Iniopterygians: The best and weirdest chondrichthyans ever!

It is impossible to currently find any extensive popular science literature on the Iniopterygians. If you type their name into google it is very hard to even find any technical publications about them. I intend fill in this small niche in the Internet by doing a post all about this fascinating and weird group of fish. Iniopterygians were a group of chondrichthyans (shark relatives) that lived in the Carboniferous period of the late Paleozoic era. They are generally preserved as flattened specimen in black Pennsylvanian shales, but more recently have been found as 3 dimensional fossils in 300 million year old concretions from Kansas and Oklahoma. Iniopterygians have rather shortened snouts, sometimes adorned with tubercles. Their pectoral fins articulate far dorsally to the scapulacoracoid and in males the anterior ray of the pectoral fin bears denticles that form a rasp like surface. The patterning and structure of the clasper mechanism in the pelvic region of these fishes is usually the distinguishing feature between species. The paired clasper mechanism functions a bit like a penis; and are a great way for palaeontologists to distinguish males from females. Females lack these structures; respectively. The iniopterygians were initially placed with the holocephalans (the group of chondrichthyes that contains creatures like the chimera and rabbit fish). This was due to an observation in the iniopterygian genus Sibrynchus that the palatoquadrate (upper jaw) was autostylic; a feature found in modern holocephalans and not elasmobranchs. That is; the palatoquadrate was fused to the neurocranium (braincase and snout). Elasmobranchs such as sharks have non autostylic palatoquadrates. This allowes their upper jaw to extend forewards and backwards during a bite sequence. If you watch a great white shark bite you can see the non autostylic jaw articulation at work as the upper jaw extends forewards. However more recent work by Sthal 1980 suggests that 2 known iniopterygians did not have autostylic jaw articulations. She found that in Iniopteryx and Promexyele there was a projection on the posterolateral face of the neurocranium that articulates with the separate palatoquadrate. It therefore seems that not all iniopterygians were autostylic and that opened up the possibility that the iniopterygians evolved autostyly seperately from the chimaeriformes. It also suggests that autostyly is not a diagnostic character of the iniopterygia or the holocephali as was previously suggested. The Iniopterygia genera come mainly from Pennsylvanian black shales of North America. These represent coal forests that became inundated by oceanic waters. The mud was toxic and contained high levels of hydrogen sulfide, so most of the animals probably inhabited the water column and floating vegetation. The unusually large number of male specimens from these black shale environments suggests that this was not the usual environment for the iniopterygians, and that most of the time they inhabited deeper oceanic waters. It is clear that the males must have had more of a tendency to enter these environments than the females. I have heard (read) of indications that iniopterygians may have glided like flying fish, but because of the proportionally small tail fins and heavy denticles and teeth these animals are unlikely to have done so. They presumably moved like modern chimaeroids; with dorsoventral movements of their enlarged pectoral fins. The diet of these fish was presumably predatory as indicated by their sharp teeth. The varied dentition of sibrhynchids suggests that they may have been more generalised and opportunistic than the iniopterygids. Basic systematics covering the most well known genera and species: Iniopterygia: Small chondrichthyes (about 30 cm in length) with large dorsally articulating pectoral fins. The anterior fin ray is the thickest. Snout is short. Tail thin and caudal fin small and circular in lateral view. Males posses denticles on the anterior rim of the pectoral fin. The operculum (bone supporting the gill flaps) possesses radials emanating from its posterior margin. Iniopterygidae: Generally non autostylic upper jaw. Dentition mainly consisting of triangular denticles. Lower jaws unfused at symphasis. Iniopteryx rushlaui: This was a rather unornamented iniopterygian; lacking external rugosities on the head and skin. Its mouth was full of small pointed single cusped denticles arranged in labial, lingual rows. In the symphasis of the lower jaw was a simphyseal tooth worl consisting of unfused denticles. The pectoral fin of Iniopteryx males consisted of a broad, rectangular basipterygium that articulates dorsally to the scapulacoracoid. Posterior to this is a smaller basal element bearing a small antler like complex as in Cervifurca. The anterior fin ray is the thickest of the fin rays and in males bears a series of 13 curved denticles on its anterior margin. The clasper consists of one proximal long cartilage rod followed by a series of smaller sections that taper to a point. Iniopteryx is interesting because one specimen has a trace of the gut preserved. It is a spiral gut; a key feature of chondrychthians. The visceral mass contains plant fragments, conodonts and arthropod fragments; suggesting a varied diet. It seems that Iniopteryx was an oppourtunust; feeding on whatever it could find. The small teeth suggest that it was not adapted to feeding on large prey. Promexyele peyeri: In overall body form Promexyele is similar to Iniopteryx; however the teeth are mainly tricusped. The pectoral fins on the males have many more denticles than those of Iniopteryx. They are proportionally larger, but consist of fewer and thinner rays. This suggested to Zangrel and case that the fins were more flexible and therefore resistent to lower forces; making Promexyle slower moving than Iniopteryx. The claspers consist of 5 short sections of cartilage followed by a single, long tapering rod tipped with small hooks. Cervifurca nasuta: This is a very unique iniopterygian that presumably lived on the sea floor. It had remarkably enlarged pectoral fins and a dorsoventrally flattened head. The nasal capsules were enlarged considerably. The eye sockets were not on the side of the head as in other iniopterygians; dorsally rimmed by a supraorbital shelf, but dorsolaterally oriented as in batomorphs for keeping a look out for predators cruising above. Cervifurca has been placed in the iniopterygidae because of its non autostylic palatoquadrate and lack of fused tooth whorls. The teeth are generally single cusped and possess enormousley wide roots. The pectoral fin is proportionally large and has 2 basipterygia (a feature also seen in Iniopteryx rushlaui). The anterior fin ray is constricted considerably in its most distal half. The most posterior fin ray spans out into an unusual "antler" complex.

