull (rather than one), and scleral rings unfused to the skull (rather than fused).[1]Qilinyu differs fromBianchengichthys in possessing a longer rostrum while lacking dentary denticles, a lateral line branch on the postmarginal, and a preorbital plate on either side of the rostral.[2]
Qilinyu,Entelognathus, and bony fish all have three distinctive non-jaw bones flanking the upper jaw: thelacrimal,jugal, andopercular. InQilinyu, the lacrimals are hidden under the rostrum and link between the equally hidden nostrils and upper jaw bones. The jugal is visible on the side of the skull, directly above the maxilla and behind the eyes. The opercular is a slender gill cover which directly attaches to the back of the jugal rather than a cartilaginous intermediary. It also has a deflected internal lamina akin to the jaw bones. In both respects, the opercular most closely resembles that ofyunnanolepidid antiarchs.[1]
The mainlateral line groove extends from the postorbital straight back across the marginal and paranuchals. At its front terminus it sharply bends down as the infraorbital line groove, which passes behind the eye along the anterior postorbital and jugal. An additional lateral line groove (the posterior pitline) branches inwards from the anterior paranuchal to the central plate.[1]
Like other placoderms,Qilinyu has extensive trunk armor narrowly separated from the skull. The front edge of the trunk armor closely traces the rear edge of the skull along a series of grooves and ridges, with no major gap. There are threemedian dorsal plates in a row along the top surface of the trunk. The second plate is hexagonal and particularly elongated, and is most likely homologous to the main median dorsal plate of other advanced placoderms. It is smoothly convex, but otherwise the trunk armor lacks sharp ridges or crests. The third plate is an enigmatic feature which may have originally been an enlarged body scale, as seen inSigaspis. The first plate is also unusual; few other placoderms have a first median dorsal plate smaller than the second. The first plate is tentatively considered homologous to the (large) first median dorsal plate of antiarchs, as well as an unattached neck plate inEntelognathus (where it is known as a postnuchal),Sigaspsis, andEurycaraspis (where it is known as an extrascapular).[1]Silurolepis andBianchengichthys are the only other placoderms with median dorsal plate proportions similar toQilinyu.[3][2]
Four plates are primarily exposed on either side of the trunk: theanterior dorsolateral (upper front),anterior lateral (lower front),posterior dorsolateral (upper rear), andposterior lateral (lower rear).[1] Of these four, the anterior dorsolateral plate occupies the greatest area, in contrast toSilurolepis.[3] The anterior lateral plate is irregularly shaped and expands forward. It acts as the base for a prominent blade-likespinal plate which projects outwards from the side of the body. The lateral line continues from the skull onto the dorsolateral plates. The lower surface of the trunk has two small pentagonal midline bones (the anterior and posteriormedian ventral plates) surrounded by two pairs of large rectangular bones (the anterior and posteriorventrolateral plates). Both ventrolateral plates form a small contribution to the side of the trunk along a bent edge, and the posterior ventrolateral plates are much larger than their anterior counterparts.[1]
Qilinyu has a unique form of interlocking neck joint which has not been previously reported in placoderms. The front edge of each anterior dorsolateral plate hosts a U-shaped groove (the obstantic groove) edged by a pair of sharp ridges. The rear edge of the skull's posterior paranuchal plate fits into this groove, a form of articulation similar to that of antiarchs, where it is called a 'reverse ginglymoid' joint. Unlike antiarchs, the underside of the posterior paranuchal plate also has a shallow depression defined by a shelf-like ridge. This allows the lower ridge of the trunk armor to slot into the skull, similar to the 'ginglymoid' neck joint of certain arthrodires. The 'dual articulation' ofQilinyu may also be present inSilurolepis (based on its trunk armor anatomy)[3] andBianchengichthys (based on the external appearance of its head and trunk armor).[2]
Qilinyu was the first maxillate placoderm to be discovered with preservedpectoral andpelvic fins. The pectoral fins fit into a broad gap excavated between the anterior lateral and anterior ventrolateral plates. The pelvic fins attach to a small isolated plate behind the trunk armor. The base of each fin is a fleshy lobe covered with small scales, though the trailing edge of the fin was presumably broad and naked.[1]
Qilinyu rostrata, together withEntelognathus, demonstrates additional evidence that modern gnathostomes evolved from placoderms.[1][4]
Maximum parsimony andBayesianphylogenetic analyses in the original description by Zhu et al. (2016) placedQ. rostrata as thesister taxon to a clade containingEntelognathus,Janusiscus, andcrown-group gnathostomes (i.e.,bony andcartilaginous fishes and their descendants).[1] A redescription ofSilurolepis by Zhu et al. (2019) found thatQilinyu andSilurolepis were sister taxa, though otherwise the results were the same as the earlier paper.[3]Bianchengichthys, a maxillate placoderm described by Li et al. (2021), is also similar toQilinyu. It occupies apolytomy withQilinyu,Silurolepis, and crownward taxa.[2]
Below is acladogram showing the results of Zhu et al. (2016):[1]
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