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Late K, NA <--> SA Paleobiogeography (LONG)



Hi All -

        Recently, the issue of a Late Cretaceous land bridge linking North
and South America has surfaced.  I've had a little bit of experience with
the issue and thought some of you might be interested in what I learned, so
I'll try and summarize! 

        First, a little history:  students of paleogeography and
paleobiogeography have long known that North American and South America
were conncted during the Triassic and Early Jurassic (as part of Pangaea)
and again in the Late Pliocene-present (via the Panamanian Isthmus).  Some
question has arisen as to whether or not they were connected at any point
in between, particularly during the Cretaceous, when we see similar
vertebrate faunas on both continents.  Paleobiogeographically, it seems
certain that such a land bridge existed, but tectonicists have, largely,
reconstructed the tectonics of the time period without any connection
between the two, based on structura and paleomagnetic data.

        Some recent paleontologic analyses have indicated that there are
some faunal similarities between North and South America in the Early
Cretaceous (e.g., carcharodontosaurid theropods [_Acrocanthosaurus_ and
_Giganotosaurus_], primitive titanosaurian sauropods [_Andesaurus_ et al.
and _Pleurocoelus_ plus unnamed, new forms], and primitive iguandontians
[_Gasparinisaura_].  The question about these remains as to whether these
are relics of an earlier, Jurassic connection, the vicariant remnants of a
broadly distributed, Pangaean population of organisms, or due to an actual
terrestrial connection persisting into the Early Cretaceous-early Late
Cretaceous.  By the late Late Cretaceous, the faunas are singularly
different, with abelisaurid theropods and titanosaurian sauropods
predominating in South America, while ceratopsian, ankylosaurian,  and
hadrosaurid ornithischians and tyrannosaurid, dromaeosaurid, troodontid,
etc. theropods predominating in North America.  By the
Campanian-Maastrichtian, similarities reappear, with nyctosaurid pterosaurs
(Price, 1953), avisaurid enantiornithines (Brett-Surman & Paul, 1985;
Chiappe, 1993; Varricchio & Chiappe, 1995), saltasaurine titanosaurids
(Gilmore, 1946; McIntosh et al., 1992), troodontid(???) and alvarezsaurid
("mononykine") theropods (Novas, 1994), ankylosaurid ankylosaurs (Gasparini
et al., 1987), hadrosaurine hadrosaurids (Bonaparte, 1984), and possibly
even ceratopsians (I don't have a ref handy for this one, but I recall
reading about it somewhere...) occurring on both continents (and
Antarctica).  Small, non-volant tetrapods seem, for the present,  to have
remained endemic (e.g., gondwanatherian multituberculates [Krause et al.,
1992; Krause & Bonaparte, 1993]).

        It seems fairly clear that _some_ vector existed for the exchanges,
but few tectonic scenarios presented one (although even fewer considered
paleontologic evidence at all...).  Focus has been on Central America, as
it has been for the Pliocene-present exchange, particularly since
Cenomanian-age ornithopod material has been found in Costa Rica (Horne,
1994).  Reconstructing the nature of the bridge based on paleontological
material is, sadly, a virtual impossibility, since outcrops of Mesozoic
terrestrial sediments are few and far between and generally covered either
by lush jungle or Cenozoic igneous rocks.  Thus, answers must be sought
elsewhere, and this is where tectonicists have focused their research.

        First, we must understand that Central America is composed of
several tectonic microplates (distinguishable, recognizeable, and delimited
along marked faults by different metamorphosed rocks on either side). 
North America _sensu stricto_ extends as far south as the Sant Cruz fault
in southern Mexico (Donnelly and Lopez-Ramos, 1990).  The southern tip of
Mexico, Belize, and most of Guatamala consist of what is called the Maya
Block (Donnelly and Lopex-Ramos, 1990); the rest of Guatemala, El Salvador,
Honduras, and northern Nicaragua are on the Chortis Block (Horne et al.,
1990); southern Nicaragua, Costa Rica, and the northern tip of Panama are
on the Chorotega Block; and the rest of Panama and a bit of northern
Columbia are on the Choco Block (Escalante, 1990). The Maya Block was
already grafted onto North America by the Late Jurassic-Early Cretaceous
(Pindell & Barrett, 1990).  Of particular interest to us is the Chortis
Block.  

