The Stenophlebiidae is an extinct family of medium-sized to large fossil odonates from the Upper Jurassic and Cretaceous period that belongs to the damsel-dragonfly grade ("anisozygopteres") within the stem group of Anisoptera. They are characterized by their long and slender wings, and the transverse shape of the discoidal triangles in their wing venation.
The head is similar to that of Recent Gomphidae with two large and globular compound eyes that are distinctly separated. The three ocelli are arranged in a triangle on the globular vertex. The mandibles are strong. The legs have short and strong spines.[1]
The largest species Stenophlebia amphitrite reached a wingspan of about 17.3 centimetres (6.8 in), while the smallest species Hispanostenophlebia barremiana reached only a wingspan of about 7.7 centimetres (3.0 in).[2]
Stenophlebiidae are distinguished by the following set of eight derived wing characters (synapomorphies):
(1) a long not zigzagged secondary longitudinal convex vein in the postdiscoidal area, parallel to MP, the base of this vein being just distal of the discoidal triangle; (2) Cr long or very long, covering more than one or two cells between RA and RP; (3) pterostigma shifted basally; (4) pterostigma very long; (5) hindwing subdiscoidal space transverse and crossed by two veins or more; (6) the four wings elongate and more or less falcate; (7) forewing discoidal triangle long transverse; (8) numerous and well defined straight intercalary secondary longitudinal veins reaching posterior wing margin.[3]
Furthermore they share the following four synapomorphies with Prostenophlebiidae in the superfamily Stenophlebioidea:
(1) a long and not zigzagged (or slightly zigzagged) secondary longitudinal convex vein in postdiscoidal area, parallel to MP, the base of this vein being about two or three cells or just distal of discoidal triangle; (2) a long and not zigzagged concave Mspl; (3) oblique vein "O" absent; (4) wings elongate.[3]
And they share these four synapomorphies with Prostenophlebiidae and Liassostenophlebiidae in the suborder Stenophlebioptera:
(1) nodal crossvein Cr very oblique, i.e. angle Cr-RA > 140°; (2) subnodus Sn very oblique, with or without cross-veins reaching it; (3) CuAa with a broad area between the two most distal posterior branches; (4) presence of straight supplementary longitudinal veins in the areas between IR2 and MP near posterior wing margin.[3]