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Piscivorenantiornis inusitatus is a bird, belonging to the Enantiornithes, which lived during the early Cretaceous in the area of present-day China. Piscivorenantiornis is the first species from the Enantiornithes thought to be a fish-eater. On the slab of stone showing the fossil, there is a spot with fish bones that the animal would have spit up at the time of its death.

Find and name
In 2016, the discovery was reported at Dapingfang, in Caoyang prefecture in Liaoning province, of the fossil of a fish-eating bird.

In 2017, the type species Piscivorenantiornis inusitatus was named and described by Wang Min and Zhou Zhonghe. The genus name links a reference to the piscivorous, i.e. fish-eating, lifestyle with belonging to the Enantiornithes. The species designation means "unusual" in Latin because a fish-eater had never been found within that bird group before.

The holotype, IVPP V22582, is believed to have been found in a layer of the Jiufotang Formation dating to the Aptian. It consists of a partial skeleton with skull, compressed on a single slab. The skeleton is not in bandages. The skull has fallen apart and pieces are missing. Also missing is the pygostyle, the fusion of the posterior caudal vertebrae. The fossil does contain a kind of vomit ball with fish remains. Due to the fragmentation of the skeleton, several loose parts are clearly visible.

A new specimen was described in 2020.

Size and distinguishing characteristics
Piscivorenantiornis'' is a relatively small enantiornithe. It has a wingspan of about forty centimetres.

The descriptors were able to establish some characteristics in which Piscivorenantiornis differs from other Enantiornithes. Some of these were autapomorphies, unique derived features. The anterior snout bones, the praemaxillae, are notched on the anterior surface of the furrow. The anterior facets of the centres of the caudal vertebrae are saddle-shaped: concave in lateral view and convex across. The lower edge of the anterior surface of the ilium is straight.

In 2020, an additional autapomorphy was reported: there are paired grooves next to the octopus, which is unique to the entire Aves.

In addition, there is a unique combination of features. On the sternum, the front edge has paired protrusions at the outer corners, while the troughs for contact with the raven bones are widely separated. The outer protrusions of the sternum's posterior edge end in large triangular protrusions that do not extend beyond the rear of the sword-shaped protrusion. The ilium has a prominent hump on the inside above the hip joint. The pubic bone has a large "foot" with a strongly curved backward point.

Skull
The shape of the head is not entirely clear due to the separation of the skull. The snout cannot have been very long. The paired praemaxillae are visible from below and show that the snout profile is tongue-shaped and not very pointed. In each praemaxilla, there are four regularly spaced small teeth. In front of the nostrils lie a pair of veinlets. The frontal bone is triangular with a tapering elongated anterior branch. In the quadratum, the inner lower articular tubercle is larger than the outer one. The quadratum does not seem to be pneumatized; in any case, a foramen at the back is missing. The occiput has detached as a whole and is visible from behind, albeit of course completely flattened. The latter prevents us from determining whether the apparent trapezoid shape of the large occipital hole is authentic. The bones of the occiput are seamlessly fused. The hollow exoccipitalia protrude laterally into small slender processus paroccipitales. At the bottom, short processus basipterygoidei extend laterally at right angles to the long basal phenoid. It is not clear whether they have been forced out of a normal oblique position by deformation.

Jaws
The dentarium of the left mandible has been preserved. It shows seven dentary teeth in side profile. The anterior tooth is quite small. The second is abruptly larger. The third and fourth are about the same height but have a wider base in lateral view. The second, third and fourth teeth are dagger-shaped with a backward-angled tip. The fifth, sixth and seven teeth are much smaller and form short triangles. They are also further apart. On the outside, the dentarium has a horizontal row of slit-shaped veins running along its upper edge. On the inside of the dentarium is the os spleniale. It is triangular in lateral profile with an elongated anterior branch about three times as long as the posterior portion. Fairly in front is an oval foramen on the upper edge. This was not considered homologous to that of more basal Theropoda because basal birds lack it. At the back of the mandible, the surangulare is relatively robust.

