Etymology:
In reference to the occurrence on a precipitous site known locally as Cardiac Hill.
Historical notes: Distinguishing features: A member of the M. miquelii complex, separated by its large stature, usually darker green leaves with spines for only up to two-thirds of the petiole length, and the large size of the male and female cones. Allied to M. douglasii, from which it differs in the pale yellow callous bases of the leaflets, the proximal leaflets being more closely spaced (20-28 mm apart), and the greater number of reduced spine-like leaflets on the petiole (18-30). Distribution and habitat: Endemic in south-eastern Queensland, where it is restricted to Mt Walsh National Park. Plants form extensive colonies on precipitous slopes in open forest dominated by Eucalyptus acmenoides and E. major. M. cardiacensis is restricted in its known occurrence, but is very common at the two known localities, both in Mt Walsh National Park. Conservation status: M. cardiacensis is restricted in its known occurrence, but is very common at the two known localities, both of which are conserved in Mt Walsh National Park (IUCN Red List category LR,lc; ROTAP category 2RC-t). Description:
Plants acaulescent or arborescent (less commonly), stem 0-0.4 m tall, 20-40 cm diam. Leaves 10-20 in crown, deep green, highly glossy, 150-200 cm long, flat (not keeled) in section, with 100-140 leaflets; rachis not spirally twisted, straight, lax and pendulous; petiole 30-40 cm long, straight, spinescent, 13-20 mm wide at lowest leaflets; basal leaflets reducing to spines. Leaflets simple, strongly discolorous; margins flat; apex entire, spinescent; median leaflets 230-310 mm long, 9-11 mm wide. Pollen cones fusiform, 35-40 cm long, 6.5-8 cm diam.; microsporophyll lamina 22-32 mm long, 10-15 mm wide; apical spine 2-30 mm long. Seed cones narrowly ovoid, 32-36 cm long, 11-14 cm diam.; megasporophyll lamina 35 mm long, megasporophyll with an expanded peltate apex 45 mm wide, 25 mm high; apical spine 7-48 mm long. Seeds oblong, 22-35 mm long, 12-20 mm wide; sarcotesta orange to red.
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