        The Chortis Block has long been recognized as a migrant portion of
the North American plate that was sheared off the Pacific side of the plate
by an alteration in movement direction of the Pacific Plate against the
Farallon and North American Plates.  Malfait & Dinkleman (1972) placed this
shearing in the Tertiary, but Gose & Swartz (1977) and Pindell & Barrett
(1990) moved the origin of shear in the Cretaceous; its initiation has been
implicated even as far back as the Middle Jurassic (Anderson and Schmidt,
1983).  The shear origin of the plate, and in fact of most other Central
American plates related in a complex way to the origin of the Caribbean
Plate, which was a "peninsula" of the Farallon Plate (this plate is almost
completely subducted beneath the North American plate today; the Juan de
Fuca plate seen off the coast of Washington and Oregon is a tiny piece of
this former giant).  The Farallon was subducting nicely beneath both the
North and South American plates in the Mesozoic, but between North and
South America, an ocean-ocean plate collision occurred; eventually, a
portion of the Farallon overrode the Atlantic portion, creating a complex
series of island arcs and faults; traces of this can be seen today in the
Aves Swell, the Beata Ridge, and in Cuba.  Significant faulting at the
western edge of this effectively created the Caribbean Plate.

        Anyway, back to the Chortis Block.  Between the mid-Cretaceous and
early Tertiary, paleomagnetic data from this block show it underwent a
remarkable series of rotations and motions (Gose, 1985; Gose and Swartz,
1977).  Gose (1985) found the block to be adjacent to southern Mexico and
Guatemala by the Late Cretaceous.  The plate position reconstructions of
Anderson and Schmidt (1983) indicate that the Chortis Block was perfectly
situated to serve as a terrestrial land bridge between North and South
America around the Cenomanian (remember that North and South America
weren't as far apart then as they are now). Possibly the dating of this
connection is off, and the bridge existed later in the Cretaceous, in time
to serve as the connection in the Campanian-Maastrichtian; as before,
finding suitable rocks to date, especially those with fossils in them, is
difficult under the present-day vegetation and volcanic rock.  (Late
Cretaceous subduction of the Farallon beneath the new Caribbean plate
begins to create layered igneous rock and marine sedimentary masses and
island arcs whose later accretion creates the Chorotega and Choco Blocks
and wil be incorporated into the future Panamanian isthmus.  Late
Cretaceous marine sediments and fossils are known on these blocks; this,
coupled with their paleopositions, indicated that they could not serve as
terrestrial land bridges in the Cretaceous.)

        So there's a good possibility of an early Late Cretaceous
connection between North and South America, and a more remote though still
conceivable possibility of one again in the Campanian-Maastrichtian.  I'm
much less familiar with theories of paleobiogeographic vectors in the Late
Cretaceous across a proto-Antillean arc (if anyone's got references, I'd
love to have some!), but island-hopping by sauropods, hadrosaurs, and
ankylosaurs seems less parsimonious to me than a sturdy, if short-lived,
land bridge!  Nor have I looked into this issue more recently than about
1995, so if anyone knows of newer tectonic references to the problem, I'd
be interested in those, too!  Hope this helps some people...

Refs: 

Anderson, T.H. and Schmidt, V.A.  1983.  The evolution of Middle America
and the Gulf of Mexico - Caribbean Sea Region during Mesozoic time.  _GSA
Bulletin_ 94:  941-966.

Bonaparte, J.F.  1984.  Nuevas pruebas de la conexion fisica entre
Sudamerica y Nortemareica en el Cretacico Tardio (Campaniano).  _III
Congreso Argentino de Paleontologia y Biostratigrafica_:  141-149.

_____.  1994.  Approach to the significance of the Late Cretaceous mammals
of South America.  _Berliner Geowissenschaften Abhandlungen_ 13:  31-44.

Brett-Surman, M.K. and Paul, G.S.  1985.  A new family of bird-like
dinosaurs linking Laurasia and Gondwanaland.  _Journal of Vertebrate
Paleontology_ 5:  133-138.

