Abstract
Globacrochordiceras transpacificum gen. et sp. nov. is an ammonoid (Ammonoidea, Cephalopoda) with a shell characterized by plicate ribbing (rounded and undulating ribs strengthening on the venter without interruption), increasing involution through ontogeny, overhanging and deep umbilical wall, absence of tuberculation, subtriangular whorl section, globose adult shape with a closed umbilicus followed by an abrupt egressive coiling, and a subammonitic adult suture line. This new taxon occurs in Nevada (USA) and in Guangxi (South China). It has its typical occurrence within the Neopopanoceras haugi Zone of late Spathian age (Early Triassic). The plicate ribbing, suture line and general shell shape are diagnostic of the family Acrochordiceratidae. The large adult size, high degree of involution and subammonitic suture line of Globacrochordiceras markedly contrast with the next younger genus of the family (Paracrochordiceras of early Anisian age, Middle Triassic), which is evolute and displays a ceratitic suture shape. Shell coiling and suture line of Globacrochordiceras are closer to that of the youngest member of the family: Acrochordiceras carolinae (late middle Anisian). The latter is the end-member of a long-term morphological evolutionary trend of the family during the early and middle Anisian. This trend composed of classical increases in adult size (Cope's rule), shell involution and suture indentation, lasted ca. four Myr. The sudden morphological evolutionary jump between Globacrochordiceras and Paracrochordiceras at the Spathian/Anisian (Early/Middle Triassic) boundary may correspond to a generalized morphological reset of long-term trends, a process that differs from classic paedomorphic transformations. A dramatic global sea level change and carbon isotope positive excursion at the Early/Middle Triassic boundary both indicate stressful environmental changes that may have triggered this evolutionary jump. doi:10.1002/mmng.201300010
Highlights
The family Acrochordiceratidae (Ammonoidea, Cephalopoda, Mollusca) was established by Arthaber (1911, p. 179) and represents one of the main components of ammonoid faunas during the Anisian
This study describes a new genus of Acrochordiceratidae (Ammonoidea, Cephalopoda), namely Globacrochordiceras, which records the oldest known member of the family
This new taxon occurs in the Neopopanoceras haugi Zone of Nevada (USA) and Guangxi (South China)
Summary
The family Acrochordiceratidae (Ammonoidea, Cephalopoda, Mollusca) was established by Arthaber (1911, p. 179) and represents one of the main components of ammonoid faunas during the Anisian (ca. 248–241 Ma; Middle Triassic). Large-sized acrochordiceratid characterized by compressed to depressed shell, moderately evolute to involute coiling, rounded and low-arched venter, abrupt umbilical shoulders, slightly sinuous to straight, rectiradiate ribs, plicate ribbing, a single row of tubercles from which may arise several ribs, presence of parabolic nodes on inner whorls, and ceratitic to subammonitic suture line with smoothly crenulated saddles and deeply indented lobes. Acrochordiceras covers a large morphological spectrum because of its long-term evolutionary trends during the middle Anisian (see Monnet et al 2010, 2012) It is a medium- to large-sized shell with moderately evolute to involute coiling, rather subquadrate, compressed to depressed whorl section, ceratitic to subammonitic suture line, and the presence of one row of tubercles. The species occurs in the Neopopanoceras haugi Zone (late Spathian, Early Triassic) from the NW Guangxi (South China) and from the northern Humboldt Range (Nevada, USA). Suture line subammonitic with weakly indented, rounded, narrow, elongated saddles, and deeply crenulated lobes with the first one being deeper and larger than the first saddle (Fig. 4)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