Postcrania
Nine separate cervical vertebrae have been preserved from the neck and the total was at least ten because the turner is no longer present. The first cervical vertebra, the annular atlas, has obliquely downward-pointing short protrusions for ribs at the long low vertebral arch, a basal feature. The ribs themselves were probably no longer there. The groove for the joint plane with the occipital protuberance is angled upwards. This surface is not notched but shows a central low ridge between lying oval hollows. The following cervical vertebrae can be clearly seen in front or rear view because they are loose. The anterior cervical vertebrae are keeled and have saddle-shaped joint surfaces on the anterior facet but hollow surfaces on the posterior. In some relatives, both facets are heterocoelous. In the posterior cervical vertebrae, the centre is wider than the top. The front joint facet is strongly heterocoelous and the curvature is exactly the reverse of that of modern birds: concave from top to bottom and convex across. The curvature thus corresponds to that of the back of modern birds and is also unknown in all other fossil birds. The shape of the vertebrae, however, seems to rule out the possibility that the front and back were mistaken. In many Enantiornithes, the morphology of the cervical vertebrae cannot be checked because the neck is still bandaged or the vertebrae are only visible from the side. The function of this inversion was unclear to the describers. However, it would not have affected the mobility of the neck.

The, at least nine, vertebrae do not show any pleurocels. They have the centrally located parapophyses, anterior rib facets, typical of Enantiornithes. The synsacrum includes seven or eight sacral vertebrae. They have a longitudinal groove on the underside. Towards the back, the orientation of the lateral protrusions changes from perpendicular to oblique.

Six caudal vertebrae have been preserved. These are the "loose" vertebrae in front of the fused pygostyle that is missing from the fossil. These anterior caudal vertebrae do not have posterior joint protrusions, but the anterior joint protrusions protrude forward by half a centimetre. The lateral protrusions extend backwards at an angle and each is longer than the width of the vertebral body.

At the scapula, the upper end is bent backwards instead of straight. The processus acromialis has a crescent-shaped facet for contact with the wishbone. This furcula is Y-shaped with the anterior tip, the hypocleidum, having one third of the length of the branches that make an angle of 50° with each other. The sternum, which has become one large bony plate, is clearly visible and forms a complex structure that has its own specific shape in each species of Enantiornithes. In Piscivorenantiornis, the anterior margin does not have a sharp projection in the form of a central point or cristospina. However, the anterior edge forms a blunt point. The anterior edge does not gradually bend into the lateral edges, but in a sudden bend so that there are sharp anterior angles. The sides have a slightly concave profile. Towards the back, they run out in long protrusions, the "trabeculae laterales". These are widened at their ends, but the widening is mainly on the inside. This results in sharp, inclined points that are directed inwards and backwards. Between these outer protrusions, the hind edge has two large inlets that run for about 40% of the length of the sternum. On the anterior edge of each inlet is a second, much smaller, protrusion or trabecula medialis, forming an inward slanting tooth. Compared to many of its relatives, it is relatively short and situated close to the lateral protrusion. In the middle of the posterior margin is a central sword-shaped protrusion, the xiphoid which, however, as the sternum is actually made up of paired sterna, is also seen as a fusion of two median trabeculae. In Piscivorenantiornis this extends as far back as the lateral protrusions and ends in a gradual widening that is cut off straight. It is therefore not actually "sword-shaped". The middle ridge of the sternum, the carina that anchors the flight muscles, is relatively low, no higher than the lateral protrusion is wide at the thinnest point of its shaft. Towards the back, the carina extends over the entire sword-shaped protrusion and towards the front over half of the sternum, splitting forked, a typical feature of Enantiornithes. The fossil also shows abdominal ribs.