Sibrhynchidae: Lower jaws fused at symphasis. Dentition mainly consisting of labial-lingual tooth rows fused into whorls. Autostylic palatoquadrate: May be an adaptation along with tooth plates to dealing with hard shelled prey. Sibrhynchus denisoni: Sibrhynchus has a unique display of hetertodonty that is very interesting. In the front of the lower jaw is a single pointed denticle. At the front of the upper jaw is a flat denticular plate; either side of it are 2 pointed denticles. Behind it are symmetrical rows of tooth whorls. 6 on each side in the upper jaw and lower jaws. On the lower jaw is a symphyseal tooth whorl behind the front pointed denticle. On the palate and mucos membrane are sets of flat denticular plates. This heterodonty suggests a very varied diet: With the front tooth on the lower jaw perhaps being used to dislodge molluscs or sessile prey from hard surfaces. The tooth worls may have been used for cutting and grabbing animal prey and the denticular plates may have been for crushing arthropods and other shelled creatures. All in all Sibrhynchus seems to be a very well adapted oppourtunist. Amazingly 300 million year old concreations containing 3d skulls of this genus have been discovered and scanned. One specimen preserves an mineralized brain. Many features of the skull relating to the patterning of foramina on the skull and other neurologcal details suggest that the iniopterygia were members of the holocephala. The mouth line is slanted upwards as indicated by Zangrel and Case and proven by the 3d specimens from Kansas and Olklahoma. The pectoral fins consist of about 6 rays. The anterior ray in males is proportionally less broad than that in Iniopteryx or Promexyele, but still has a series of pointed denticles on its anterior surface. The claspers consist of one relatively long cartilage rod, followed by 5 shorter sections and finally a long tapering rod at the end. Iniopera richardsoni: This is a very unusual iniopterygian from the black shale distinguished by a double symphyseal tooth whorl on the lower jaw. It also seems to have posessed small denticles on the skin that have a snowflake appearance. It is to my knowledge the only iniopterygian known to possess postcranial denticles. The most interesting part of this animal is its shoulder girdle and pectoral fin. The pectoral fin consists of 8 rays. The anterior ray has denticles on its anterior margin as in other iniopterygians, but is not the longest ray on the fin. The third ray is the longest. Associated with the pectoral fin is a sack containing a fossilised substance that appears to be opaque to x rays. These sacs were presumably attached to the fins and may have contained some kind of ink similar to the ink used by squid and octopi for defensive purposes. The substance appears to be fiberous. The clasper mechanism consists of 4 basal cylindrical elements followed by a long cartialage rod, and finally a long tapering element consisting of dentine or bone (also seen in a little known iniopterygian; Inioxyle). On this element can be seen the central canal through which the sperm would have passed. Recently 2 new iniopterygians were described in 2009 by Grogan and Lund. I have yet to read the full paper, but the abstract of the paper names them as Rainerichthys zangreli and Papilionichthys stahle. There is a good specimen photo of Rainerichthys in the book; "Rise of Fishes" second edition by John Long. For more information see: Skull and brain of a 300 million year old chimaeorid fish revealed by synchroton holotomography (Pradel and co 2009), Cervifurca nasuta an interesting member of the iniopterygidae from the pennsylavnian of Indiana (Zangrel 1997), Iniopterygia, a new order of chondrichthyan fishes from the pennsylvanian of North America (Zangrel and Case 1973), Non-autostylic pennsylvanian iniopterygian fishes (Stahl 1980), Two new iniopterygians (Chondrichthyes) from the Mississippian (Serpukhovian) Bear Gulch Limestone of Montana with evidence of a new form of chondrichthyan neurocranium (Grogan and Lund 2009).