Over the past three decades, similar to other branches of the tree of life, the taxonomic system of lycophytes and ferns has undergone significant changes due to the accumulation of vast amounts of new information, particularly molecular data. In 2016, these phylogenetic hypotheses were synthesized and presented in a community-derived classification for extant lycophytes and ferns, known as the system of the Pteridophyte Phylogeny Group I (PPG I, Figure 1) [2] . This classification categorizes ferns and lycophytes into 14 orders and two classes: Lycopodiopsida (lycophytes) and Polypodiopsida (ferns). Globally, with an estimated 10,578 extant species, ferns (Polypodiopsida) alone constitute the second most diverse group of vascular plants, spanning 12 orders: Equisetales, Psilotales, Ophioglossales, Marattiales, Cyatheales, Gleicheniales, Osmundales, Salviniales, Polypodiales, Hymenophyllales, and Schizaeales (Figure 1) [2] . Here, we took this fern phylogeny as the phylogenetic frame to see the distributions of three exceptionally preserved sporangium types among the main lineages of ferns corresponding to orders to indicate the fern diversity in the mid-Cretaceous amber forests.

Historically, fern taxonomy has primarily relied on the morphology of sorus, and associated structures continue to play a crucial role in classifying higher hierarchical levels (such as order classifications) for ferns [12] [24] . Special emphasis has been placed on sporangia exhibiting the morphologically unique catapult mechanism, representing the apomorphy of polypod ferns [12] [25] . This distinctive sporangium type (the sporangium with a vertical annulus interrupted by a stalk (Figure 1(A)) is present in over 95% of Polypodiales but absent in other ferns. Consequently, the presence of such sporangia serves as compelling evidence for the occurrence of polypod ferns in mid-Cretaceous amber forests, as demonstrated by all polypod ferns identified thus far from mid-Cretaceous Myanmar amber [5] - [16] .

The sporangium annulus refers to a row or patch of partially or entirely thickened, usually darkened cells of the capsule that contract or break, allowing the capsule to open and discharge its spores (Figures 1(A)-(C)). In addition to the vertical annulus characteristic of Polypodiales, there are two other distinct

types of annuli corresponding to Cyatheales and Schizaeales. In Cyatheales, the annuli are oblique and complete, bypassing the sporangium stalk (Figure 1(B)), whereas in Schizaeales, the annuli are apical or subapical, representing a synapomorphy for Schizaeales (Figure 1(C)) [24] [26] [27] .

The amber specimens investigated in this study were from Hukawng Valley in Tanai Township, Myitkyina District of Kachin State, Myanmar, and were collected in 2016 (Table 2). Previous research by [28] estimated the Burmese

Kachin amber to be about the Cenomanian–Turonian based on the stratigraphic distributions of Cretaceous insect families. Furthermore, Cruickshank and Ko [29] reported an ammonite Mortoniceras of Middle or Upper Albian age. More precise dating was provided by [30] , who assigned an earliest Cenomanian age (98.79 ± 0.62 Ma) based on U–Pb zircon dating to the sedimentary matrix of the amber-bearing beds. More recently, this dating was further refined by [31] , who, using biostratigraphic and radioisotope data, constrained the age of the Burmese Kachin amber to approximately the Upper Albian to Lower Cenomanian period. The amber specimens are deposited in the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences. For examination, the specimens were prepared by trimming it with a water-fed saw and grinding and polishing with a lap to expose the inclusions. Photographic documentation of the fossil inclusions was performed using a Zeiss Stereo Discovery V18 microscope system. To capture the details of these inclusions, both incident and transmitted lighting techniques were employed. The resultant images were then compiled, arranged, and annotated using Adobe Photoshop Pro DC for presentation and analysis.

The Polypodiales, comprising approximately 80% of extant fern species, is strongly supported as monophyletic by molecular studies [24] [32] - [38] . Within this order, two distinct morphological synapomorphies characterize the sporangium: a three-rowed stalk (Figure 1(A), Figure 2(C)) and a vertically interrupted annulus at the stalk (Figure 1(A), Figure 2) [1] [12] [25] . Despite some exceptions such as the Lindsaeaceae [39] and Athyriaceae [40] , most families within Polypodiales lack a well-documented fossil record traceable to the Mesozoic from stratigraphic deposits [41] [42] [43] until the discovery of the first polypod fern fossil in mid-Cretaceous Myanmar amber [12] . One challenge stems from the scarcity of diagnostic characteristics, particularly the sporangium’s vertical annulus and distinct stomium, which are often lost in fossilized material.