The humerus is over four centimetres long; the ulna is a little longer. The humerus has a fairly short deltopectoral ridge, covering only a third of the shank length. The ridge is also quite low. The joint surface is strongly bevelled. The lower end is strongly widened transversely, with both articular tubercles protruding transversely and the one for contact with the radius protruding more forward. The ulna is almost as robust as the humerus. Its medial end curves strongly towards the radius. The formula of the phalanges is uncertain. The first phalanx of the first finger seems to reach less far than the end of the second metacarpal.

The pelvis is strongly fused. The ilium has a front blade with a straight rather than concave lower edge as in many relatives. The hump immediately above the hip joint is also present in specimen PVL 4042 which is assigned to an Enantiornis sp. Piscivorenantornis has the more basal hump again above it, which is secondarily absent in PVL 4042. The pubic bone is inclined backwards at an angle of 57°. Its "foot" has a basal shape like a golf stick. In most relatives the foot is much smaller. The ischium is perfectly straight. The lower third is suddenly narrowed. At the upper posterior edge, a protrusion extends upwards to the posterior surface of the ilium so that a foramen ilioischiadicum is enclosed there.

The calf bone has a flat inner side without a hollow. The second and fourth metatarsals have a more convex sole than the third. Second and third metatarsals have a hinge joint at the bottom but the fourth is simplified to a simple point.

Fracture ball
In the fossil, a structure is visible under the right upper arm bone that was described in 2016 as a vomit ball. If that identification is correct, it is, according to the describers, the oldest pellet known of a bird and the first from the Mesozoic. They themselves detracted from this claim by stating at the same time that a pellet had been found in a specimen of Piscivoravis. The structure is an oval zone with a length of 22.6 and a width of 7.1 millimetres. A brown colour makes the zone stand out against the otherwise light grey colour of the slab and suggests that the vomit was secreted as a compact whole. Fish bones, including vertebrae and spinous processes, are visible in the zone. Most of it consists of small fragments. The bones appear to be from the small fish species Lycoptera' which is commonly found in the Jiufotang formation. It was assumed that the pellet was regurgitated either just before or during death.

Phylogeny
Piscivorenantiornis was placed in the Enantiornithes as a possible sister species of Pterygornis. A cladistic analysis also found probable close relationships to Dunhuangia with that sometimes forming a narrower clade with Pterygornis. Piscivorentiaornis was, according to the analysis, at least above Eoenantiornis and below Zhouornis in the family tree.

In 2020, Piscivorenantiornis was found to be the sister species of a clade consisting of Pterygornis dapingfangensis and Mirusavis parvus'.

Paleoecology
In 2016, it was concluded from the presence of the pellet that the bird was a fish eater. It would have swallowed its prey whole. The regurgitation indicated that the stomach was divided into a forestomach or ventriculus and a gizzard far back in the abdominal cavity. The forestomach would have compressed those parts that could not pass through the narrow passage to the intestines and pushed them forward in one mass through the oesophagus, which would then have removed them again in a process of antiperistalsis, a reverse peristalsis. This is not surprising, as crocodilians also have such a division, which can be directly observed in the more basal coelurosaur Scipionyx. However, from the fact that coprolites from dinosaurs often contain bone remains, the writers inferred that the crocodiles would have developed this system separately from the birds.

Literature

 * Wang, M., Z. Zhou, and C. Sullivan, 2016, "A fish-eating enantiornithine bird from the Early Cretaceous of China provides evidence of modern avian digestive features", Current Biology 26: 1170-1176
 * Min Wang & Zhonghe Zhou, 2017, "A morphological study of the first known piscivorous enantiornithine bird from the Early Cretaceous of China", Journal of Vertebrate Paleontology, '37: 2, e1278702
 * Min Wang & Zhonghe Zhou. 2020. "Anatomy of a new specimen of Piscivorenantiornis inusitatus (Aves: Enantiornithes) from the Lower Cretaceous Jehol Biota. Journal of Vertebrate Paleontology. 40(3): e1783278

Category:Extinct Birds

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