Psarolepis
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| Psarolepis Fossil range: Late Silurian to Early Devonian |
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 Psarolepis romeri
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| Psarolepis romeri Yu, 1998 |
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 Psarolepis range (in blue)
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Psarolepis (Greek: psaro = ψάρι ("fish") and lepis = λέπια ("scales"); meaning "Fish with scales") is an extinct, possibly basal sarcopterygian fish.
Many relatives of Psarolepis have been found with other groups of fishes and palaeontologists are not certain in which clade Psarolepis belongs.
Fossils have been found from Late Silurian (Pridoli) to Early Devonian (Lochkovian) in South China of Qujing, Yunnan.
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[edit] Description
At the front of the lower jaw, Psarolepis has two parasymphysical teeth. Also, Psarolepis has the skull and jaws like those of primitive lungfishes and other Devonian forms.
Psarolepis has a huge pectoral spine extending back from the shoulder girdle, and there is a median spine presumably located in the midline behind the head.
A pectoral spine like this is known in some placoderms and in acanthodians, and the median is known in sharks and acanthodians, but neither has been seen in other bony fishes.
[edit] History and classification
Psarolepis is the most primitive sarcopterygian fish, and probably the missing link between actinopterygians and sarcopterygians.
When Psarolepis was described for the first time, it was assigned to the sarcopterygians as result of the skull and jaws relations with the group of the sarcopterygian fishes. Zhu (et al., 1999) were unable to locate Psarolepis in the Cladogram with certainty. They thought it was either the basalmost sarcopterygian or the basalmost osteichthyan. The second interpretation has been confirmed as more likely by Zhu and Schultze (2001), because Psarolepis lacks the closed pineal opening and large quadratojugal of other osteichthyans, and it has a posterior pectoral process and paired pectoral spines, which are absent in other bony fishes. Long (2001) points out many similarities between Psarolepis and the Onychodontida, and places both as basal sarcopterygians.
[edit] Fossils
Most of the Psarolepis specimens were found at the same locality in Qujing of the Yulongsi Formation (Pridoli), the Xishancun Formation (early Lochkovian) and the Xitun Formation (late Lochkovian).
[edit] Characteristics
[edit] The skull
The head of the Psarolepis was made of several thick dermal plates containing a layer of porcelain-like cosmine. The head was covered with deep pock-marks with large, irregular pores. The rostrum, a strange humped structure with sharp ventral curvature and anterior nares is situated above the orbit. It seems that Psarolepis had a shark-like form with an elongated rostrum and subterminal mouth. The anterior nares possibly contained a dermintermedial[1] process. This is significant, since Psarolepis clearly had two posterior nares and lacked a choana. This proves that the choana developed not by division of the anterior nares but by the migration of the posterior nares onto the palate.
The maxilla, which bears the teeth, is under the median rostral, and appears to reach the orbit. It is difficult to surmise the exact osteology of Psarolepis because the thick cosmine layer obscures most suture lines. It is sure that the premaxilla is largely a palatal bone. Also, Psarolepis had a sharp, horizontal dermal intracranial division. The otoccipital[2] shield is quite short.
Besides the ventral part of the otoccipital region, the braincase of Psarolepis is very well ossified. One consistent feature of the early sarcopterygian palate is the small, thick parasphenoid pierced by a prominent, central hypophyseal canal and flanked by foramina for the internal carotids.
An open bucco-hypophyseal[3] canal is present, although, this canal is closed in almost all living gnathostomes. Another feature is the deep internasal cavities on either side of the sphenoid. These provided room for the large parasymphysical tooth whorls which are now believed to be basal osteichthyan. Although this feature is often found in chondrichthyans, it is not known from the most basal forms.
[edit] Snout and skull table
The anterior of the snout curves down and even posteriorly. The anterior nares are in anterodorsal position and the bone above anterior nares has a ventral projection like dermintermedial process. Also, the posterior nares are ventrally enclosed by dorsal lateral flanges of premaxilla which meet medially under the medial rostral.
The median rostral participates in the jaw margin. The maxilla dorsal and ventral expands posteriorly and in broad contact with the preopercular. The skull roof is flat except for eminence around the anterior nares. The otoccipital shield is rectangular and short. The sphenethmoid[4] shield reaches back to the trigeminal exit, far behind the basipterygoid process and the notochordal pit.
[edit] The palate
The upper jaw margin is M-shaped in both anterior and palatal views. The premaxillae meet medialy forming a small part of the anterior palate behind the median rostral teeth. There are large internasal cavities for "parasymphysical tooth whorls"[5]. The internasal cavities flank the dorsolateral margins of parasphenoid (which is small and rhomboid) and extending posteriorly beyond it.
The parasphenoid has a median longitudinal gutter which contains the hypophyseal canal, so that the sides project ventrally beyond the central region. In Psarolepis, the parasphenoid ascending process is absent. Finally, in the palate, the internal carotid does not penetrate the parasphenoid.