To date, the majority of ferns identified from mid-Cretaceous Myanmar amber belong to Polypodiales, particularly to its basal families, including Dennstaedtiaceae [5] [12] [13] , Lindsaeaceae [14] [16] , Cystodiaceae [8] [9] , and Pteridaceae [10] . For the more divergent lineage Eupolypods within Polypodiales, only two compelling fossils, Holttumopteris burmensis [11] and Cretacififilix fungiformis [6] [7] , have been discovered. Additionally, some dispersed polypod sporangia have been reported [12] [28] . In addition to these published findings, new dispersed sporangia exhibiting characteristic features of Polypodiales (Table 2, Figure 2) have been uncovered in mid-Cretaceous Myanmar amber, including one specimen potentially affiliated with Eupolypods (NPA-PB-XFY05 of Table 2, Figure 2(F)). This affiliation is supported by two spores with a lophate perine within the sporangium and a few acicular hairs protruding from the sporangium, both characteristic features indicative of a connection to Eupolypods [11]

[12] . These recent findings contribute further evidence to support the notion that the Cretaceous amber forests of Myanmar boasted a diverse fern flora, likely dominated by polypods based on current data [11] [12] .

Traditionally, Cyatheales known as “tree ferns” was confined to the families Cyatheaceae, Dicksoniaceae, Lophosoriaceae, and Metaxyaceae [22] [44] . Recent phylogenetic studies have significantly expanded this understanding, revealing that these families form a coherent clade with the Loxomataceae and Plagiogyriaceae. Such relationships were previously unanticipated and underscore the power of molecular data in revealing evolutionary linkages [33] [45] . Additionally, these analyses have led to the placement of Lophosoriaceae into Dicksoniaceae and Hymenophyllopsidaceae, endemic to the tepui region of southern Venezuela, into Cyathea [46] . Apart from the conspicuous feature of the rhizome, tree ferns exhibit two distinct common characteristics in their sporangia, in contrast to the Polypodiales, the Cyatheales exhibit a unique sporangial morphology, characterized by a greater number of stalk cells and complete, ring-like annuli that bypass (not interrupted by) the sporangium stalk (Figure 1(B), Figure 3). These features facilitated the identification of a tree fern inclusion within Myanmar amber as Thyrsopteris Cretacea of the Thyrsopteridaceae, marking the first record of the order from this locale [17] . More dispersed sporangia with oblique annuli from the mid-Cretaceous Myanmar amber were recognized in this study (Table 2, Figure 3). Annulus cells of these sporangia are usually more than that in Polypodiales (Table 2), as observed in extant Polypodiales and Cyatheales [47] .

The current investigation extends the knowledge of Cyatheales-type sporangia in the mid-Cretaceous Myanmar amber, introducing additional sporangial types and thereby highlighting the complexity and diversity of the fern flora within these ancient forests. These findings indicate that the mid-Cretaceous forests of Myanmar were not only rich in polypods, as previously documented but also harbored a significant diversity of tree ferns, pointing towards a more complex ecosystem than currently recognized. Furthermore, the extant tree ferns frequently constitute a locally dominant component of the wet southern temperate rainforests across Australasia, southern Africa, and regions adjacent to the tropics in South America. Historically, most fossil records of Cyatheales have been discovered in the Southern Hemisphere, leading to the widely accepted view that they are of Gondwanan origin [44] [45] [46] . However, this current investigation has identified Cyatheales-type sporangia within the mid-Cretaceous amber forests of Myanmar, a region part of Laurasia, thereby broadening our understanding of the historical biogeography of tree ferns. Previously considered to be predominantly Gondwanan, these findings suggest that the distribution of Cyatheales was more extensive than currently recognized, indicating that they once thrived in Laurasia as well.

The Schizaeales, a monophyletic group encompassing the families Lygodiaceae, Schizaeaceae, and Anemiaceae, primarily distributed in warm, moist environments near water sources such as riverbanks, represent a fascinating case study in the field of plant phylogenetics [32] [33] [48] [49] [50] . The Schizaeales are distinguished by their unique sporangial characteristics, notably the complete apical or subapical annulus [22] [51] . Based on the sporangium characters, three amber pieces (NPA-201738, NPA-201914, and NPA-201918 of Table 2) from our collections could be put into Schizaeales (Table 2, Figure 4). With naked sporangia and trilete spores, specimens in amber piece of NPA-201738 (Table 2) could be considered as affinity with Anemiaceae; similarly, with naked sporangia and a bigger distal plate (with more than one cell) on the apical end of the sporangia, specimen in an amber piece of NPA-201914 (Table 2), could be considered as affinity with Anemiaceae too. These investigations have revealed the presence of diverse schizaeoid ferns within the rich fern flora of mid-Cretaceous Myanmar amber forests. Additionally, the ecological preferences of Schizaeales, which favor warm, humid environments close to water sources, are consistent with the inferred depositional environment of the Myanmar amber forests. This suggests a proximity to a nearshore marine setting, such as a bay or estuary [28] [29] [30] [52] , or alternatively, a dynamic coastal forest environment for the mid-Cretaceous Myanmar amber forests [31] . This alignment between the ecological characteristics of Schizaeales and the depositional context of the amber indicates a complex ecosystem where these ferns thrived.