The largely intact skeleton of LACM 128319, 5.67 m long, has been prepared in oblique left lateral to dorsolateral view (). The pelvic girdle and basal part of the tail are somewhat disturbed and some elements in these areas have suffered slight crushing; otherwise, the skeleton is in nearly perfect articulation, preserving the gently curved neck, the broadly hunched back, and the acutely downturned distal half of the tail. Because the skeletal anatomy of Platecarpus
is reasonably well known 
, the osteology of LACM 128319 will be dealt with elsewhere, and the exceptionally preserved soft tissue morphology and overall body outline are the focus of this report.
Platecarpus tympaniticus, LACM 128319, upper Santonian–lowermost Campanian, Kansas, USA.
In the head region of LACM 128319, purplish matter in the sclerotic ring aperture of the left eye may represent remnants of the retina, as this tissue was presumably pressed down against the inner surface of the underlying scleral ossicles when the head collapsed during the decay of the carcass (). This interpretation is corroborated by the presence of loosely packed, aligned bodies, about 2 µm long, with a morphology that is comparable to that of retinal melanosomes (i.e., lysosome-related organelles of pigment cells) in the eyes of extant tetrapods () 
. We refute the possibility that the oblate microstructures represent replacement bacteria because they are embedded inside the fossilized tissues (probably representing their in situ
position) rather than forming a superficial coating, as would be expected had they instead been part of a microbial biofilm 
. Moreover, melanosome-like microstructures were not found in any other part of the mosasaur examined under scanning electron microspectroscopy (SEM), including scales, visceral traces, intestinal content, surrounding matrix, and the film that defines the former body outline. Energy dispersive X-ray (EDX) analysis showed that phosphate predominates in the diagenetically mineralized tissue, and hence it is possible that the micrometer-sized bodies are simple microcrystalline apatite aggregates 
. However, calcium phosphate crystallites often nucleate on already existing soft structures 
, which, in this case, may have been melanosomes. Additionally, apatite aggregates are normally either spherical or cubic in shape 
, not oblate with rounded termini, and, again, would be expected to be found in other phosphatized tissues of LACM 128319 (although we acknowledge the possibility that various types of phosphatization may occur within the same fossil 
Portions of the respiratory tube, as represented by its reinforcing cartilaginous rings, are visible in the temporal region of the skull through the posterior portion of the neck, immediately anterior to the pectoral girdle. Tracheal rings [diameter
26.2–34.4 mm (taphonomic compression gives widely varying measurements)] first appear along the lower half of the lateral temporal fenestra (). They are concealed under the left quadrate, but reappear between the retroarticular processes of the lower jaws. Following an abrupt upward turn suggesting dislocation prior to burial, another section of the trachea occurs some distance ventral to cervical vertebrae four and five (). Unfortunately, the segment in which the trachea bifurcates into right and left bronchus was lost during quarrying. Nonetheless, two sub-parallel strings of bronchial rings (average diameter about 20 mm) situated below the first dorsal vertebra suggest that mosasaurs, similar to extant limbed squamates, had two functional lungs (). Snakes, on the other hand, only have one functional lung (the left one is either vestigial or absent altogether) because their tubular bodies require their internal organs to be reduced in thickness and/or in number 
. Apparently, the tracheal bifurcation occurs anterior to the forelimbs in mosasaurs, unlike the condition in terrestrial lizards in which the bifurcation occurs in the chest region at the level of the forelimbs 
. Among mammals, whales also exhibit a short trachea (due to their abbreviated neck) supported by heavy cartilaginous rings, followed by paired bronchi that in some taxa extend sub-parallel to each other rather than diverging from one another 
Selected soft tissue structures in the head and neck region of LACM 128319.
In the area of the lower rib cage, a reddish stain extends from the third to fifth thoracic rib (). The colored area is bounded anteriorly by the third rib, which, in turn, is located posterior to a thin sheet of mineralized tissue (possibly the remains of an intercostal plate; ‘pl’ in ). Consequently, it is possible that the trace continues forward some distance inside the rock. Ventrally, the pigmentation disappears underneath the calcified cartilage of the sternal ribs to suggest that it was something inside the body cavity that remained within the rib cage as the animal decomposed (thus excluding the possibility that the red matter is a residual of microbial mats that grew on the underside of the body). Its present dorsal margin, however, is an artifact produced by plaster-filling in the space between the long ribs. Likewise, it is currently not possible to confidently determine the original posterior edge of the stained area due to cement infill, although a reddish coloration on the succeeding long ribs would suggest that it may have been at the level of the eighth thoracic rib.
Putative visceral traces in the thoracic and abdominal cavities of LACM 128319 (arrows indicate preserved body margin).
Another ruddy patch is located ventromedial to the second and third lumbar vertebra, lining the medial surface of the left rib head of the second lumbar (). The colored layer is bounded above by the vertebral centra, and is interrupted anteroventrally by the left rib of the first lumbar (presumably an artificial perimeter because there is cement infill in the area immediately anterior to this rib). Ventroposteriorly, the colored matter gradually thins out and fades away; hence, there are no precise edges.
