Climatically driven biogeographic provinces of Late Triassic tropical Pangea

Abstract

Although continents were coalesced into the single landmass Pangea, Late Triassic terrestrial tetrapod assemblages are surprisingly provincial. In eastern North America, we show that assemblages dominated by traversodont cynodonts are restricted to a humid 6° equatorial swath that persisted for over 20 million years characterized by "semiprecessional" (approximately 10,000-y) climatic fluctuations reflected in stable carbon isotopes and sedimentary facies in lacustrine strata. More arid regions from 5-20 °N preserve procolophonid-dominated faunal assemblages associated with a much stronger expression of approximately 20,000-y climatic cycles. In the absence of geographic barriers, we hypothesize that these variations in the climatic expression of astronomical forcing produced latitudinal climatic zones that sorted terrestrial vertebrate taxa, perhaps by excretory physiology, into distinct biogeographic provinces tracking latitude, not geographic position, as the proto-North American plate translated northward. Although the early Mesozoic is usually assumed to be characterized by globally distributed land animal communities due to of a lack of geographic barriers, strong provinciality was actually the norm, and nearly global communities were present only after times of massive ecological disruptions.

Fig. 1.

(Upper) Map of basins studied. (Lower) Time-geography nomogram showing correlation of key rift basin sections in eastern North America, typical facies, and distribution of traversodonts and procolophonids. Time scale and paleolatitudes are based on the Newark basin section (–8, 30). The gray curved lines are lines of equal paleolatitude assuming rift basins are within a rigid plate and all drift with Pangea. Red arrows show the position of the studied sections (SI Text): (A) Vinita Formation; (B) Cumnock Formation; (C) lower member Cow Branch Formation; (D) upper member Cow Branch Formation; (E) Lockatong Formation; (F) Balls Bluff Formation; (G) Passaic Formation.

Fig. 2.

Examples of traversodont cynodonts from equatorial latitudes (Left) and procolophonoid parareptiles from higher tropical latududes (Right). Skull is above, and mandible showing teeth is below. Scale bar, 1 cm. See SI Text for specimen data.

Fig. 3.

Frequency spectra of the lacustrine sections. Measured sections and data curves of depth ranks, color, , and TOC are given in the SI Text. Darker gray bands show the range of frequencies expected for specific periods (purple).

Fig. 4.

(Left) Magnitude of maximum insolation (black), insolation at the vernal equinox (blue), and insolation at the autumnal equinox (red) from the equator to 23°N based on the La2004 solution (see Methods). (Right) Frequency spectra of maximum insolation showing the prevalent semiprecessional peak at frequency 0.10 near the equator, and the strong obliquity peak at frequency 0.05 farther north.

Fig. 5.

Conceptual model showing relationship between the latitude of the coincidence of perihelion and the solstice through time. Width of the sinusoidal line is proportional to the insolation intensity at the coincidence of perihelion and solstice following the eccentricity cycles. A, B, and C represent hypothetical lacustrine sections showing lithological variations caused by lake level cycles produced by changes in precipitation tracking the yearly insolation maximum. A and C have pure approximately 20-ky cycles, whereas B at the equator has only an approximately 10-ky cycle.