Cheirolepidiaceae

Cheirolepidiaceae Temporal range:
Leafy shoot ofTomaxelliashowing details of epidermis and stomata
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Gymnospermae
Division:	Pinophyta
Class:	Pinopsida
Order:	Pinales
Family:	†Cheirolepidiaceae Turutanova-Ketova 1963
Genera
See text
Synonyms
Hirmeriellaceae

Cheirolepidiaceae(also spelledCheirolepidaceae) is an extinctfamilyofconifers.They first appeared in theTriassic,and were a diverse and common group of conifers during most of theMesozoicera, primarily at low latitudes,where they often formed a dominant element of the vegetation.They are united by the possession of a distinctive pollen type assigned to the form genusClassopollis(the distinctive characters of which include "distal cryptopore, proximal scar often with filaments, striate equatorial girdle, subequatorial rimula, tegillum (outer tectum of a lesser electron density), and columella-like infratectal elements") The name Frenelopsidaceae (as a separate family) or "frenelopsids" has been used for a group of Cheirolepidiaceae with jointed stems, thick internode cuticles, sheathing leaf bases and reduced free leaf tips. The leaf morphology has been noted as being similar to that ofhalophyteSalicornia.Several members of the family appear to have been adapted for semi-arid and coastal settings, with a high tolerance of saline conditions.Cheirolepidiaceae disappeared from most regions of the world during theCenomanian-Turonianstages of theLate Cretaceous,but reappeared in South America during theMaastrichtian,the final stage of the Cretaceous, increasing in abundance after theK-Pg extinctionand being a prominent part of the regional flora during thePaleocene,before going extinct.Survival into the Paleocene in North America and China has also been suggested based on pollen.

Pararaucariacone

The habit of cheirolepidaceous confers is likely to have varied widely, from large trees (some with trunks over 3 metres (9.8 ft) thick at their base) to shrubs.Theirarchitectureis poorly known, though some are thought to have haddecurrentspreadingcrowns,while others had conical crowns. Many seem to have plagiotropic lateral branches that developed in whorls.

The relationships of Cheirolepidiaceae to other conifers are uncertain. A close relationship withAraucariaceaeandPodocarpaceaehas been proposed, based on the similarities of their reproductive structures,though other studies have suggested that they may fall outside thecrown groupof modern conifers among variousvoltzialeanlineages.

At least some species of Cheirolepidiaceae have been suggested to have been pollinated by insects, due to the construction of the reproductive organs and the fact that insects have been found associated withClassopolispollen grains.

The family nameHirmeriellaceaeis a junior synonym of Cheirolepidiaceae.Some authors have suggested Hirmeriellaceae is the valid name for the family, due to nomenclatural issues with the originalCheirolepisgenus, which is a junior homonym of a member ofAsteraceae,withCheirolepidiumsuggested to be an invalid replacement. Both genera are likely synonyms ofHirmeriella.

Genera

• †Agathoxylon(wood, in part)
• †Brachyoxylon(wood)
• †Classopollis(pollen)
• †Classostrobus(pollen cones)
• †Dicheiropollis(pollen)
• †Frenelopsis(foliage)
• †Hirmeriella(whole plant)
• †Pararaucaria(ovulate cones)
• †Pseudofrenelopsis(foliage)
• †Watsoniocladus(foliage)
• †Brachyphyllum(foliage, in part)
• †Tomaxiella(foliage and ovulate cone)
• †Kachaikestrobus(ovulate cones)
• †Alvinia(ovulate cones)
• †Pseudohirmeriella(ovulate cones)