Going Upstream

The Upstream Progression of Ecosystem Development
during the Devonian Transformation

The span from the Middle Silurian to the Late Devonian exhibited a progressive increase in the diversity and productivity of estuarine, freshwater and terrestrial ecosystems. It was also a time during which many freshwater and terrestrial lineages first appeared in the fossil record.

Early Desolation

Meager and often inconclusive fossil evidence indicate that pre-Silurian terrestrial and freshwater habitats were poor to nonexistent. There are some possible vascular plant spores and suggestive invertebrate trace fossils, but that's about it. Several researchers have reported estuarine fossils of invertebrates and jawless fishes, but their estuarine status is inconclusive. In any case, they were not clearly differentiated from marine forms.

Mid-to-Late Silurian Beginnings

Estuarine animals become significantly more plentiful and diverse in the Middle and Late Silurian. In addition, they’re more clearly differentiated from contemporary marine assemblages. Confirmed freshwater assemblages are found in some Late Silurian deposits, but most of the freshwater forms are indistinguishable from their estuarine counterparts.

Perhaps the most conspicuous animals are chelicerate arthropods, including eurypterids (water scorpions), xiphosurans (horseshoe crabs) and aquatic scorpions. Vertebrates are represented by a variety of jawless fishes (anaspids, thelodonts, osteostracans and heterostracans). Lingulid brachiopods, ostrocod crustaceans, gastropods (snails) and bivalves (clams) are also notable faunal elements.

middle silurian aquatic faunal development

The increase in abundance and diversity among estuarine and freshwater animals is probably attributable to the expansion of vascular plants in aquatic and terrestrial habitats. These early vascular plants—collectively known as rhyniopytes—first appear in the fossil record in the late Early Silurian and diversify considerably through the Middle and Late Silurian. Their rhizomes would have helped stabilize banks and shallow water habitats. Moreover, decaying plant matter would have increased the amount of food available to estuarine and freshwater animals.

Feeding dynamics in Middle to Late Silurian estuaries appeared to be relatively simple. Most animals probably fed on benthic (bottom) detritus and/or algae. Eurypterids and aquatic scorpions were the dominant macropredators. Most aquatic animals are relatively small (<20 cm), but some eurypterids exceeded a meter in length. In fact, the largest known arthropods were eurypterids.

Progress in the Early Devonian

The expansion and diversification of vascular plants in the Early Devonian led to further increases in the diversity and abundance of estuarine and —especially— freshwater animals. Many of the Silurian groups were still important in Early Devonian environments, but several other groups either become substantially more abundant and diverse or appear for the first time in the fossil record.

Osteostrachan and heterostrachan jawless fishes diversify considerably and expand further into freshwater habitats. But perhaps the biggest changes in the vertebrate fauna occur among the jawed fishes. The previously uncommon acanthodians become abundant in both estuarine and freshwater localities. Arthrodire placoderms and some lobe-fin fishes invaded estuarine and freshwater habitats; lungfishes were apparently restricted to transitional marine-brackish habitats.

early devonian aquatic faunal development

Early Devonian freshwater faunas are also notable for the appearance of a variety of small branchiopod crustaceans, including Conchostraca (clam shrimp), Anostraca (fairy shrimp), Lipostraca and Acercostraca. The appearance of these swimming microcrustaceans suggest increasingly stable stream flows.

Feeding dynamics become increasingly complex. Food webs were probably based on aquatic algae and detritus that originated from both terrestrial and aquatic macrophytes. However, because vascular plant tissues are more resistant to decomposition, their detritus probably persisted longer that the more readily degraded algal and bryophyte detritus that predominated during the Silurian. The presence of swimming branchiopod crustaceans suggests that there was an increase of small food particles in the water column. Presumably, active vertebrate swimmers such as acanthodians fed on these crustaceans. Benthic predatory niches were increasingly occupied by other vertebrates (i.e., heterostrachans, arthrodires and lobe-fins), possibly at the expense of eurypterids, which declined throughout the Devonian. Some previously aquatic arthropods (e.g., scorpions) became increasingly terrestrial in habit.

