Early Land Animals
Bill
Shear (Hampden-Sydney College, Hampden-Sydney, Virginia, USA)
Department of Entomology & Department of Biology,
Ohio State University
23 November 1999
Some
remarkable early terrestrial animals were found by paleobotanists (Birmingham
Campus of SUNY) recently in the Devonian of New York. The evidence for
the earliest terrestrial ecosystems is scattered and fragmentary - most of it
is in the Afro-Arabian area of Gondwana and southern Laurentia (Early Paleozoic
coordinates).
The
earliest evidence is fossil spores. Sporopollenin is very resistant,
including being resistant to bacterial attack. The Middle Ordovician of
Arabia has spore tetrads, which are markers for embryophytic plants (correlates
with terrestriality) - were dispersed as tetrads (groups of four) - modern
liverworts still do this. Trilete marks indicate that a single spore was
originally from a tetrad. Very few other remains are available.
There are no macrofossils in the Ordovician, though, that contribute to our
understanding of the earliest terrestrial ecosystems.
We
see a shift from dispersed tetrads to dispersed individual spores (moss grade)
by the Silurian. Tracheids are found in Silurian rocks, as well as plant
cuticle, including cuticle from enigmatic plants called pneumatophytes, plants
consisting of trunks or bundles of tiny tubes. By the Silurian, most
evidence for terrestrial ecosystems is from either side of the Acado-Caledonide
Mountains. Animal evidence begins to show up. Middle Silurian
sediments (including the Rose Hill Formation of western Virginia) contains
arthropod cuticle (Shear showed a Rose Hill setate podomere from some arthropod).
In
the Upper Silurian, there is richer evidence of terrestrial plants and animals,
including Cooksonia (basically living astroturf) - Cooksonia
comes in vascular and nonvascular forms which are otherwise indistinguishable
at the generic level. Looking at the type locality for the Ludlow in
Wales (in town of Ludlow), we have a succession of Ludlovian rocks containing a
fish bed which is traditionally chiseled way back into the outcrop, but
has been recently bulldozed in order to more easily sample nowadays. This
fish bed contains tea-leaf-plant-fragment hash and a nice eurypterid claw
chelicera. It also has yielded a coalified spider-like arachnid, a
trigonotarbid (an arachnid without spinnerets and no silk production and the
abdomen is still segmented). This trigonotarbid is the earliest known
land animal (Late Silurian). In the Upper Silurian of Scotland, there is
a millipede-like fossil called Archadesmus which has verifiable
diplosegments; so, millipedes were also early invaders of the land.
The
Old Red Sandstone of southern Scotland was partly deposited near hydrothermal
activity near the town of Rhynie. The paleo-hot springs would
occasionally overflow into surrounding vegetated areas, resulting in the local
terrestrial flora and fauna being preserved in fine-grained chert, the Rhynie
Chert. Rhynie Chert has long been used by locals as building stone, and
has been called noodle rock. The noodles in the rock are actually plant
stems with wonderfully preserved details. The Rhynie Chert is ~405 my
(Devonian), and is a potential gold (Au) field. In the Devonian, contrary
to most reconstructions, the land probably had vast tracts of monospecific
stands of plants, rather than the mixed diversity scenes we often see.
The Devonian land plants probably used a turfing strategy. The Rhynie
Chert also has animal fossils, including trigonotarbids (with compound eyes and
book lungs preserved, as well as exceptional preservation of carapace and legs,
etc.). There is an exceptionally good specimen in 1 translucent chip that
is reminiscent of amber - the trigonotarbid has 3-D preservation that shows up
beautifully in transmitted light.
Near
Gilboa, New York state, a pump storage facility was recently being constructed,
and the excavation uncovered a lens of dark gray shale packed with plant
remains. The construction crew was good enough to stop and allow
collecting by SUNY Birmingham paleobotanists for 1 day - they came with a
pick-up truck and collected several tons of rock.
By
the Mid-Devonian, vegetation was more layered than before - had tall trees to
low shrubby things. The Gilboa material included trigonotarbid arthropods
- only about a dozen are complete specimens. Trigonotarbids are
fundamentally spider-like, but they lack many spider apomorphies.
Found
a spinneret in the Gilboa macerations with spigots from which silk was issued -
this was obviously from a real spider. Cellular impressions were found on
this spiders cuticle. It has been reconstructed based on additional
macerated material from this site to be similar to the most primitive living
spiders. This Devonian spider is called Attercopus [Note:
does anyone recognize the inspiration for the genus name of this fossil
spider? Bilbo Baggins in The Hobbit used the name
"Attercop" as an insult when confronting the mongo spiders of
Mirkwood Forest!]
Attercopus has silk
spigots that appear to have been scattered over the ventral surface of the
abdomen, rather than concentrated at the posterior tip of the abdomen.
Showed a chelicera of Attercopus - < 1/10 mm across - wonderful
preservation; a spinneret for silk occurs on the chelicerae.
The
1st Paleozoic pseudoscorpions also occurs in this material, and is called Dracochela
- a representative of a primitive clade of pseudoscorpions. Pseudoscorpions
lack a stinger, and their relation to true scorpions is dubious.
Also
have orobatid mite material in the Gilboa collections, some of which is very
well preserved with setal hairs. Gilboa has also yielded centipede
remains. There is also potential evidence of Devonian insects in this
material - cuticle with coffin-shaped sockets reminiscent of bristletails -
flightless insects living today; or, perhaps, they represent silverfish.
A
later Devonian site near Gilboa (South Mountain locality) has arthropleurid leg
material (forms which by the Late Carboniferous were 6' long and 2' across) and
Attercopus spider cuticle & silk spigots and also a hemipteran
insect-like head (there is no smoking gun for insect material - but there is
mounting evidence for Devonian insects). This locality also has Archaeospermatopsis,
probably the first tree - had spore & seed bearing forms, indistinguishable
from each other at the generic level.
The
Red Hill locality in central Pennsylvania has yielded a scorpion leg with
individual setae in place, etc.; also part of a scorpion pincer; also large
patches of unidentified arthropod cuticle.
Vertebrates
were real latecomers to the land - they were more aquatic than terrestrial -
they may still have had internal gills. They were capable of land
excursions, but to what extent is unknown. Ichthyostega has 7
digits on each front leg and 8 digits on each hind leg (not the traditional 5
each). The earliest demonstrated tetrapod that was a herbivore is from
near the Pennsylvanian-Permian boundary.
The
earliest evidence for herbivory is probably Middle Carboniferous insects.
There is a problem with looking at insects mouth parts in judging herbivory
based on morphology - a praying mantis mandible & a locust mandible will
not inherently indicate to you that the mantis is predatory and the locust is
herbivorous. In the Middle and Upper Devonian, there is evidence of
suctorial damage to plants (by aphid-like things?). Many mites are
pollenivores. Spiderlings are also - they will make an orb web that
catches pollen, and the web with its pollen is eaten.