Why are some of the largest animals vertebrates?

Life on Earth comes in an astonishing number of forms, shapes and sizes. The evolutionary milestone of forming a notochord at least 530 mya marks the beginning of a new lineage. Not only does a notochord provided structural support to some of the earliest invertebrate chordates; it also acted as a site for muscle attachment and is the precursor for vertebral development.

 

Aside from being a crucial element that forms the strong spine you and I would need to stand upright; the development of a vertebra column also allows vertebrates to grow larger without needing to deal with issues that stemmed from having an exoskeleton. These include periodic vulnerability post-molting and a self-limiting factor that prevented parts of the body from growing too thick and too heavy during growth. According to the Square-cube law, any object that has been doubled in body size would have its mass and volume increased by a factor of eight, whereas its ability to support itself, as in the cross-sectional area, would have only increased by a factor of four. Although this rule also applies to the endoskeleton of vertebrates, a combination of bones, muscles and tendons working together has provided stronger structural integrity than what an exoskeleton alone is capable of offering.

 What are ...

 What are ...

Kingdom: Animalia

Phylum: Annelida

Reference(s): 1, 2

Traditionally grouped under 3 classes, but have since underwent on-going major revisions as phylogenomic and other molecular phylogenetic analyses show errors in grouping based on morphology alone. Some taxa have been excluded to form new phyla, whereas some have been included into Annelida.

Subphylum

-None-

Class

Order

Family

Genus

Species

22000

Phylum: Arthropoda

Reference(s): 1

Subphylum

Chelicerata
Crustacea
Hexapoda
Myriapoda
†Trilobitomorpha

Class

Order

16

Family

Genus

Species

1257040

Image Credit: GorissM (CC BY-SA-2.0)

Phylum: Brachiopoda

Reference(s): 1, 2, 3, 4

Most species are known primarily from fossils

Subphylum

Craniiformea
Linguliformea
Rhynchonelliformea

Class

Order

3

5

Family

29

Genus

Species

116

391

Phylum: Bryozoa

Reference(s): 1, 2

Subphylum

-None-

Class

Order

3

4

Family

187

Genus

Species

808

5869

Phylum: Chaetognatha

Reference(s): 1, 2, 3, 4, 5, 6, 7, 8

Subphylum

-None-

Class

Order

1

2

Family

9

Genus

Species

26

132

Phylum: Chordata

Reference(s): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

Almost all are free-living, although many species of agnathans (lampreys and hagfish) and several species of teleost (e.g., toothpick fish and pearlfish) are parasites to other vertebrates and invertebrates. Some males from the Lophiiformes order are sexual parasites to their female counterparts. About 1% of all bird species are brood parasites. Chordata consists of members that are usually on top of the food chain, reigning over members from other phyla across a multitude of ecosystems worldwide. Species richness peaks on the continents and in the marine environment of tropical and subtropical regions. Although distributed unevenly, chordates are found in most, if not all ecosystems on Earth.

Subphylum

Urochordata/Tunicata
Cephalochordata
Vertebrata

Class

Order

16

165

Family

1100

Genus

Species

10000

69000

Phylum: Cnidaria

Reference(s): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

Subphylum

Anthozoa
Medusozoa
Myxozoa

Class

Order

8

27

Family

350

Genus

Species

1800

12300

Image Credit: Bruno C. Vellutini (CC BY-SA-3.0)

Phylum: Ctenophora

Reference(s): 1, 2, 3, 4, 5, 6

All free-living. Superficially resembles jellyfish in the Cnidaria phylum, but possesses 8 rows of cilia arranged in stacks of combs (known as comb plates or ctenes) for locomotion. With the exception of members from the Beroidea order which lack tentacles, and Haeckelia rubra which possesses stinging cells, all other species possess colloblast cells which secrete a glue-like substance to capture preys. Exclusively marine.

