Amphibians ("double life" in Greek) are a class of primitive terrapods (vertebrates with four limbs) that spend at least part of their lives in water and part on land, although some are entirely aquatic or terrestrial. Amphibians provide a biological link between fishes and the true land-living vertebrates, as they are the first group of tetrapods that invaded the land some 370 million years ago. Living amphibians are classified in the Subclass Lissamphibia (with their closely related ancestors in fossil forms). They include not only the familiar forms such as the frogs and toads (Order Anura), salamanders and newts (Order Urodela), but also the less familiar iorms such as the limbless caecilians (Order Gymnophiona). Extinct archaic amphibians include labyrinthodonts and lepospondyls that flourished in the Late Paleozoic, from either of which the ancestral stocks of the living amphibians may have evolved.
Compared with other vertebrate groups, amphibians, especially the lissamphibians (having thin and fragile bones unfitted for fossilizarion), arc-generally less well documented in the fossil record. This is especially, and painfully, true of the Mesozoic Bra, during which important events of origin and early diversification of modern amphibians took place. Therefore, discoveries of any liss&mphibian fossils of this time are usually newsworthy, if not outright sensational. In the past few years, many well-preserved specimens of lissamphibians were recovered from the Mesozoic beds in western Liaoning, northern Hebei and southeastern Inner Mongolia in China. Most of the fossils are important components of the Jehol Biota that flourished in East Asia about I30~ 1 10 million years ago. The recovered fossils constitute a diverse lissamphibian fauna in the late Mesozoic of Asia, and provide crucial information on understanding the biogeographic evolution ol early salamanders and frogs of modern amphibian affinities.
Before these discoveries, fossil amphibians from China were rather limited in both quantity and taxonomic diversity. Prior to 1998, all the known fossils were the Cenozoic in age (early Miocene to middle Pleistocene), totaling ten species of five genera from five major localities in northern China. Since then, new findings from northeastern China have added nearly as many species of the rare Mesozoic lissamphibians to the Chinese record. The fossils include several hundred well-preserved skeletons, some with clear impres sions of soft parts such as eyes, gills, and skins. They also include the earliest known record of several taxonomic groups. These incredible finds have nor only rejuvenated amphibian paleontology but also drawn a worldwide attention beyond the scientific community.
Frogs Frogs are tailless amphibians classified in the order Anura (belonging to the superorder Salientia, which also includes the primitive proanurans). They have unusual body structures that are specialized for jumping. Such structures include a rodlike urostyle formed by fusion of tail vertebrae and the greatly elongated hind limbs with highly modified tarsal elements, The earliest fossil representatives of Salientia were from the Early Triassic of Madagascar and Poland. After a long rime evolution, now the order Anura includes some 4,800 living species with a global distribution except for extreme northern latitudes, Antarctica, and most oceanic islands.
The fossil frogs known from the Liaoning beds include several archaic forms that documented an important diversification of anurans during the Early Cretaceous time in the area, One oi the frogs known from Liaoning is Callobatrachus sanyanensis (Fig. 104, 105), named and described on the basis ol a nearly complete skeleton from the Sihetun site. The fossil beds yielded a radiometric date of 125 million years BP, indicating the Early Cretaceous age of the fossil. Taxonomic studies have revealed that Callobatrachus sanyanensis is a primitive member of the Discoglossidae, a basal family group of the living anurans. Callobatrachus is basal (or primitive) in having nine opisthocoelous presacral vertebrae, differing from eight in other discoglossids. It further differs from other members of the family in the combination of the following characters: lacking a dorsal protuberance but having a weak dorsal crest on ilium; having bicondylar sacro-urostylar arriculation; lacking dermal sculptures on skull roof, and having anteriorly expanded sacral diapophysis.
Living discoglossids in China include five species in a single genus Bombina, with Bumbina ovicntalis popularly known as the "oriental fire-bellied toad' (Fig. 106). Before the discovery of Callobatrachus, no discoglossid fossils were known in the same range of the extant group in Bast Asia. Consequently, Callobatrachus represents the first discoglossid frog known from China, and the earliest fossil record of the group from Asia.
Mesapbrytie beipiaoensis (Fig. 107) is another Mesozoic frog reported from in I
■«105 Skeletal reconstruction of Callobotrochus sanyanensis. I. sacrum: 2, presacral vertebrae; 3, urostyle: 4. ilium. (Art: Yuan Wang/ IVPP)
106 Bombina arientalis, a living discoglossid frog mainly distributed in Fast Asia, and an extant relative of Callobcilrachus, (Courtesy: Er-mi Zhao/CIB)
the Liaoning beds. It is represented by a nearly complete skeleton split on part and counterpart of a shale slab. It has obviously different osteological structures from Callobatrachus, including the presence of procoelous presacrals and greatly shortened vertebral column. It is worth noting here that Mesophryne also has nine presacrals, albeit having a short vertebral column. The number of presacral vertebrae is among the few crucially significant anatomical features in the evolution of frogs. In the earliest known frog, Triadobatrachus (a proanuran) of the Early Triassic of Madagascar, this number is as many as 14; Vieraella, an Early Jurassic frog from Argentina, has ten presacrals. Nine presacrals can be found in the Middle to Late Jurassic Argentine frog Notobatrachus, as well as in the aforementioned two Early Cretaceous Chinese taxa. This number is eight or fewer in all living frogs, except for two very primitive forms, Ascaphus (North American tailed frogs) and Leiopelma (New Zealand frogs), both having nine presacrals. Thus, the decrease in presacral number has been recognized as an obvious trend in the frog evolution.
