The earliest turtles evolved up to 300 million years ago, branching off from a group of reptiles more closely related to crocodiles and birds than to lizards and snakes. Lots of turtle species have come and gone since then, including some spectacular ones like the car-sized "coal turtle" or the Koopa-like Meiolania damelipi.

But today's turtles face an unusually widespread danger, with about 40% of Earth's 335 known species listed as either vulnerable or threatened or endangered with extinction. They're largely under siege from humans, yet unlike King Koopa, they didn't bring this on themselves — and they're up against more than just a few plumbers and princesses.

Poaching for food and the international pet trade are two of turtles' biggest threats, especially for high-demand species like the American bog turtle or the Roti Island snake-necked turtle. Habitat loss is another problem, since many turtle and tortoise species depend on specific ecological niches. Land turtles are also often killed by vehicles on roads, while sea turtles face a litany of risks such as oil spills, boat strikes and fishing gear. Slow growth and reproduction rates can make it even harder to overcome these hurdles.

In light of all this, May 23 is World Turtle Day, an annual holiday founded in 2000 by American Tortoise Rescue to raise awareness of turtles' troubles. According to ATR co-founder Susan Tellem, the holiday is meant to generally boost interest in turtles while also specifically reducing ecological pressure from the pet trade.

"World Turtle Day was started to increase respect and knowledge for the world's oldest creatures," she says in a press release. "These gentle animals have been around for about 200 million years, yet they are rapidly disappearing as a result of the exotic food industry, habitat destruction, global warming and the cruel pet trade." The ultimate goal, she adds, "is to stop the illegal trade in turtles and tortoises around the world."

That's a daunting task, but ATR knows slow and steady wins the race. The group rescues "abandoned or lost" turtles on an individual basis, having helped more than 3,000 since its inception in 1990. The idea of World Turtle Day is to expand this effort globally, spreading the notion of turtles as wildlife rather than pets. And in that spirit, here are some amazing turtle photos, plus a few tips on helping these ancient animals in your local ecosystem:

The hawksbill sea turtle has been hunted for its beautiful shell. (Photo: Rich Carey/Shutterstock)

Hawksbill sea turtle (Eretmochelys imbricata)

Range: Tropical oceans worldwide

Status: Critically endangered

Threats: Bycatch in fishing gear, boat strikes, oil spills, disturbance of eggs in nest

An Indian narrowheaded softshell turtle. (Photo: Krishna Kumar Mishra [CC BY 3.0]/Wikimedia Commons)

Indian narrow-headed softshell turtle (Chitra indica)

Range: India, Nepal, Bangladesh

Status: Endangered

Threats: Habitat loss, hunting (for food, materials and folk medicines)

A baby bog turtle found in northern New Jersey. (Photo: Rosie Walunas [public domain]/USFWS/Flickr)

Bog turtle (Glyptemys muhlenbergii)

Range: Eastern U.S.

Status: Critically endangered

Threats: Habitat loss, pet trade, invasive plants

A snakeneck turtle. (Photo: reggie35 [CC BY-SA 2.0]/Flickr)

Eastern long-necked turtle (Chelodina longicollis)

Range: Australia

Status: Least concern

Threats: Road mortality, pet trade

Diamondback terrapin (Malaclemys terrapin). (Photo: Jay Ondreicka/Shutterstock)

Diamondback terrapin (Malaclemys terrapin)

Range: Eastern U.S., Gulf Coast

Status: Lower risk/Near threatened

Threats: Habitat loss, hunting for food, bycatch in crab traps, pet trade, roads

And old male wood turtle. (Photo: Richard Bonnett [CC BY 2.0]/Flickr)

Wood turtle (Glyptemys insculpta)

Range: Northeast U.S., southeast Canada

Status: Endangered

Threats: Habitat loss, pet trade, roads, farming accidents

Gopher tortoise (Gopherus polyphemus)

Range: Southeast U.S.

Status: Vulnerable

Threats: Habitat loss, hunting for food, pet trade, road

Alligator snapping turtle (Macrochelys temminckii). (Photo: Ryan M. Bolton/Shutterstock)

Alligator snapping turtle (Macrochelys temminckii)

Range: Southern U.S.

Status: Vulnerable

Threats: Habitat loss, hunting for food, pet trade, bycatch in fishing gear

A Kemp's Ridley sea turtle returns to the water after laying a clutch of eggs on the beach at Padre Island. (Photo: JB Manning/Shutterstock)

Kemp's ridley sea turtle (Lepidochelys kempii)

Range: Western Atlantic Ocean, Gulf of Mexico

Status: Critically endangered

Threats: Bycatch in fishing gear, boat strikes, oil spills, disturbance of eggs in nest

A Midland painted turtle (Chrysemys picta marginata) basks on a log at Pinery Provincial Park, Ontario, Canada. (Photo: Brian Lasenby/Shutterstock)

Painted turtle (Chrysemys picta)

Range: Canada, U.S., Mexico

Status: Least concern

Threats: Habitat loss, pet trade, roads.

Yellow-spotted Amazon river turtle (Podocnemis unifilis)

Range: Amazon River Basin

Status: Vulnerable

Threats: Pet trade, hunting for food, collection of eggs for food

Green sea turtle (Chelonia mydas) swims to the surface to breath air with remora fish. (Photo: Hans Gert Broeder/Shutterstock)

Green sea turtle (Chelonia mydas)

Range: Tropical and subtropical oceans worldwide

Status: Endangered

Threats: Bycatch in fishing gear, boat strikes, oil spills, disturbance of eggs in nest

If you'd like to pitch in this World Turtle Day, here are five tips from American Tortoise Rescue (ATR differentiates between turtles and tortoises, but for simplicity they're all called turtles here.) Check out the group's website for more ways to get involved.