Transformation

Dramatic changes in the composition, productivity and complexity of estuarine and freshwater ecosystems occurred throughout the Devonian. By the end of the period, eurypterids become uncommon and jawless fishes are absent. Diverse assemblages of jawed vertebrates including acanthodians, sharks, arthrodire and antiarch placoderms, ray-fin fishes, lobe-fin fishes and tetrapods are found in many estuarine and lowland river localities. Indeed, some formerly marine groups (e.g., lungfishes) become increasingly restricted to estuarine and freshwater habitats following the Late Devonian.

The record for aquatic invertebrates is scarce. Two groups of small freshwater crustaceans first recorded in the Early Devonian (conchostracans and anostracans) are extant, so they must have been present in the Late Devonian. Freshwater clams have been reported from several freshwater localities, including some from the Catskill Delta.

late devonian aquatic faunal development

The variety of Late Devonian estuarine and freshwater vertebrates indicate increasingly complex food webs. Antiarch placoderms (e.g., Bothriolepis) were probably bottom-feeding detritivores, but numerous small-to-medium sized bottom feeding predators (e.g., lungfishes and groenlandapsid placoderms) suggest that benthic invertebrate provided an abundant and reliable food supply. Similarly, the abundance of active swimmers such as Limnomis and reports of freshwater clams indicated that food in the water column was also plentiful; plankton may have also been consumed by another Red Hill vertebrate, Gyracanthus. An assortment of lobe-fin fishes probably fed on other vertebrates.

The increased productivity of Late Devonian estuaries and freshwater ecosystems is also indicated by an increase in body size. Several of the Late Devonian vertebrates were substantially larger than their Silurian and Early Devonian counterparts. For instance Ctenacanthus, Gyracanthus, tetrapods and several lobe-fins typically approached a meter in length, whereas few of the earlier vertebrates exceeded 30 cm. Moreover, two groups of Late Devonian lobe-fins, rhizodonts and tristichopterids, often exceeded 2-3 m in length.

The expansion of terrestrial vascular plants during the Middle and Late Devonian is probably the single most important contributor to these changes in non-marine aquatic environments. Extensive riparian and wetlands vegetation enriched streams and estuaries with their organic matter exports. These plants also helped moderate flooding and otherwise stabilize and diversify stream habitats

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Web:
Palaeos.com's web pages on eurypterids, scorpions & xiphosurans:
www.palaeos.com/Invertebrates/Arthropods/Eurypterida/
www.palaeos.com/Invertebrates/Arthropods/Scorpionida/Scorpionida.htm
www.palaeos.com/Invertebrates/Arthropods/Xiphosura/Xiphosura.htm
Palaeos.com's web pages on jawless fishes:
www.palaeos.com/Vertebrates/Units/020Craniata/100.html
U.C. Museum of Paleontology web page on eurypterids and xiphysurans:
www.ucmp.berkeley.edu/arthropoda/chelicerata/eurypterida.html
www.ucmp.berkeley.edu/arthropoda/chelicerata/xiphosura.html
Scientific Papers:
Beerbower, J.R. 1985. "Early development of continental ecosystems." pp. 47-92. In: B.H. Tiffney (ed.), Geological Factors and the Evolution of Plants. Yale Univ. Press: New Haven.
Beerbower, J.R., J.A. Boy, W.A. DiMichele, R.A. Gastaldo, R. Hook, N. Hotton, III, T.L. Phillips, S.E. Scheckler, and W.A. Shear. 1992. "Paleozoic terrestrial ecosystems." pp. 205-235. In: A.K.Behrensmeyer, J.D. Damuth, W.A. DiMichele, R.Potts, H.-D. Sues and S.L. Wing (eds.) Terrestrial Ecosystems throught Time. Chicago: Univ. Chicago Press, Chicago.
Image Credits:
The three images above are copyrighted © 2002, Dennis C. Murphy, (see Terms of Use).

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