Subphylum

-None-

Class

Order

2

10

Family

33

Genus

Species

74

200

Image Credit: Ulf Jondelius (CC BY-NC-SA 4.0)

Phylum: Cycliophora

Reference(s): 1, 2

Subphylum

-None-

Class

Order

1

1

Family

Genus

Species

2

Image Credit: Fossa (YouTube Creative Commons License)

Phylum: Dicyemida

Reference(s): 1, 2, 3

Subphylum

-None-

Class

Order

1

1

Family

3

Genus

Species

9

124

Phylum: Echinodermata

Reference(s): 1, 2, 3, 4, 5, 6

About 13,000 species were extinct as documented in fossil record

Subphylum

Asterozoa
†Blastozoa
Crinozoa
Echinozoa

Class

Order

5

Family

Genus

Species

7300

Image Credit: John Turnbull (CC BY-NC-SA-2.0)

Phylum: Entoprocta

Reference(s): 1, 2, 3, 4, 5, 6, 7, 8

Most if not all adults are sessile, filter-feeding organisms (although some solitary species are capable of slow movement). Some species produce eggs which would hatch into planktonic larvae. These are free-living at first, but will soon settle down into sessile adults. About 156 species are classified under Loxosomatidae, the only family consisting solely of solitary species. The remaining 39 species are colonial. Solitary species are commonly found attaching commensally to various hosts, including sponges, sessile tube-dwelling polychaetes, and bryozoans. The distribution of colonial species is more diverse, as they are found attaching to a wide variety of substrates, including rocks, shells, and algae. Only two species have been documented to live in freshwater. A handful of species live in brackish water, while the remaining large majority are exclusively marine.

Subphylum

-None-

Class

Order

4

2

Family

4

Genus

Species

14

195

Image Credit: Proyecto Agua (CC BY-NC-SA 2.0)

Phylum: Gastrotricha

Reference(s): 1, 2, 3, 4, 5, 6, 7

All free-living. About 366 species live in freshwater, whereas the other 494 species are exclusively marine. Commonly found in the benthic zone and amongst other periphytic communities, ocassionally found in interstitial water.

Subphylum

-None-

Class

Order

0

2

Family

18

Genus

Species

69

860

Image Credit: Nicobola123 (YouTube Creative Commons License)

Phylum: Gnathostomulida

Subphylum

-None-

Class

Order

2

Family

12

Genus

Species

27

101

Image Credit: Necrophorus (CC BY-SA-3.0)

Phylum: Hemichordata

Subphylum

-None-

Class

Order

4

Family

Genus

Species

Image Credit: Alvaro Esteves Migotto (CC BY-NC-SA 3.0)

Phylum: Kinorhyncha

Reference(s): 1, 2, 3, 4, 5, 6

All free-living. Many species inhabit the muddy and sandy sediments of seafloors in both shallow and deep seas. Some are also found living in the sandy regions of seacoasts, as well as in the sediments of brackish water.

Subphylum

-None-

Class

Order

2

2

Family

9

Genus

Species

23

222

Phylum: Loricifera

Reference(s): 1

Subphylum

-None-

Class

Order

1

Family

Genus

Species

9

37

Image Credit: Nicobola123 (YouTube Creative Commons License)

Phylum: Micrognathozoa

Reference(s): 1, 2

Subphylum

-None-

Class

Order

1

1

Family

1

Genus

Species

1

1

Phylum: Mollusca

Reference(s): 1, 2

Subphylum

Aculifera
Conchifera
Placophora

Class

Order

13

Family

Genus

Species

117358

Phylum: Nematoda

Reference(s): 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

Approximately 40% are free-living, the remaining 60% are either endoparasite, ectoparasite, or a combination of both. The diversity of parasitic animal nematodes surpasses that of parasitic plant nematodes with a ratio of 3 to 1. Free-living species are distributed in extremely diverse landscapes (from freshwater to marine to terrestrial environment; from mountainous terrains to ocean depths; from polar regions to tropical zones), though many are found in soils and sediments. Many known parasitic species are highly host-specific, some are even beneficial to us as they infect and kill pests. Although 26,000+ species have been described, total diversity is estimated to be at 100,000 to 10 mil species.

Subphylum

-None-

Class

Order

3