A previous chemical analysis of the aforementioned pigmentations, using mass spectrometry and X-ray diffraction techniques, detected sufficient amounts of iron and porphyrin-derived compounds suggestive of the presence of hemoglobin decomposition products, and thus indicative that the traces may represent residues of visceral organs derived from the decaying animal 
. The heavily pigmented matter does have a substance and consistency that are noticeably different from those of the surrounding bones and matrix. Our SEM-EDX analysis demonstrated that the stained areas contain iron, oxygen and carbon, to suggest a partial replacement of the organic matter with either siderite or pyrite (which, in turn, may have altered to iron oxyhydroxides 
), i.e., diagenetic minerals commonly associated with exceptional soft tissue preservation 
. Decomposition and subsequent compaction during burial may have displaced the organic residues toward the lower part of the body wall, thereby explaining the discoloration of the adjacent ribs ().
High in the abdomen, well in front of the pelvis, lie the contents of the gastrointestinal tract. These consist of partially digested remains of moderate-sized fish packed into a dense mass with an outline that appears to follow the course of the digestive tract (). It is possible that the ingested bones derive from the anterior portion of the digestive system (they would then represent displaced stomach contents); however, given that the longitudinal axis of the well-delimited skeletal accumulation runs dorsally beneath and parallel to the vertebrae in the lumbar region, it is more likely that the residues represent processed food from within the colon. Given this, the incompletely digested bones would suggest that mosasaurs, similar to e.g., tyrannosaurid theropod dinosaurs 
, had short gut-residence times and/or low gastrointestinal absorption rates (which would then also explain the sporadic finds of massed bivalve shell pieces in gastric residues and alleged coprolites of the durophagous mosasaur Globidens 
). Alternatively, the resistant skeletal elements were rapidly transported through the digestive system as waste material poor in nutrients 
The most remarkable features of LACM 128319 are the preservation of skin structures from all parts of the body and the undistorted posture of the caudal fin. The squamation is preserved as articulated sections of phosphatized matter and faint impressions along the neck, abdomen, and upper and lower surfaces of the tail (, ), and as a reticulated pigmentation on the bone surfaces (, ). Some parts of the integument, particularly on the head, are poorly discernable in normal light but fluoresce under ultraviolet light (e.g., ).
Integumentary structures of LACM 128319.
Fluke area of the tail of LACM 128319.
The scales covering the tip of the snout are large (approximately 10 mm when measured transversely), non-imbricated, and sub-hexagonal in outline. Along the upper and lower jaws the scales are longitudinally rhomboidal (measuring up to 20 mm in length), and they are obliquely arrayed into an alternating pattern where neighboring scales overlap one another. Large, rhomboidal scales also cover the posterior, gently domed portion of the left osseous nasal aperture (), to suggest that the fleshy nostrils were situated far anteriorly, as well as somewhat peripherally in mosasaurs, as they are in most extant squamates and archosaurs 
. The scales in front of the orbit are longitudinally oval in shape (), whereas those bordering this cranial concavity from above and below have more-or-less rhomboidal outlines (). At the top of the skull the scales appear as shallow, hexagonal depressions (). The scales covering the anterior end of the frontal are reasonably large (about 7 mm when measured diagonally); however, they diminish in size posteriorly and gradually attain the dimensions of the body scales.
Whereas the head scales include a blend of morphologies, the body scales are all rhomboidal, well imbricated, and arranged in an alternating pattern where adjacent rows are diagonally offset from one another. As in the advanced mosasaurine Plotosaurus 
, the posterior scale margin appears to be acutely medially angled (); yet there are no apparent keels or other external surface ornamentations. The body scales measure about 3.6×3.3 (width×length) to 4.4×4.4 mm along the dorsal surface of the neck, 5.5×5.2 mm above the anterior tracheal rings, 4.4×3.0 to 4.8×3.1 mm along the ventral margin of the rib cage, 4.3×3.9 to 4.7×4.3 mm over the posterior dorsal region, and 4.4×4.6 to 5.5×4.0 mm at the tail base; hence not showing any significant size gradation across the body. The size (up to 10.5×4.7 mm) and morphology (tall, almost columnar) of the caudal scales in the hypaxial region of the tail fin differ somewhat from those of the body scales (), whereas the epaxial caudal scales are comparable in size (about 3.0×3.4 mm) and morphology (regular rhomboidal) to the body scales (). Paddle scales are also comparable in morphology and organization to the body scales but appear to become progressively larger distally. Putative color markings, in the form of darkly pigmented, irregular stains and narrowly spaced, oblique stripes (), are found on the premaxilla, third cervical vertebra, and along the gum line. A dark, phosphatic film defines the fleshy outline of the neck ( – black arrows) and right body margin ( – arrows). Unfortunately, most skin structures surrounding the tail fin were lost during collecting and/or initial preparation of the specimen before the recognition that soft tissues were preserved. It is noteworthy, however, that scale impressions above the neural spines extend to the edge of the block that contains the posterior tail portion of LACM 128319 (). Additionally, the terminal caudal segment is structurally downturned due to the presence of wedge-shaped vertebrae (i.e., vertebrae where the dorsal centrum edge is longer than the ventral centrum edge; , ) 
, preserving the original configuration of the arcuate fringe formed by the neural spines in the anterior terminal caudal series near the bend ().
Morphometric data for LACM 128319, an extant shark (Squalus), and the wolf eel Anarrhichthys.