Chiappe, L.M.  1993.  Enantiornithine (Aves) tarsometatarsi from the
Cretaceous Lecho Formation of northwestern Argentina.  _American Museum
Novitates_ 3083:  1-27.

Donnelly, T.W. and Lopez-Ramos, E.  1990.  The Maya Block and Motagua
Suture Zone, pp. 37-55 _in_ Dengo, G. and Case, J.E. (eds.)  _The Caribbean
Region:  The Geology of North America Volume H_.  Boulder:  GSA.

Escalante, G.  1990.  The geology of southern Central America and western
Columbia, pp. 201-230 _in_  Dengo, G. and Case, J.E. (eds.)  _The Caribbean
Region:  The Geology of North America Volume H_.  Boulder:  GSA.

Gasparini, Z., Olivero, E. Scasso, R., and Rinaldi, C.  1987.  Un
ankylosaurio (Reptilia, Ornithischia) Campaniano en el continente
Antartico.  _Anais do X Congresso Brasileiro de Paleontologia_ 1:  131-141.

Gilmore, C.W.  1946.  Reptilian faun of the North Horn Formation of central
Utah.  _U.S.G.S. Professional Paper_ 210-C:  29-52.

Gose, W.A.  1985.  Paleomagnetics results from Honduras and their bearing
on Caribbean tectonics.  _Tectonics_ 4:  565-585.

Gose, W.A. and Swartz, D.K.  1977.  Paleomagnetic results from Cretaceous
sediments in honduras:  tectonic implications.  _Geology_ 5:  505-508.

Horne, G.S.  1994.  A mid-Cretaceous ornithopod from central Honduras. 
_Journal of Vertebrate Paleontology_ 14:  147-150.

_____, Finch, R.C., and Donnelly, T.W.  1990.  The Chortis Block, pp. 55-76
_in_  Dengo, G. and Case, J.E. (eds.)  _The Caribbean Region:  The Geology
of North America Volume H_.  Boulder:  GSA.

Krause, D.W., Kielan-Jaworowska, Z., and Bonaparte, J.F.  1992. 
_Ferugliiotherium_ Bonaparte, the first known multituberculate from South
America.  _Journal of Vertebrate Paleontology_ 12:  351-376.

_____. and Bonaparte, J.F.  1993.  Superfamily Gondwanatheroidea:  a
previously unrecognized radiation of multituberculate mammals in South
America.  _Proceedings of the National Academy of Science_ 90:  9379-9383.

Malfait, B.T. and Dinkelman, M.G.  1972.  Circum-Caribbean tectonics and
igneous activity and the evolution of the Caribbean Plate.  _GSA Bulletin_
83:  251-272.

McIntosh, J.S., Coombs, W.P., and Russell, D.A.  1992.  A new diplodocid
sauropods (Dinosauria) from Wyoming, USA.  _Journal of Vertebrate
Paleontology_ 12:  158-167.

Novas, F.E.  1994.  Patagonykus puertai n. gen. et sp., and the
phylogenetic relationships of the Alvarezsauridae (Theropoda, Maniraptora).
 _VI Argentine Congress on Paleontology and Biostratigraphy_.
        
Pindell, J.L. and Barrett, S.F.  1990.  Geological evolution of the
Caribbean Region; a plate-tectonic perspective, pp. 405-432 _in_  Dengo, G.
and Case, J.E. (eds.)  _The Caribbean Region:  The Geology of North America
Volume H_.  Boulder:  GSA.

Price, L.I.  1953.  A presenca de Pterosauria no Cretaceo Superior do
Estado de Paraiba.  _Ministerio da Agricultura Notas Preliminares e
Estudos_ 71:  1-10. 

Varricchio, D.J. and Chiappe, L.M.  1995.  A new bird from the Cretaceous
Two Medicine Formation of Montana.  _Journal of Vertebrate Paleontology_
15:  201-204.

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                     Jerry D. Harris
                 Fossil Preparation Lab
          New Mexico Museum of Natural History
                   1801 Mountain Rd NW
               Albuquerque  NM  87104-1375
                 Phone:  (505) 899-2809
                  Fax: ; (505) 841-2866
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