The quadrate bone forms the lower jaw articulation in all classes except mammals. Evolutionarily, it is derived from the hindmost part of the primitive cartilaginous upper jaw.
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Megarachne was very similar to other mycteroptid eurypterids in appearance, a group distinguished from other mycteropoids by the parabolic shape of their prosoma (the head plate), hastate telsons (the hindmost part of the body being shaped like a gladius, a Roman sword) with paired keel-shaped projections on the underside, and heads with small compound eyes that were roughly trapezoidal in shape.
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The mouth opens on the underside between the ends of the groove, while the anus opens into the hindmost part. Like chitons, monoplacophorans possess a sensory subradular organ, as well as a rasping radula. A fold of ciliated tissue surrounds the mouth to the front and sides, while a smaller fold, bearing a number of tentacles, lies just behind it. The stomach contains a style, projecting from a diverticulum, or "style sac".
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The height of the dentary diminished towards the hindmost extend of the tooth row, whereafter it sharply fanned out to contact the bone behind it; by contrast, the hind part of the dentary in Erlikosaurus gradually approached the surungular in a gentle arc. Right half of the holotype mandible in outer and inner view, with component bones marked by different colors; the (green) bore the teeth. Segnosaurus was distinct among therizinosaurs in that the hindmost part of the dentary was toothless.
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The undersides and legs of the animal are paler than the rest of the body, and the bare skin of the wings varies from light brown to almost black, depending on the individual. The bat has a rounded head with narrow, pointed ears, and unusually large feet. The membranes of the wings attach to the toes at their hindmost part, while the membrane between the legs is partially supported by a long calcar that actually extends beyond its edge to end in a small rounded lump.
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Flying fish combine swimming movements with the ability to glide in air using their long pectoral fins. A typical teleost fish has a streamlined body for rapid swimming, and locomotion is generally provided by a lateral undulation of the hindmost part of the trunk and the tail, propelling the fish through the water. There are many exceptions to this method of locomotion, especially where speed is not the main objective; among rocks and on coral reefs, slow swimming with great manoeuvrability may be a desirable attribute. Eels locomote by wiggling their entire bodies.
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The hindmost part of the surangular had a small foramen placed in the same position as similar openings in the mandibles of non-bird theropods and modern birds. The splenial bone was three-pronged (as in some modern birds, but unlike the simple splenial of Archaeopteryx), and its lower margin followed the lower margin of the mandible. There was a large rostral mandibular fenestra and a small, rounded caudal fenestra behind it. Though only five specimens preserve parts of the beak's keratinous covering, these show that there would have been differences between species not seen in the skeleton.
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More bones bearing the specimen number IGM 100/82 were located but were not mentioned in Perle's description, while the whereabouts of some paratype elements was unknown. In a 2016 re-description of the holotype mandible, which had been little studied since 1979, Zanno and colleagues reported the majority of the tooth crowns had been damaged after collection, and most of them were missing their tips. Of the two (halves of the lower jaw), the right is nearly complete; only the hindmost part and the upper front of its mandibular symphysis (the area where the halves of the mandible meet) was missing. The left hemimandible is fragmented and preserves the front part with some displacement of bone due to crushing.
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It includes the (lower jaws), an incomplete , a complete and (lower arm bones), of the fingers, a forelimb (claw bone), an almost-complete , an incomplete right , six , ten from the front of the tail, fifteen from the hindmost part of the tail, the first , and fragments of the dorsal ribs. Two more specimens were designated as paratype specimens; specimen IGM 100/82 from the Khara Khutul locality includes a femur, and (leg bones), and , five toe phalanges including a foot ungual, rib fragments, complete , the upper portion of an , and the lower portion of a . Specimen IGM 100/83 includes a left (shoulder girdle), a radius, an ulna, forelimb unguals, and a fragment of a (neck) vertebra. In 1980, Perle and the paleontologist Rinchen Barsbold assigned another specimen to Segnosaurus; IGM 100/81 from the Amtgay locality included a left tibia and fibula. In 1983, Barsbold listed additional specimens GIN 100/87 and 100/88.
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