Psarolepis
A somewhat free reconstruction of Psarolepis. Length of the individual 25 cm
- Asia (China, Vietnam)
- Psarolepis Romeri YU 1998
Psarolepis is a very original genus of meat -finned fishes ( Sarcopterygii ) from the latest Silurian and earliest Devonian. The type and only known species is Psarolepis Romeri. Their fossilized remains were first discovered in 1984 in China ( Yunnan Province) (X. Yu 1998) and are among the oldest almost complete findings of Sarcopterygiern.
Etymology
The genus name is derived from Psarolepis ψαρός of the ancient Greek words ( Psaros ), which means " star " "Read pied " in the sense of " speckled " means and λεπίς ( Lepiš ) for fish scale. The epithet of the type species is a reminder of the anatomist and paleontologist Alfred Sherwood Romer ( 1894-1973 ).
Significance of the find
Psarolepis was the first Fund, which was received well enough to give finally an idea of the whole pristine Sarcopterygiern, as these were still quite similar to the contemporary representatives of actinopterygians, their sister group. This sister group relationship was isolated because of findings of traditional teeth, spines and scales ( Ichthyolithen ) for some time well protected, but now it was possible to make such around 415 million years ago (ie obersilurisch - Devonian ) to examine fish whole. Meanwhile, fossils have been identified from Vietnam as Psarolepis and only the tail region is still unknown. Initial positions hope Psarolepis will bring a lot of light in the lineage relationships of the basal fish, has not fulfilled. Since 2009, we know from the same locality in southern China the last approximately five to ten million years older than sarcopterygians Guiyu oneiros, who also again clearly shows some characteristics of Actinopterygiern.
Features
The most striking feature of the Psarolepis is a tight group of long parasymphysialer teeth at the tip of the lower jaw. The whorl opposite the unpaired rostral has moved slightly in the upper jaw. Such Zahnwirtel you already knew, in unauffälligerer expression of Onychodontiformes, which one has therefore Psarolepis also attributed to long (Long 2001). Similar Unterkieferbezahnung can be found in the tart about Umberfischgattung Macrodon, it can be as an adaptation to the crustacean diet ( eg crabs - then perhaps Merostomiden ) indicate. Zahnwirtel at the tip of the instrument had long known particularly from fossil sharks. The rest of the mouth edge teeth are smaller ( of different sizes ). The dentition in the oral cavity itself is at best insignificant ( bruising ). The teeth themselves are from polyplocodonten type, that is, the dentin is inside crinkled in a characteristic manner as with other early Sarcopterygiern also and still (similar ) in the early tetrapods, the " Labyrinthodontia ". The gill basket was probably cartilaginous, which is why he is not delivered even with similar findings. Based on the narrow operculum can be derived, however, that there was little space available for the gills. The oxygen consumption, i.e., the activity of the fish was apparently generally less than the current.
Remarkable are also rigid bone spines in front of the pectoral fins and the back fin, with the exact location can not be clearly established. This feature is also unusual, both for Sarco as well as for actinopterygians, and can be found rather in placoderms, cartilaginous fish and Acanthodii. As an indication of a closer relationship with these groups, however, this can not be used because it may very well be apomorphies, similar to recent armored catfishes ( Loricariidae ). At Actinopterygierverhältnisse remember especially the bones of the snout and lower jaw ( five Infradentalia; Ahlberg 2001).
The skull, front and top is largely ossified, as with all original Sarcopterygiern, clearly divided into two equal parts as large: Ethmosphenoid and Otoccipitalblock. This improves the probability of success at catching prey, but is free for swimming adversely, so this condition does not occur with the Actinopterygiern, though it may have been present in the common ancestor of all Osteichthyes. The intracranial joint is located at the level of the exit of the trigeminal nerve from the brain. The Onychodontiformes were so pretty sure Benthopelagic ( and had to probably a swim bladder ). The small eyes are located to the side of the nose, just behind the Praemaxillaria - this position is maintained in Palaeozoic fish for a long time. The " rear " Narinen ( nostrils ) were inside before the eyes, between rostral and premaxilla, both of which were strongly forward bent downward. Internal nares were missing in the oral cavity ( choanae ). Ventrolaterad the nasal sacs, the paired Rostralorgan was for the electrical detection of prey. The " premaxilla " seems merged with the " palatine ". The short, wide parasphenoid has a central foramen ( "hole" ) for the derivation of the Hypophysensekrets and two lateral foramina of the aortic roots. The labyrinth region was ventral not ossified. The Ethmosphenoid Shield (which can not be further differentiated ) has dorsally on a large Parietalforamen. Psarolepis and the somewhat younger and clearer " sarcopterygische " Achoania, possessed, as well as more original Gnathostomen early actinopterygians, eye stalks ( pedunculated cartilage cup in the orbit, eyeball - to flutter; tart in sharks; Zhu, Yu and Ahlberg 2000).
The mouth was as usual deeply divided, (not included! ) The hyomandibular oriented backwards and down and how the front part of Suspensoriums quite weak. Since the two heads on Hyomandibel Otoccipitalblock steered, the front skull was when opening the jaw "automatically" lifted - sets him yes no dorsal musculature; Basicranialmuskeln the other hand, were active when closing the jaw. The lower jaw has been advanced here ( he guided indirectly by the [ fossil not preserved ] palatoquadrate at the front skull: you can still see the joint sockets ) similar to Malacosteus ( the " Fang mask bat " - of course not so excessive, because of the Gularplatten ). However, the decomposition of the skull is not a very good solution to the old problem, jaw opening and swimming direction to bring in the open water to cover - but it does not require complex nervous feedbacks in the area of the maze.
Since the skull was squashed during diagenesis by the compaction of the surrounding sediment, the skull bones are clearly only partially in their relative positions. Of the recent fish shapes most similar to him the skull of Polypterus ( Flösselhecht, the " primitive " living actinopterygians ). The " squamosal ", a cover of the bone sarcopterygians which is the Praeoperculare the actinopterygians teilhomolog, shows portions of the lateral line canal in the characteristic sarcopterygians way. The cover bones have a thick layer of enamel. They are perforated by numerous pores, which are likely to be passages for nerves of the skin senses. This gives the skull of Psarolepis a " pockmarked " look. The shoulder girdle shows a certain similarity to that of placoderms, but received only incomplete. The hull of Psarolepis was of elongated Cosmi scales ( smaller than in the above figure! ) Covered gephyrocerk the tail fin highly probable.
System
Psarolepis is a basal representative of the sarcopterygians. In his first description has been speculated that Psarolepis may be older than the separation of the two main lines of bony fish, but this is true even for not too Guiyu. The systematic position within the basal sarcopterygians according to current knowledge situation shows the following cladogram:
Rhipidistia, including lungfish ( Dipnoi ) and terrestrial vertebrates ( Tetrapoda )
† Styloichthys
Coelacanth ( Coelacanthiformes )
† Onychodontiformes
† Achoania
† Psarolepis
† Guiyu
† Meemannia
† Ligulalepis