[edit] The lower jaw
In some Silurian specimens, the left dentary overlaps right at the ventral part of the symphysis. There is a postsymphysial pit for the ligament to retract the tooth whorls. The dentary is elongated with four infradentaries.
There are three large foramina which mark the boundaries between infradentaries. The infradentary fossae continued posteroventrally by furrow for articulation of the submandibular. The lower jaw has at least five coronoids. The kidney-shaped articular surface is divided by a ridge, probably for double-headed quadrate.
[edit] Dentition
The teeth had very free pulp cavity. The median rostral was toothed, with large fangs on tooth plate. There are parasymphysical tooth whorls. The premaxilla and the dentary had large inner teeth and irregular array of tiny outer teeth.
[edit] The braincase
The endocranium[6] is almost completely ossified, including the periosteal[7] bone layer. The otoccipital region is very short and the ventral part is unossified or missing. The hyomandibular[8] is single-headed, based on only one known articulation on the braincase. The posterior entrance to notochordal pit is ventrally lined with curved grooves.
The basipterygoid process[9] is poorly developed. The postorbital (posterior-orbital) pillar, with posterior and dorsal branches flanks the lateral cranial wall and jagular[10] foramen. The intracranial joint is slanted, running through the trigeminal foramen antero-ventrally.
The wide ledge under the orbit bears the palatoquadrate[11] articulation. The complete, vertical post nasal wall seals off internasal cavities. The sphenethmoid with long lateral anterior buttresses reaches the premaxilla. Also, the sphenethmoid has a large, open internal carotid foramina posterior to the parasphenoid. The sphenethmoid has long, thin internasal cavities for tooth whorls.
[edit] Orbit and cheek region
The premaxilla reaches the orbit with no separate lacrimal element. The ridged sphenethmoid is above the orbits. Also, the posterior and the dorsal rims of the preopercular have three large foramina.
Little is known of the cheek region. The homologies in all early sarcopterygians are unclear between the jugal, quadratojugal, squamosal, preopercular and opercular which cover the gill region. The maxilla and the dentary are short anteriorly but expand behind the orbit.
[edit] Fin spines
The most spectacular findings were the fin spines. Two are known: a posteriorly-directed pectoral spine associated with the scapulocoracoid and another believed to be associated with the anterior dorsal fin. As result of this, fin spines now are believed to be primitive in gnathostomes.
The fin spines are apparently absent from the forms believed to be the most primitive chondrichthyans, but present in abundance in more derived forms.
[edit] Relationships
The cheek bones and the shoulder girdle, however, seem to share features of both osteichthyan groups.
In the skull, Psarolepis shows a mix of actinopterygian and sarcopterygian features. In the lower jaw, for example, the teeth are sarcopterygian in showing multiple infoldings and in bearing cosmine. The fossil of Psarolepis from the Upper Silurian to Lower Devonian has shown related features of both groups.
In the cheek region, Psarolepis has actinopterygian features: The squamosal elements, seen in sarcopterygians, are absent, and the canal running along the preopercular bone has a ventral extension towards the maxilla. The three large openings in the preopercular bone (foramens), near the canal, however, are more sarcopterygian like.
Psarolepis has a huge pectoral spin extending back from the shoulder girdle, and there is a median spine presumably located in the midline behind the head. A pectoral spine like this is known in some placoderms and in acanthodians, and the median is known in sharks and again in acanthodians, but neither has been seen in other bony fishes.
[edit] References
- Benton, M. J. (2005). Vertebrate Palaeontology (Third Edition). Blackwell Publishing.
- Palaeos Vertebrates 140.100 Sarcopterygii [1]
- Nature 397, 607-610 [2]
[edit] Notes
- ^ Dermintermedial process: A bump on the floor or wall of the naris in some sarcopterygians.
- ^ The otoccipital: In sarcopterygians and basal tetrapods, relating to the posterior half of the divided braincase, containing the otic and occipital regions. The anterior half is referred to as the ethmosphenoid or sphenethmoid.
- ^ Bucco-hypophyseal canal: A short canal linking the pituitary and digestive tract.
- ^ The sphenethmoid: The combined sphenoid and ethmoid regions of the braincase. The anterior half of the braincase, physically separate from the posterior, otoccipital unit in sarcopterygian fishes.
- ^ Parasymphysial tooth whorl (or spiral): A possible gnathostome synapomorphy.
- ^ The endocranium covers the ventral, lateral and posterior parts of the brain as well as closely related sensory structures.
- ^ The periosteal is an acellular bone which coats the surface of endochondral bones.
- ^ The hyomandibular: The upper main element of the hyoid arch. The same bone as the stapes, columella, and epihyal.
- ^ The basipterygoid process joins the braincase to the palate.
- ^ The jagular: An important landmark on the occiput. It is a small hole where the jugular vein exits the braincase.
- ^ The palatoquadrate: The primitive upper jaw formed from the hypothetical mandibular arch.
[edit] External links
- Psarolepis at Palaeos
- Psarolepis classification - The Taxonomicon
- A primitive fossil fish sheds light on the origin of bony fishes - Nature 397, 607-610 (1999)
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