Monographs Details: Radula steerei Grolle
Authority: Buck, William R. 1987. Bryostephane Steereana: A Collection of Bryological Papers Presented to William Campbell Steere On The Occasion of His 80th Birthday. Mem. New York Bot. Gard. 45: 1-749.
Description:Latin Diagnosis - R. faurianae Steph. similis, sed foliis minorious 0.23-0.29 mm (non 0.4-0.5) longis differt.
Species Description - Only two sterile fragments known, light chocolate, axes (probably branches torn from a nonpreserved main axis) 1.58 mm and 0.71 mm long, respectively, to 0.46 mm and 0.38 mm wdde, respectively, with leaves reduced in size towards the apex, hence the apical leaves microphyllous. Branching mostly athecal of the Radula type (4 branches in the available material), rarely gyrothecal of the Lejeunea type (1 branch in the available material); lowermost leaf in the 4 athecal branches abaxial, in the gyrothecal branch adaxial; branches at the base of every (third-) fourth or fifth leaf of the primary axis, all five in the available material unbranched, in the whole length microphyllous (each with 4-9 microphyllous leaves), length 0.25-0.55 mm, about twice the length of the respective leaf. Stem opaque, to 60 µm wdde, cortical cells shortly rectangular with around equally slightly incrassate walls; rhizoids about 10 µm wide, single or rather few, the latter ± diverging or ± fasciculate, only in a suprabasal area of the lobule close to the keel. Leaves with U-shaped insertion leaving dorsally no insertion free stripe and ventrally an insertion free stripe of 1 cell width only, slightly imbricate, a little lifted dorsally, nearly planodistichously patent under an angle of about 45°; lobe with a nearly straight line of insertion, somewhat curved only at the dorsal end and with a length of about half the lobe width, obliquely ovate to (in the largest leaf) obliquely triangular-ovate, (0.21-) 0.23-0.29 mm long, entire-margined; margin at the dorsal base in the largest leaf broadly rounded ampliate reaching beyond stem margin, in the other leaves hardly or slightly ampliate and often hardly crossing the middle line of stem; lamina equally very shallowly concave (in ventral view); transition to the keel ± straight or (in the largest leaf) very widely curved; cells with colorless thin walls without trigones, penta- and hexagonal, ± isodiametric, 10-15 x 10-15 µm, cuticle smooth; keel in length at least 1/2 of the lobe length, longitudinally equally slightly convex, transversely with narrow dorse; lobule about half as long and wide as the lobe, rounded shortly rectangular, entire-margined, almost flat, merely the area 01 the rhizoids origin ± slightly pouched, apical margin slightly curved, flat, transition to the lateral free margin of the lobe abrupt, rectangular, rarely with slightly downward drawn lobe margin, transition to the free lateral margin of the lobule a broadly rounded comer; free lateral margin in the distal 1/3-1/2 ± straight, rarely very slightly concave, in the proximal half slightly convexly curved, without ampliation at base; cells smaller than in the lobe, otherwise similar. Leaves of microphyllous branches about half in length and width as normal leaves, lobe and lobule -+ equal in size, both at their apex transversely trun cate with a slit-like opening between them. Special means of vegetative reproduction lacking.
Radula (sect. Faurianae) steerei Grolle sp. n. is described and illustrated from Dominican amber (Miocene). It is the first fossil record of Radulineae from the New Worid.
Type: Inclusion in Dominican amber; Museum fur Naturkunde Berlin (Palaontologisches Museum) inventory no. 1985/6a (holotype), inventory no. 1985/ (paratype).
The holotype and paratype were originally in the same amber piece, though already separated from each other when I received the material. To get a better aspect of the two Radula fragments, the amber piece was cut between them. Thus, they now lie in separate pieces.
Taxonomy: The most conspicuous feature of R. steerei is the microphyllous branches in the restricted sense of Yamada (1979, p. 219), who excluded amentulose branches. Microphyllous branches in Yamada's sense were hitherto known only from the Japanese species R. fauriana Steph., which was placed into a monotypic sect. Faurianae by Yamada (1979). Since R. steerei shows surprisingly close similarities to R. fauriana in other ways, especially through the rather small thin-walled cells without trigones, it seems to have its natural place in sect. Faurianae. Whether sect. Faurianae is also represented among the numerous extant neotropical species of Radula must remain an open question at present.
Paleontology: The prevailing substrate of extant Radula species is bark. Thus R. steerei fits well into the assemblage of hepatics in Dominican amber, for which generally the assumption of bark inhabitancy seems well founded. Radula steerei is the first fossil record of Radulineae in the New World. The few other fossil records of this worldwide monogeneric suborder with many more than 100 extant species are from the European Tertiary, where R. oblongifolia Caspary (Caspary, 1886) and R. sphaerocarpoides Grolle (Grolle, 1980) have been described from Baltic amber (Upper Eocene to Miocene) and R. bitterfeldensis Grolle (Grolle, 1985) from Bitterfeld amber (Lower Miocene). All these are without close relationships to R. steerei.
In Dominican amber at least one further Radula species occurs. In the Museum fur Naturkunde Berlin is a specimen of Dominican amber (inventory no. 1985/5a), in which a Radula fragment of sect. Caducifoliae Schust. is represented. It is however so scrappy and badly preserved that a description is not warranted.