In addition to its large presacral number, Mesophryne also has primitive features such as the presence of free ribs on anterior three presacrals, and the retaining of an intermedium bone in the carpal region. Thus, it is not surprising that a recent phylogenetic study has recognized it as a distinct clade of basal anurans (Fig. 108).
Liaobatrachus grabaui is the first Mesozoic frog described from the Liaoning beds; however, its systematic position is still questionable at present. Liaobatrachus grabaui was named and briefly described in early 1998 based on an incomplete skeleton with a disarticulated and poorly preserved skull. It has a snout-pelvic length of about 75 mm, intermediate between Callobatrachus sanyanensis (94 mm) and Mesophryne beipiaoensis (71.3 mm). The taxonomic status of this animal remains to be investigated, as the referral of it to the family Pelobatidae (by the original researchers) was based on several characters in question, such as the presence of procoelous presacrals and the lack of free ribs, which cannot be confirmed due to poor preservation.
Salamanders Salamanders are tailed amphibians classified in the order Urodela (belonging to the superorder Caudata, which also includes some primitive non-urodeles). In the past few years, several hundred of salamander fossils have been found from the fossil beds in northern China. Some were recovered from the Fengshan fossil bed in northern Hebei Province, and the Jiufotang Formation in western Liaoning. These strata can
, 107 Holotvpe of Mesophryne beipiaoensis (Slab A. dorsal view, snout-pelvis length about 71 mm), a primitive frog representing a distinct basal anuran clade, from Heitizigou locality (lower part ofYixian Formation) in Beipiao, Liaoning. (Photo: IVPP)
Caudata (M Jur-Rec) Tnadabatrachus (E Tri) Ciatkobatrachus (E Tri) Prosahrus (E Jur) Notobatrachus (M-L Jur) Vieraella (E Jur) Gob/ales <L Cre)
Mesophyme (E Cm) Ascaphus (Raci Leiopelma ([email protected]~Rec) CaMobstiaclms (E Crg) Eodiscoglossus (M Jur?, E Crei Alytes (Mlo?. Ph-Rec) Discoglossus (Mio~Rec) Barbourula (Rec) Bombina (Mio~Rec) Pelobates (Mio-Rec) Eopelobates (Eoc-Pli) Megophrys (Rec) pBlodytas IMto-Rec) Pipa (Oua-Rec) Xenopus (Pal~Rec) Rhtnophrynus (Oli-Rec) Palaeobatrachus (L Cre-Pb)
ali> 109 A specimen of Loccotriton subsolanus (dorsal view, snout-pelvis length about 40 mml, a small, primitive metamorphic salamander, from Fengsltan locality in Fengning, Hebei. I Photo: Mick Ellison/ AMNH)
1110 Holotype ofSmerpeton fengshanensis (dorsal view, snout-pelvis length 47 mm), a primitive salamander with neotenic features indicated by the ossified eeratobranchi.il (denoted by a red arrow) and ossified carpals (denoted by yellow arrows), from Fengshan locality in Fengning, Hebei. (Photo: Mick Ellison/ AMNH)
Cladogram showing the hypothesized phylogenetic relationships of major lineages of archaic anurans and the relationships within the Discoglossidae, with twojehol frogs included lin red).
■fyWI Holotype ofJeholotriton paradoxus (Slab A, ventral view, body length about 140 mm), a primitive neotenic salamander similar to living cryptobranchoids in some skeletal structures, from Daohugou locality in Ningcheng, Inner Mongolia. (Photo: IVPP)
112 One of the three paratypes of Jeholotriton paradoxus (Slab A. lateral view, body length about 120 mm). Note aquatic adaptation as shown by the presence ofexternal gills (denoted by a red arrow), laterally compressed tail, unossitled carpals and tarsals, and some other osteological features. (Photo: 1VPP)
be included in the Jehol Group (sensu lato) that has yielded the Jehol Biota. Some other fossils were recovered from the Daohugou locality of southeastern Inner Mongolia, the strata older in age (refer to Chapter 2 for stratigraphie information). The discovery of Mesozoic salamanders from China has special implications, as they are the earliest known representatives of modern salamander groups, and thus, provide important insights into the early evolution of some anatomical structures and the biogeographic history of these tailed amphibians.
Living salamanders form China have been grouped in three families: the Hynobiidae, the Cryptobranchidae, and the Salamandridae. None of the fossil salamanders reported from northern China can be assigned to any of these families
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