The hyperdiverse conifer flora of the Baltic amber forest

Baltic amber represents one of the most important Cenozoic fossil Lagerstatten, providing numerous fossils including plenty of invertebrates. Although it is essential for understanding the past habitats and ecology of these fossil organisms, the Baltic amber forest and its palaeoecology is not satisfactorily known for answering questions on the identity of the amber-producing conifer(s) and the specific habitat and climate conditions under which the resins were exuded. Knowledge of the Baltic amber flora mostly dares back to the 19th century and the early decades of the 20th century and needs revision on a larger scale. Here, we continue our revision and monographic treatment of conifer inclusions based on historical and new collections. On this account, type specimens and original material of the classical monographs on Baltic amber conifers were identified from museum collections, and newly discovered material complemented the study. The studied material encompasses portions of foliage shoots, single leaves and pollen cones. The preservation mode of these conifer inclusions is briefly described and discussed. Identification of the conifer remains is based on gross-morphological (shoot architecture, leaf shape and size, phyllotaxis) and leaf epidermal characters (distribution and arrangement of stomata, cells structure of leaf margin). For taxonomic purposes, we use both the modern classification system of conifers but also artificial fossil-genera, when available characters were inadequate to accommodate the fossils into 'natural' genera. Based on our present and previous revisions of holotypes from Baltic amber conifers, we confirm five families, 17 genera including two fossil-genera and 26 fossil-species of conifers, including the following taxa: Cupressaceae [eight genera, 14 species; Quasisequoia couttsiae, Sequoia sp. aft & abietina, Taxodium sp., aft Glyptostrobus europaeus, Cryptomeria sp., Cupressinanthus (four spp.), Cupressinocladus (four spp.), Calocedrus sp.], Pinaceae [six genera, nine species: Pinus (four spp.), Pseudolarix sp., aff. Tsuga, Cathaya sp., Nothotsuga protogaea, Abies sp.], Geinitziaceae (Cupressospermum saxonicum), Sciadopityaceae (Sciadopytis sp. cf S tertiaria) and Cephalotaxaceae (Cephalotaxus sp. cf . C messelensis). In particular, cupressaceous foliage shoot portions are herein reconsidered and affiliated to the fossil-genus Cupressinocladus SEWARD 1919, and new combinations are proposed, C. borealis (CASP. et R.KLEBS 1907) comb. nov., C. breynianus (Gorr. et BERENDT 1845) comb. nov. et emend., C kleinianus (GOPP. et BERENDT 1845) comb. nov., and C. lamellifirmis (CASP. in CASPARY & KLEBS 1907) comb. nov. et emend. To sufficiently accommodate cupressaceous pollen cones, we amend the fossil-genus Cupressinanthus CASP. in CASPARY et KLEBS 1907 and designate Cupressinanthus magnus CASP in CASPARY & KLEBS 1907 emend. as type. Besides, Cupressinanthus hartmannianus (Gorr. et BERENDT 1845) comb nov., C. linkianus (Gorr. et BERENDT 1845) comb. nov. et emend. and C klebsii sp. nov. can be distinguished. Inclusions of Cryptomeria (Cupressaceae), Cephalotaxus (Cephalotaxaceae) and aff. Tsuga (Pinaceae) represent first evidence of these genera from the Baltic amber flora. We discuss palaeoecological, palaeofloristic and palaeoclimatic implication for the Baltic amber forest based on data from other fossil occurrences and co-occurrences of the fossil conifers. Our results show a hyperdiversity of conifer taxa which is unique among European Eocene fossil assemblages. The conifer flora from Baltic amber is distinct from both paratropical/subtropical floras from Central Europe as well as from cool-temperate floras in the Arctic zone of the early and middle Eocene. Assemblages from early-middle Miocene coastal lowland vegetation of Central Europe are most similar in composition and their palaeoecology, implying that the Baltic amber forest might have been part of the Atlantic-Boreal phytoprovince. Our survey substantiates that the Baltic amber forest was growing under a warm-temperate and humid palaeoclimate. Based on the autecology of individual fossil-species, the Baltic amber forest represented heterogeneous vegetation with a mosaic of forests and open habitats in alluvial plains, coastal swamps and mesic hinterland. In light of these new results, we discuss the age of Baltic amber and its Lagerstatte and conclude that the revealed conifer diversity supports the palynostratigraphic late Eocene age estimations.

Automated summary

Baltic amber is an important Cenozoic fossil Lagerstatten, containing numerous fossils including invertebrates. However, the identity of the amber-producing conifers and the specific habitat and climate conditions under which the resins were exuded are not well known. This study revises and analyzes conifer inclusions in Baltic amber, identifying five families, 17 genera, and 26 fossil species of conifers. The results reveal a hyperdiversity of conifer taxa and suggest that the Baltic amber forest grew under a warm-temperate and humid palaeoclimate.